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Astronomy Notes, Page 1
I. Astronomy and Time
A. Coordinate Systems
- systems in which numbers are used to give the locations of bodies or events
1. Coordinates
- set of numbers used to locate something
- must agree on a Zero Point, the spot from which the coordinates are measured
2. Great Circle
- circle that divides a sphere into two parts (Ex. = the Equator)
3. Terrestrial Coordinate System
- system used to locate places on the Earth
- the Zero Point is located on the equator, directly south of Greenwich, England
a. Longitude
- describes distance east or west from the zero point
- angle, measured east or west, around the equator from the point where the Prime Meridian
intersects the equator
- Prime Meridian (at 0° longitude) is the longitude line passing through Greenwich, England; at
180° E and W longitude is the International Dateline
b. Latitude
- angle measured north or south from the equator
- Equator is at 0° latitude; North and South Pole at 90°N and 90°S latitude respectively
B. The Celestial Sphere
- the imaginary sphere surrounding the Earth upon which celestial bodies appear to carry out their
motions
1. Angular Measurement
- used to describe the positions and sizes of objects seen in the sky
a. Degrees
- one degree is 1/360 of a circle
- the most commonly used system for measuring angles uses degrees
- at arms length your index finger is about 1° across, your fist about 10° across the knuckles, and
your outstretched hand about 18° across from the tip of your thumb to the tip of your little finger
b. Minute of arc
- is 1/60 of a degree
- some planets have angular sizes almost as large as a minute of arc
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c. Second of Arc
- 1/60 of a minute
- equals the angular diameter of a penny at a distance of 4 kilometers (2.5 miles)
- angular sizes of stars are all smaller than 1 second of arc
2. The Horizon System
- coordinate system used to locate the positions of objects in the sky, using altitude and azimuth
as coordinates
a. Zenith
- point on the celestial sphere directly above your head
b. Celestial Horizon
- the great circle dividing the celestial sphere into an upper (visible) and lower (invisible) half
- the celestial horizon is situated 90° from the Zenith
c. Meridian
- circle on the celestial sphere that passes from the south celestial pole to the north celestial pole
and passes through the observer's zenith
- north and south points on the observer's horizon occur where the meridian crosses the horizon
- celestial equator is the circle midway between the north and south celestial poles; it divides the
celestial sphere into northern and southern halves
d. Altitude
- angular distance above the celestial horizon
- the horizon has an altitude of 0° and the zenith has an altitude of 90°
e. Azimuth
- angular distance measured eastward from north around the celestial horizon to the point directly
below the chosen position on the celestial sphere
- the azimuth of the east point on the celestial horizon is 90°, the south point is 180°, and the
west point 270°
- altitude and azimuth of a star depend on the time and the location where the observation is
made
C. Diurnal (Daily) Motion
- daily motion of the stars is westward on a circle that is centered on the North Celestial Pole
(situated about 3/4° from Polaris, the North Star)
- pattern of diurnal motion makes it look as if the celestial sphere is rotating on an axis passing
through the celestial poles
- motion appears to be counterclockwise if facing north and clockwise if facing south
1. Diurnal Circle
- path that a star appears to follow in the sky
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- usually you can observe only part of the diurnal circle because the rest is below the horizon
(except for stars that lie in the North Circumpolar Region or in the South Circumpolar Region)
2. Equatorial System
- coordinate system used to describe the angular location of astronomical objects
- uses right ascension and declination as coordinates
a. Declination
- north-south coordinate equal to the angular distance of a star from the celestial equator
- declination is measured in degrees, minutes, and seconds of arc
b. Right Ascension
- angular distance measured eastward along the celestial equator from the Vernal Equinox to the
point on the celestial equator nearest the star's position
- Vernal Equinox is the location of the Sun on the celestial sphere on the first day of spring
- right ascension is measured in hours, minutes and seconds (one hour equals 15 degrees)
c. Locating a Star using the Equatorial System
- right ascension (or hour angle) and declination are used to locate stars in the equatorial system,
which resembles the terrestrial system of longitude and latitude
- use Star Catalogues to find a particular star to observe
- you need to know where on the celestial sphere the vernal equinox is located; use Sidereal
Clocks to keep track of the local hour angle of the vernal equinox; sidereal clocks read 0h (zero
hours) when the vernal equinox crosses the meridian and 24h when the vernal equinox returns to
the meridian; once the vernal equinox is located, the right ascension and declination of a star can
be used to find the star
D. Motions of the Planets
1. Prograde versus Retrograde Motion
a. Prograde (Direct) Motion
- when a planet moves eastward with respect to the stars
b. Retrograde Motion
- when a planet appears to reverse its direction of motion and move westward with respect to the
stars
- Synodic Period of a Planet is the interval of time in which episodes of retrograde motion occur
2. Conjuction and Opposition
a. Conjunction
- time when a planet is nearly aligned with the Sun
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- for Mercury and Venus, retrograde motion can only take place when the planet appears to pass
near the Sun in the sky (near every other conjunction)
b. Opposition
- when the planet is opposite the Sun in the sky
- retrograde motion for all planets (except Mercury and Venus) happens when planets are at
opposition
E. Keeping Time
- time keeping is based primarily on celestial events
1. The Ecliptic
- the path that the Sun appears to follow among the stars
a. Zodiacal Constellations
- the constellations through which the Sun appears to move during a year
- includes Virgo, Libra, Scorpius, Sagittarius, Capricornus, Aquarius, Pisces, Aries, Taurus,
Gemini, Cancer and Leo
b. Year
- time it takes the Sun to move through the zodiacal constellations and return to the same spot on
the celestial sphere (complete one circle around the Zodiac)
- the year is 365.242199 days or 365 days, 5 hours, 48 minutes and 46 seconds long
2. The Seasons
a. Sun Declination
- angle between the equator and the ecliptic is 23.5°, so the Sun's declination varies from +23.5°
(north of the celestial equator) to -23.5° (south of the celestial equator) during the year
- Tropic of Cancer is at 23.5° north latitude and Tropic of Capricorn is at 23.5° south latitude
b. Summer Solstice
- the point on the ecliptic where the Sun's declination is most northerly (directly overhead at
23.5° north latitude)
- marks the beginning of Summer, around June 22
c. Winter Solstice
- the point on the ecliptic where the Sun's declination is most southerly
- marks the beginning of Winter, around December 22
d. Autumnal Equinox
- the point in the sky where the Sun appears to cross the celestial equator moving from north to
south
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- marks the beginning of Autumn, around September 22
e. Vernal Equinox
- the point in the sky where the Sun appears to cross the celestial equator moving from south to
north
- marks the beginning of Spring, around March 21
- is the zero point from which right ascension is measured in the equatorial coordinate system
- Tropical Year is the length of time it takes the Sun to return to the vernal equinox; this is the
unit of time associated with the annual cycle of the seasons
3. The Month
- the interval from New Moon to New Moon
- therefore there are about 12 lunar cycles per year and 12 months
- Julian Calendar made months 30 or 31 days long; Roman politics resulted in the formulation of
our current Calendar
- Religious Calendars (Muslim, Jewish) are often based primarily on lunar cycles
a. Sidereal Month
- the length of time it takes for the Moon to return to the same place among the stars (about 27.3
days)
b. Synodic Month
- the length of time required for the Moon to return to the same position relative to the Sun
(about 29.5 days)
- because the Sun moves eastward among the stars, it takes more than a Sidereal Month for the
Moon to return to the same position in the sky relative to the Sun
4. Days of the Week
- seven days probably due to 7 visible objects in sky seen by ancient peoples that move with
respect to stars (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn)
- weekdays are named primarily for astronomical objects or ancient gods
5. The Day
- the time from sunrise to sunrise
a. Day Length
- approximately 24 hours (Mean Solar Day)
- Mean Solar Time is the time kept according to the average length of the Solar Day (24 hours)
b. Apparent Solar Day
- the amount of time that passes between successive appearances of the Sun on the meridian
- Apparent Solar Time is time kept according to the actual position of the Sun on the meridian
(that is, the difference between the apparent and mean solar time)
- the Solar Day is longer when the Earth is near the Sun and shorter when farther away
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c. Sidereal Day
- the length of day it takes for a star to return to the meridian
- the length of a Sidereal Day is 23h 56m 4 s (23 hours, 56 minutes, 4 seconds)
6. Time Zones
- regions of the Earth, roughly 15° wide in longitude, where everyone keeps the same standard
time
- the Earth has 24 time zones of one hour each
a. U.S. with Eastern, Central, Mountain, and Pacific Standard Time Zones
- when you move West to East across a time zone add an hour; East to West subtract
b. Daylight Savings Time
- set clocks ahead of Standard time; provides more recreation hours and saves energy
- Spring forward (2nd Sunday in March at 2 A.M.) and Fall back (first Sunday in November at 2
A.M.)
c. International Date Line
- at 180 degrees longitude where the day shifts
d. Universal Time (UT)
- time kept in the time zone containing longitude 0 (through Greenwich, U.K.)
- provides a "standard" World time
2. Daylight Length
- due to Earth's tilted rotational axis
a. Northern Hemisphere with sunshine more direct in Summer than Winter; therefore longer
day
b. Equinoxes
- with Northern and Southern Hemispheres equally lit and day/night of equal length everywhere
on Earth
4. A.M. and P.M.
- noon sun lies overhead (at the meridian) at noon
a. Antemeridian (A. M.) - sun lies before (ante) the meridian
b. Postmeridian (P. M.) - sun lies past (post) the meridian
5. Calendars
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a. In order for seasons not to get out of sequence with the calendar add one day every fourth
year (Leap Year)
b. Gregorian Calendar
- because the Tropical Year is shorter than 365 1/4 days, Century Years are not leap years unless
they can be evenly divided by 400 (1900 was not a leap year, but 2000 was)
c. B. C. and A. D.
B. C. - before Christ
A. D. - anno Domini, "in the year of our Lord"
- another system uses BCE (Before the Common Era) and CE (Common Era)
II. Early Cosmological Models
A. Prehistoric Astronomy
Ethnoastronomy – anthropological study of skywatching in contemporary cultures
Archeoastronomy/Archaeoastronomy – how ancient cultures interpreted and utilized sky
phenomena
- the religion of many prehistoric peoples included cosmological elements
- humans have kept track of the Moon's phases for possibly 30,000 years
- many prehistoric cultures have kept track of the equinoxes and solstices, probably for religious
and/or economic (Ex. = agricultural) purposes
- famous proposed examples include solar alignments in the Neolithic (New Stone Age) tombs at
Newgrange (3200 BC) and Knowth (2500-2000 BC) in the Republic of Ireland and at Maeshowe
(2800 BC) on the Mainland of Orkney, Northern Scotland
- both solar and lunar alignments have been proposed for Stonehenge (3100-1600 BC) and other
prehistoric sites in southern England, and Anasazi/Pueblo sites (900-1150 AD) within Chaco
Canyon, northwestern New Mexico
B. Mesopotamian Astronomy
- the Mesopotamians, situated in the region of the Tigris and Euphrates Rivers of modern Iraq,
were the first astronomers to make long-term written records of their observations
1. Ziggurats
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- pyramid-like towers with seven terraces, each representing one of the wandering celestial
bodies visible to the naked eye (Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn)
- represent the first "observatories", first built about 6,000 years ago
- the Babylonians' ability to predict celestial events marked the beginning of astronomy in the
scientific sense (by about 2700 years ago they were able to predict some lunar eclipses)
2. Geometry/Mathematics
- the Mesopotamians originated the idea of dividing the circle into 360° and further dividing each
degree into 60 minutes and each minute into 60 seconds
3. Zodiacal Constellations
- the 12 Mesopotamian zodiacal constellations are essentially those we use today to mark the
annual passage of the Sun with respect to the stars
4. Astrology
- Mesopotamians invented the pseudoscience of astrology in the belief that the positions of the
celestial objects influenced events on Earth
C. Egyptian Astronomy
- Egyptians used astronomy to develop a calendar, for predicting the Nile flood, and in the design
of temples and monuments
D. Greek Astronomy
- the Greeks were the first people to raise astronomy from the level of prediction to that of
explanation and understanding
1. Eratosthenes (c. 276-195 BC)
- used the relationship between the latitude and altitude of the midday Sun to find the difference
in latitude between Syene and Alexandria
- found that the circumference of the Earth is 50 times as large as the distance between Syene and
Alexandria, which differed from the true value (about 25,000 miles) by only 15% (or just 2% if
using Egyptian measurements!)
2. Hipparchus (ca. 190-120 BC)
- often said to be the greatest astronomer of antiquity; refined earlier methods of finding the
distances to the Sun and Moon, improved the determination of the length of the year, made
extensive observations and proposed theories concerning the motions of the Sun and Moon
- when comparing his measurements of the positions of stars versus those of earlier Greek
astronomers, Hipparchus discovered that there is a slow movement of the celestial poles with
respect to the stars (Precession); this causes the coordinates of stars to change with time
3. Ptolemy (ca. 90-168 AD)
- wrote important works on astronomy, optics, geography and music
- invented terrestrial latitude and longitude and was the first to orient maps with north at the top
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and east to the right
- his method of describing the brightness of stars (the Magnitude System) is still in use
- the Epicyclic Model perfected by Ptolemy used combinations of circular motions to reproduce
the motions of planets; it was considered that the Earth is stationary at the center of the solar
system (Geocentric Model); according to the model a planet moved in a circle on an Epicycle,
which itself moved on a Deferent; the model could predict the positions of celestial objects with
such accuracy that it was used for 1500 years
E. Chinese Astronomy
- at least by 1300 BC Chinese began recording eclipses, meteor showers, novas, and comets
- the Chinese accurately determined the length of the year, discovered precession, and used
eclipse cycles to predict when eclipses would occur
F. Mesoamerican Astronomy
- the Olmec Culture (ca. 1250 - 400 BC), situated on the Isthmus of Tehuantepec, Mexico,
developed the 52 year-cycle "Calendar Round" system with a 260 day "Almanac Year" and a 365
day "Solar Year" (this was the standard calendar for all later Mesoamerican cultures)
- the Maya of Guatemala and Mexico (ca. 1000 BC - 1225 AD) aligned many of their buildings
and monuments with the position of sunrise and sunset at the equinoxes and solstices and the
position of Venus; built observatories, prepared extensive tables of the movements of
astronomical objects, and perfected the Mesoamerican calendar to keep track of the movements
of the Sun, Moon and Venus
III. The Birth of Scientific Astronomy
- during the Renaissance, the assumptions of the ancient astronomers was challenged and an
entirely new view of the solar system was formulated
A. Arabic Astronomy
- from the seventh to the fifteenth centuries AD, the center of astronomical studies was in the
Islamic world; the Arabs translated and preserved the works of the ancient astronomers
- Arabs were the first to build observatories that were "modern" in organization
- many of the Arabic names for the stars (often containing the prefix "al") are in use today
B. European Astronomy
- by the 15th century AD, the astronomy of the ancients was rediscovered by Europeans
- astronomers began to observe again and test hypotheses against observations
- although the geocentric model of Ptolemy was accepted by almost all astronomers, some had
growing doubts about the ancient theories of motion and arguments that the Earth was motionless
1. Nicholas Copernicus (AD 1473-1543)
- Polish astronomer who first fully developed the heliocentric ("Sun-centered") model of the
solar system
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- published De Revolutionibus Orbium Caelestium (On the Revolutions of the Heavenly Orbs)
two months before his death
- with the heliocentric theory, the Earth's revolution about the Sun explains the annual motion of
the Sun; retrograde motion of the planets occurs naturally whenever the Earth passes or is passed
by another planet; the daily patterns of celestial motion are explained by the rotation of the Earth
on its axis
a. Definitions
Rotation - the turning of a body, such as a planet, on its axis
Revolution - the motion of a body in orbit around another body or a common center of mass
Sidereal Period of a Planet - length of time required for a planet to complete one orbit around the
Sun
Synodic Period of a Planet - length of time it takes a planet to return to the same configuration
(for example, opposition to opposition) with respect to the Earth and the Sun
Greatest Elongation - the greatest angular distance between the Sun and the planet; occurs when
the line of sight from the Earth to the planet just grazes the planet's orbit
Astronomical Unit (AU) - average distance between the Earth and the Sun; 149.6 X 106
kilometers, or 8.3 light minutes, or 92.96 million miles
Superior Planet – a planet that has a larger orbit than the Earth's
Inferior Planet – a planet that has a smaller orbit than the Earth's
b. Consequences of the Heliocentric Theory
- the distances of the planets are arbitrary in the Ptolemaic model; in Copernicus' model the
orbital distances of the planets can be determined through observation and use of geometry
2. Tycho Brahe (1546-1601)
- Danish nobleman that made observations more accurate than any previous astronomer; his
observations of the Sun, Moon and planets were used as a benchmark by later astronomers to test
their hypotheses of cosmic motion
- Tycho's inability to detect Stellar Parallax (the annual apparent change in the position of a star
caused by the motion of the Earth around the Sun) caused him to reject Copernicus' model of the
solar system; Tycho proposed that the Sun and Moon orbited the Earth and the other planets
orbited the Sun
3. Johannes Kepler (1571-1630)
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- German mathematician and astronomer; discovered the mathematical relationship between the
distances of the planets and their periods of revolution (Kepler's Laws of Planetary Motion)
a. Kepler's First Law of Planetary Motion
- states that a planet moves on an elliptical orbit with the Sun at one focus
- at Perihelion the planet is nearest the Sun; at Aphelion the planet is farthest from the Sun
b. Kepler's Second Law of Planetary Motion
- a planet moves so that an imaginary line connecting the planet to the sun sweeps out equal areas
in equal intervals of time
Transverse Velocity - a planet moves fastest when it is nearest the Sun at perihelion and slowest
when it is farthest from the Sun at aphelion
c. Kepler's Third Law of Planetary Motion
- describes the relationship between the sidereal period of a planet and its average distance from
the Sun; it implies that there is an underlying principle that governs the orbital motions of the
planets (i.e., gravity)
4. Galileo Galilei (1564-1642)
- Italian astronomer that used an early telescope to make observations that showed that the
Ptolemaic model was false
a. Galileo's Observations
a1. Stars
- Galileo found previously unknown stars everywhere he pointed his telescope; studies of angular
sizes of stars showed that Tycho's objections to Copernicus' heliocentric model were based on
faulty data
a2. Sun and Moon
- the Sun and Moon were not perfect, smooth spheres as the Greeks had said; Galileo also
showed that the Sun rotated
a3. Jupiter's Satellites
- Jupiter's Moons (the Galilean Satellites) revolve around Jupiter, demonstrating that the Earth
was not the center of all motion in the universe
a4. The Phases of Venus
- Venus passes through the full range of phases from new to full, agreeing with the heliocentric
model
b. Galileo's Dialogue
- Galileo presented a powerful argument for the heliocentric model of the solar system in his
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book Dialogue Concerning the Two Chief World Systems, the Ptolemaic and the Copernican,
published in 1632
- the book resulted in the Church's persecution of Galileo; the Church's ban on the publication of
the Dialogue was lifted in 1822, and Galileo was cleared by the Church in 1993
c. Galileo's Experiments
- by rolling balls down inclined planes, Galileo found that:
c1. the distance a ball travels is proportional to the square of the time that it has been in motion
c2. the rate at which an object increases its speed while it falls is the same in all objects (in the
absence of air resistance, all objects fall in exactly the same way)
5. Isaac Newton (1643-1727)
- English; probably the greatest scientist of all time
- discovered the law of universal gravitation, made fundamental discoveries in optics, and
invented differential and integral calculus
- published the Philosophiae Naturalis Principia Mathematica (Mathematical Principles of
Natural Philosophy, typically referred to as the Principia) in 1687
a. Newton's Laws of Motion
a1. Newton's First Law of Motion (The Law of Inertia)
- an object remains at rest or continues in motion at constant velocity unless it is acted on by an
unbalanced external force
Velocity - physical quantity that gives the speed of a body and the direction in which it is moving
Acceleration - rate at which velocity changes with time
Mass - amount of material in a body OR measure of the inertia of a body (its resistance to
acceleration)
Momentum - a quantity, equal to the product of a body's mass and velocity, used to describe the
motion of a body
a2. Newton's Second Law of Motion (The Law of Force)
- when an unbalanced force acts on an object, it produces a change in the momentum of the
object in the direction in which the force acts
- usually a change in momentum implies an acceleration, so the law of force takes the form F =
ma (where F is force, m is mass, and a is acceleration)
a3. Newton's Third Law of Motion (The Law of Action and Reaction)
- when one body exerts a force on a second body, the second body also exerts a force on the first;
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these forces are equal in strength, but opposite in direction
- therefore there is never a single, isolated force in nature and forces always occur in pairs
- this is the basis for rocket propulsion
b. Newton's Studies of Gravity
b1. Centripetal Acceleration
- acceleration toward the center of the circle that causes the path of an orbiting object to be
continually bent away from the straight line path it would follow if no force were acting upon it
- in order for there to be centripetal acceleration, there must also be a centripetal force (a force
that acts toward the center of the circle)
b2. Newton's Law of Gravitation
- every pair of particles of matter, no matter how far apart, exert a gravitational force on each
other; the strength of the force is proportional to the product of their masses and inversely
proportional to the square of the distance between the particles
- a spherical body attracts other bodies as through its mass were concentrated at a point at its
center
Weight - gravitational force attracting an object to the Earth (or another astronomical object); a
body's weight decreases with increasing altitude and distance from the Earth's center
Acceleration of Gravity - the acceleration of a body caused by the force of gravity near the
surface of the Earth (or another astronomical object); on Earth equals 9.8m/s2 or 32ft/s2
Weightlessness - the perception experienced by people when they are in orbit or free fall so that
there is no force to balance weight
b3. Elliptical Orbits
- Newton showed that any body orbiting another under the force of gravity sweeps out equal
areas in equal times (equivalent to Kepler's Second Law); the law of equal areas is due to the
Conservation of Angular Momentum (Angular Momentum is the momentum of a body
associated with its rotation or revolution; angular momentum can be transferred from one object
to another or redistributed, but it can never be destroyed)
- Newton showed that a planet attracted to the Sun by the force of gravity will orbit an elliptical
path with the Sun at one focus; the planet will move so as to conserve angular momentum
(moving fastest when it is nearest the Sun)
- He also showed that Kepler's Third Law could be generalized to relate the period, masses, and
separation of any two orbiting bodies (this allows astronomers to find the masses of astronomical
objects)
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6. Orbital Energy and Speed
a. For orbiting bodies, two forms of energy are important:
a1. Kinetic Energy
- energy of motion; the kinetic energy of a body increases as its speed increases
a2. Potential Energy
- stored energy
Gravitational Potential Energy - the energy stored in a body subject to the gravitational attraction
of another body; as the body falls, its gravitational potential energy decreases and is converted
into kinetic energy; Gravitational Potential Energy of a planet increases with distance from the
Sun (when it approaches the Sun, gravitational energy becomes smaller but kinetic energy
increases)
b. Escape Velocity
- speed that an object must have to excape from its parent body and never return; at a given
distance from the Sun, the escape velocity is √2 times the circular velocity at that distance
- on the surface of the Earth escape velocity is about 11.2 kilometers per second (about 7 miles
per second, or about 34 times the speed of sound)
- four kinds of trajectories a body moves near the Sun are Conic Sections (named because they
are the four curves that can be made by slicing through a cone); a body moving at escape velocity
follows a parabola; at a speed greater than escape velocity, its path is a hyperbola; bodies moving
slower than escape velocity follow circles or ellipses
IV. Principles of Astrophysics and Astrochemistry
A. Waves
- a wave is composed of a regular series of disturbances that move through a medium or through
empty space
1. Description of Waves
- a wave is described by its length (wavelength), rate at which its crests pass (frequency), and the
energy flux that the wave carries (energy flux = rate at which the wave carries energy through a
given area)
2. Electromagnetic Waves
- consist of oscillating magnetic and electric fields moving at the speed of light (speed of light
equals 3 X 108 m/s or 186,000 miles per second)
- electromagnetic waves differ in wavelength and frequency (the Electromagnetic Spectrum);
waves with small wavelengths (such as gamma rays) have high frequencies, whereas waves with
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large wavelengths (such as radio waves) have low frequencies
3. The Doppler Effect
- change in the measured wavelength and frequency of a wave
- the Doppler Effect occurs when the source and observer of the wave are moving toward or
away from each other; the change in wavelength is proportional to the speed of the source
relative to the observer
4. Photons
- massless particles of electromagnetic energy; the energy of a photon is proportional to its
frequency
- light behaves both as waves and as particles
B. Reflection and Refraction
1. Reflection
- bouncing of a wave from a surface; reflection occurs for water waves, sound waves, and
electromagnetic waves
- when a wave is reflected, its new direction depends on the direction of the incident wave and its
orientation to the reflecting surface
- Reflectivity of a surface describes its ability to reflect electromagnetic waves (ranges from 0%
for a surface that reflects no light and is completely dark to 100% for a bright surface)
2. Refraction
- bending of light when it passes from one material into another
- when light travels through any gas, solid, or liquid the light moves more slowly
Index of Refraction - the ratio of the speed of light in a vacuum to the speed of light in a
particular substance
- when a wave passes from one substance to another that has a different index of refraction, it is
bent (refracted)
3. Dispersion
- when a beam of light falls on a prism it is separated according to wavelength to form a
spectrum
Dispersion - the separation of light according to wavelength; dispersion occurs because the index
of refraction in glass or other substances depends on wavelength; astronomers sometimes use
dispersion to spread out light so that narrow wavelength regions can be studied in detail
C. Atoms and Matter
1. Substances can be divided into two classes:
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a. Elements
- the most basic substances; cannot be broken by chemical reactions into simper substances
- approximately 94 elements occur in nature, but only a few dozen are common [including
Hydrogen (H), Helium (He), Oxygen (O), Carbon (C), Neon (Ne), Nitrogen (N), Magnesium
(Mg), Silicon (Si), Iron (Fe) and Sulfur (S)]
b. Compounds
- substances made of two or more elements
- some important compounds include Water (H2O), Carbon Monoxide (CO), Carbon Dioxide
(CO2), Methane (CH4), Nitrogen (N2), Ammonia (NH3), Quartz (SiO2), Iron Oxide (FeO) and
Troilite (FeS)
2. Molecule
- smallest unit of a compound that still has the chemical properties of the compound
3. Atom
- smallest unit of an element that displays the chemical properties of the element
- compounds are created when atoms join together to form molecules of the compound
a. Model of an Atom
Electrons - low-mass particles (10-30 kg) that carry a negative charge; rapidly-moving particles
that surround the nucleus (the distance between the electrons and nucleus is very great in terms of
the size of the nucleus); electron number is equal to the number of protons for a neutral atom
Nucleus - the tiny, dense center of an atom that consists of protons and neutrons; the nucleus has
a positive charge due to the presence of protons
Protons have positive charges, neutrons have no electrical charge; these particles are about 2000
times more massive than electrons
- the attraction of the positively-charged protons and the negatively-charged electrons binds the
electron to the nucleus; a strong Nuclear Force binds the protons and neutrons together in the
nucleus
b. How Atoms Differ
- elements differ from one another because their atoms differ
Atomic Number - the number of protons in an atomic nucleus; atoms of different elements
contain different numbers of protons
Mass Number - the sum of the number of protons and neutrons in an atomic nucleus
Isotope - atoms of the same element that have different mass numbers (have the same number of
Astronomy Notes, Page 17
protons, but different numbers of neutrons); Ex. = Deuterium (heavy hydrogen; with one proton
and one neutron) versus ordinary hydrogen (with one proton and no neutrons)
Ion - an electrically charged atom or group of atoms (therefore the number of electrons is less
than or greater than the number of protons, so the net charge is negative or positive); in most
astronomical situations there are positive ions (due to loss of one or more electrons)
4. Phases (States) of Matter
- matter in the universe comes in many forms, including the four common states or phases known
as solid, liquid, gas, and plasma
a. Solid Matter
- is rigid and maintains a definite shape; it strongly resists being compressed and (to a lesser
extent) resists being pulled apart; solids expand and contract only slightly with temperature
changes
b. Liquid Matter
- also resists compression and expands or contracts only slightly with temperature changes;
liquids have little resistance to being pulled apart (so are virtually formless unless occupying a
container); most substances are less dense in a liquid state than they are in a solid state
c. Gaseous Matter
- has indefinite volume and an infinitely variable and very broad density range; gas expands so as
to uniformly fill any container it is introduced into; unlike solids and liquids, gases are readily
compressible; changes in gas volume are accompanied by changes in temperature and/or pressure
d. Plasma
- a hot, ionized form of gas that has certain electromagnetic properties not possessed by ordinary
gases
D. Nature of Gases
- over 99% of the visible material in the universe is gaseous
1. Types of Gases
a. Neutral Gas
- gas containing atoms and molecules but essentially no ions or free electrons
b. Plasmas
- a fully or partially ionized gas
2. Speed and Temperature
a. Theory
Astronomy Notes, Page 18
- the average speed of the atoms or molecules in a gas increases as the temperature of the gas
increases; in a mixture of gases, the lightest atoms and molecules move fastest (on average)
- because it controls speed, temperature also controls many important processes involving
collisions between the particles of which the gas is made
- in a region of dense gas, frequent collisions prevent rapidly moving atoms and molecules from
escaping; in regions where collisions are infrequent, atoms that move faster than the escape
velocity can escape into space; if the average speed of a gas is greater than about 1/6 of the
escape velocity, all the atoms will escape in a (cosmically) relatively short period of time
b. Temperature Units and Conversion Factors
- the three temperature scales in common use are Fahrenheit (F), Celsius (C) and Kelvin (K)
°F = (1.8 X °C) + 32
°C = (°F - 32) X 0.56
°K = °C + 273.15
°C = °K - 273.15
3. Density and Pressure
- it is the balance between pressure and gravity that controls the structure of a star and the
atmosphere of a planet; pressure depends on the temperature of a gas and on its density
a. Density
- the mass of a body divided by its volume
b. Pressure
- the force exerted per unit area
- at sealevel the Earth's atmosphere is about 105 N/m2 or 14.7 lb/in2 [a Newton (N) is an amount
of force equal to about 0.225 lb]; this amount of pressure is defined as One Atmosphere of
Pressure
Ideal Gas Law - a relationship between pressure, volume, and temperature which is very nearly
true for gases that are not too dense or cold; the pressure of a gas is proportional to the product of
its density and its temperature
- when a gas is in hydrostatic equilibrium, the drop of pressure within the gas as height increases
just balances the downward pull of gravity; the rate at which pressure drops as height increases
depends on the gravity and temperature of the atmosphere as well as the masses of the gas atoms
or molecules present
E. Radiation and Matter
Astronomy Notes, Page 19
- it is the balance between absorption and emission of radiation by astronomical bodies that
controls their surface temperature
1. Blackbody Radiation
Blackbody - hypothetical body that absorbs light of all wavelengths perfectly; a blackbody also
emits electromagnetic radiation; the brightness of the radiation that a blackbody emits is the
greatest that can be emitted by an object of that size and temperature
a. Temperature and Spectral Shape
- blackbody radiation has a spectrum with a distinctive shape that depends only on the
temperature of the blackbody
- Wien's Law states that the wavelength at which a blackbody is brightest is inversely
proportional to the temperature of the blackbody (so blackbody radiation grows "bluer" with
increasing temperature)
- The Stefan-Boltzmann Law says that summed over all wavelengths, the rate at which a
blackbody emits radiation increases with the fourth power of the temperature
2. Thermal Equilibrium
- a body is in thermal equilibrium when the rate at which the body radiates energy equals the rate
at which it absorbs radiant energy that falls on it
- because the brightness of sunlight diminishes with increasing distance from the Sun,
temperatures generally fall with increasing distance
Albedo - the fraction of light striking a body that gets reflected; highly reflecting bodies (with
high albedos) are cooler, at a given distance, than dark bodies, which absorb sunlight well
F. Nuclear Reactions and Radioactivity
- the chemical makeup of the universe has been determined by nuclear reactions in stars and in
the gas that filled the universe in its earliest moments
- nearly all energy emitted by stars is produced by nuclear reactions, radioactivity produces heat
in the interiors of solar system bodies, and radioactivity allows us to determine the age of solar
system bodies
1. Nuclear Reactions
- nuclear reactions can produce energy by transforming matter into energy (according to
Einstein's Special Relativity, mass and energy are different forms of the same thing and can be
exchanged back and forth according to the formula E = mc2, where E is energy, m is mass and c
is the speed of light)
a. Fission
- nuclear reaction in which the nucleus splits to produce two less massive nuclei
b. Fusion
Astronomy Notes, Page 20
- nuclear reaction in which two nuclei merge to form a more massive nucleus
2. Radioactivity
- the spontaneous disintegration of an unstable nucleus of an atom
- in any radioactive decay, the masses of the decay products are less than the mass of the original
nucleus; the difference in masses appear as energy
a. Half Life
- half-life of an isotope is the length of time required for the isotope to decline in number by a
factor of two
- after 20 or so half-lives, the original sample of radioactive nuclei is essentially gone
b. Radioactive Dating
- the predictable rate of radioactive decay makes it possible to use unstable isotopes to determine
the ages of the objects that contain them
- 40K (potassium) decays to 40Ar (argon) with a half life of 1.3 billion years; this has been the
most useful isotope to date Earth rocks, lunar samples and meteorites
G. Internal Heat in Planets and Satellites
1. Radioactive Heating
- radioactive decay is one of the main sources of heat in the interiors of many planets and satellite
- in order to be an important energy source the isotope must be fairly abundant, release
significant energy during decay, and must have a fairly short half life [today mostly isotopes of
potassium (K), thorium (Th) and uranium (U); at the beginning of the solar system also possibly
aluminum (Al) was important]
2. Accretional Heating
- heating of a body by the impacts that occur as it grows by adding infalling material (Accretion)
- probably an important early source of heating during the formation of the solar system; as the
bodies were accreted, kinetic energy was released; accretional heating was larger for the larger
bodies
H. Flow of Heat
- heat always flows from hot regions to cooler ones
- the rate of heat flow depends on how rapidly temperature declines from the center to the surface
within a body; generally small bodies cool more rapidly than large ones
- the three processes by which heat can be transported from one place to another are:
1. Radiative Transfer
- transport of energy by electromagnetic radiation
- radiative transfer is most efficient in a vacuum or in a hot gas at low density, where photons can
travel a long distance between absorptions; radiative transfer is often the dominant energy
Astronomy Notes, Page 21
transport in gases, but is unimportant in the interior of planets and satellites
2. Conduction
- transport of energy in which heat is transferred by means of direct collisions between adjacent
atoms, molecules, or ions
3. Convection
- transport of energy in which heat is carried by hot, rising and cool, falling currents or bubbles of
liquid or gas
- whether conduction or convection is the dominant mechanism for heat transport depends on
whether the material is able to flow (which it must do if convection is to occur) and how much
heat needs to be carried
V. Astronomy Instrumentation
A. Optical Telescopes
- use lenses and mirrors to bring light to a focus
- the lens or mirror used to focus the light is called the Objective of the telescope
1. Refracting Telescopes (Refractors)
- a telescope in which the objective is a lens; the size of a refracting telescope is the diameter of
the light-collecting lens
Focal Point of a Lens - spot at which parallel beams of light striking the lens are brought to a
focus and cross
Focal Length - the distance from a lens to its focal point
Focal Plane - the surface where the objective forms the image of an extended object
The World's largest refractor is the 40-inch (1 m) telescope of the Yerkes Observatory,
Wisconsin (completed in 1895)
2. Reflecting Telescopes (Reflectors)
- one in which the objective is a mirror
- the focal length of a mirror is the distance from the mirror to the focal point
- the size of a reflecting telescope refers to the diameter of its mirror
- reflecting telescopes are preferred in astronomy because refracting telescopes cannot have any
internal imperfections in the lens, reflecting telescopes only have to have one lens surface ground
and polished (makes it easier and much less expensive to manufacture), and the weight of a large
refracting lens produces distorted images (reflecting lenses are supported along their entire back
surface and therefore do not warp as much)
Astronomy Notes, Page 22
- the World's largest telescopes include the Large Binocular Telescope (LBT) at Mount Graham
International Observatory, Arizona, consisting of two 8.4 meter multiple mirrors; the Gran
Telescopio Canarias (GTC) at Del Observatorio Rocque de Los Muchachos in the Canary
Islands, consisting of a 10.4 meter segmented mirror; the two 10-meter Keck Telescopes on
Mauna Kea, Hawaii; the Southern African Large Telescope (9.2 meter); and the Hobby-Eberly
Telescope (9.2 meters, at McDonald Observatory in the Davis Mountains of Texas)
B. Forming an Image
1. Brightness
- brightness of an image formed by a telescope depends on the amount of light collected by the
objective of the telescope and on the area of the image in the focal plane
- brightness of an image formed by a telescope depends on the square of its focal ratio, the ratio
of the focal length to the diameter of its objective
Light-gathering Power of a Telescope - the amount of light it can collect and focus; this is
proportional to the area of the objective
2. Resolution
- resolution of a telescope describes its ability to distinguish fine details in an image
- at longer wavelengths, particularly in the radio part of the spectrum, high resolution is much
more difficult to obtain (therefore radio telescopes, which operate at long wavelengths, usually
have very poor resolution compared with optical telescopes)
3. Magnification
- magnification of the combined objective and eyepiece is given by fp/fe, where fp is the focal
length of the objective and fe is the focal length of the eyepiece (using an eyepiece of very short
focal length creates higher magnification)
- an object magnified large is dim, and the field of view is small (therefore extremely large
magnifications are not often used in observing)
C. Detectors
- once light has been brought to a focus, it is measured by a detector
These include:
1. Photography
- when telescopes are used in photography they operate like a camera, except the objective of the
telescope replaces the camera lens
- long exposure times are needed to capture images of astronomical objects that may be 100
million times too faint to be seen with the naked eye
2. Photometers
- photometers precisely measure the brightness of astronomical objects
Astronomy Notes, Page 23
- many photometers, especially in the visible part of the spectrum, take advantage of the
Photoelectic Effect (collected photons strike a metallic surface inside the photometer and eject
electrons, producing an electric current that is amplified and measured)
3. CCDs
- a Charge Coupled Device (CCD) is a collection of photo-sensitive devices laid out in a
miniaturized pattern that resembles a chessboard; each piece of the pattern is known as a picture
element (Pixel)
- when an image is focused on a CCD, each pixel builds up an electric charge that depends on the
brightness of the part of the object imaged in that pixel and the length of time the image is
recorded; the amount of charge on each pixel is then recorded and stored electronically so that it
can be displayed or analyzed later using a computer
4. Spectroscopy
- the recording and analysis of spectra; in spectroscopy the light is dispersed according to
wavelength
- astronomers often measure the shapes and strengths of absorption and emission lines of atoms
and molecules to learn about important properties of celestial objects such as chemical makeup,
motion, and magnetic fields
Spectrograph - device used to produce and record a spectrum
D. Optical Observatories
1. Quality of Observing
- depends on many factors such as the amount of clear weather, the transparency and steadiness
of the atmosphere, and the darkness of the sky at night
- clearest locations tend to be in dry or desert regions (such as in the southwestern United States,
Chile, Australia) or on mountains which rise above low-lying clouds (such as on Mauna Kea,
Hawaii)
Light Pollution - light from large population centers increases the brightness of the night sky,
making it more difficult to observe faint objects
Seeing - blurring of an image caused by turbulence in the atmosphere (effects of seeing are least
noticeable where the air isn't very turbulent, such as on isolated mountain tops)
Adaptive Optics - using a flexible mirror on a telescope that has actuators to move a small
segment of the mirror up and down as much as 1000 times per second to correct for distortion
due to atmospheric turbulence
2. Modern Observatories
- Kitt Peak National Observatory in Arizona, Mauna Kea in Hawaii (probably the best observing
site), McDonald Observatory in West Texas, the European Southern Observatory and Cerro
Astronomy Notes, Page 24
Tololo Inter-American Observatory in Chile are some of the most important
E. Space Observatories
- because the Earth's atmosphere is opaque to most of the electromagnetic spectrum, it is
important to carry telescopes above the atmosphere where they can observe in the gamma ray, Xray, ultraviolet and infrared parts of the spectrum
1. Optical Observations
- the Hubble Space Telescope (HST) was lauched in 1990; an error in making one of the
telescope's mirrors was corrected by a shuttle mission in 1993; the images of the HST are now
near-perfect
2. Infrared and Ultraviolet Observations
- infrared and ultraviolet space telescopes resemble optical telescopes except their detectors are
sensitive at longer (infrared) and shorter (ultraviolet) wavelengths
- important surveys include/have included the Spitzer Space Telescope and Hershel Space
Observatory (with infrared telescopes); the Extreme Ultraviolet Imaging Telescope (EIT) and the
Galex Evolution Explorer (GALEX), which are ultraviolet telescopes
3. X-Ray and Gamma Ray Observations
- because X-rays penetrate too deeply into reflecting lenses of normal telescopes, X-ray
telescopes consist of Wolter Telescopes that are composed of a series of nested cylinders that
cause the x-rays to strike the surface at grazing incidence (like skipping stones across the surface
of a pond); grazing-incidence mirrors have been used on X-ray telescopes such as Einstein,
ROSAT, the Chandra X-Ray Observatory, and the XRT on the Swift MIDEX Mission
- Gamma-ray telescopes make use of the way that gamma rays interact with matter by producing
a pair of particles (an electron and positively-charged positron, whose paths indicate the direction
of the gamma ray that produced them); the Fermi Gamma-Ray Space Telescope was launched in
2008
- VERITAS (Very Energetic Radiation Telescope Array System) is a ground-based observatory
in Arizona where very high-energy gamma rays (with shorter wavelengths and higher
frequencies) are observed
F. Radio Telescopes
- most radio telescopes are reflectors, using metallic conducting surfaces as mirrors to reflect
radio waves to a focus; the radio waves brought to a focus can be measured using an antenna
(which operates somewhat like an automobile radio antenna)
1. Radio Interferometry
- because they are used at long wavelengths, even the largest radio telescopes have poor angular
resolution (the largest radio dish at Arecibo, Puerto Rico is 305 m across with a circumference of
about 1 kilometer!)
- interferometery is used to obtain high angular resolution, where signals from widely separated
radio dishes are combined to produce the resolution of a single telescope as large as the distance
Astronomy Notes, Page 25
between the two dishes (the signals from the dishes alternatively interfere constructively and
destructively to produce a series of Fringes, whose pattern depends on the size and shape of the
source of radio waves)
2. The Very Large Array (VLA)
- situated in western New Mexico, and consists of a Y-shaped configuration of 27 dishes which
can be moved on railroad tracks to span a region as large as 36 kilometers (22.3 miles) across
3. The Very Long Baseline Array (VLBA)
- an interferometric array consisting of 10 radio telescopes at widely separated locations in the
continental United States, the Virgin Islands, and Hawaii
- working together, the VLBA antennas achieve an angular resolution of about 1/1000 second of
arc (about 100,000 times better than the Arecibo telescope)
- a similar system is operated by the European VLBI Network (EVN); EVN and VLBA data are
combined to form the Global VLBI
4. Space Very Long Baseline Interferometry Arrays (SVLBI Arrays)
- EVN and/or VLBA Arrays may be combined with space-based VLBI antennas such as the
Japanese HALCA (Highly Advanced Laboratory for Communications and Astronomy) and the
Russian RadioAstron to create radio telescopes with huge apertures
G. High Energy Particle Telescopes
- studies astronomical objects that release highly-energetic electromagnetic radiation (includes xrays, gamma-rays, extreme UV radiation, neutrinos and cosmic rays) such as black holes, neutron
stars, active galactic nuclei, supernovae, supernova remnants and gamma ray bursts
- uses special telescopes since most energetic particles pass through metals and glasses (some
high energy particle telescopes have no image-forming optical systems, including Cosmic Ray
Telescopes and Neutrino Telescopes)
VI. Introduction to the Solar System
- the Solar System consists of the Sun, planets, their moons, other bodies that orbit the Sun, and
interplanetary dust
A. The Sun
- contains about 99.86% of the Solar System’s known mass
- the Sun is composed of approximately 71% hydrogen and 27% helium
B. The Planets
- includes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune
1. General Features
Astronomy Notes, Page 26
a. Revolution
- planets revolve around the Sun in slightly elliptical orbits in the same plane (the Ecliptic)
- they move counterclockwise when viewed from above the Earth's North Pole
b. Rotation
- planets usually spin counterclockwise and their rotational axes are usually approximately
perpendicular to the orbital plane
c. Titius-Bode Rule
- the planets are distributed at predictable distances from the Sun
- with equation (X + 4)/10 = AU from Sun; where X = 0, 3, 6, 12, etc.
d. Astronomical Unit
- the average distance between the Earth and the Sun; equals 1.496 X 108 km; approximately 93
million miles, or 8.3 light minutes
2. Types of Planets
a. Terrestrial Planets
- Mercury, Venus, Earth, Mars
- small and dense (3.9 to 5.5 gm/cm3); made of rocky material (mostly silica; also iron,
aluminum, magnesium, sulfur)
- not much hydrogen and helium, no rings, few satellites
- lie in the inner part of the Solar System
b. Jovian Planets
- Jupiter, Saturn, Uranus, Neptune
- large, low-density (0.71 to 1.67 gm/cm3) planets that lie in the outer portion of the Solar System
- Jupiter and Saturn consist primarily of hydrogen and helium
- Uranus and Neptune are composed largely of ices (water, methane and ammonia) and are often
referred to as the “Ice Giants”
- all Jovian planets have ring systems and a large number of satellites
C. Natural Satellites / “Moons”
- natural satellites are celestial bodies that orbit planets or smaller bodies (such as Dwarf Planets,
asteroids or TNO’s); the body that a satellite orbits is termed the “Primary”
- terrestrial planets have few satellites; Jovian with many
1. Regular Satellites
- Regular Satellites have approximately circular paths that are approximately in the Ecliptic plane
- they are believed to have been formed out of the same protoplanetary disc as the planets
2. Irregular Satellites
- Irregular Satellites often have highly inclined, eccentric and often retrograde orbits
Astronomy Notes, Page 27
- they are probably captured asteroids and are usually smaller in size than Regular Satellites
D. Comets
- icy bodies up to 40 kilometers across that "grow" huge tails as they near the Sun and are
vaporized by it's solar radiation
- most comets orbit far beyond Pluto in the Oort Cloud
E. Minor Planets
- Solar System bodies that directly orbit around the Sun that are not major planets or comets
- Minor Planets include Dwarf Planets, asteroids, and centaurs
- most Minor Planets are Trans-Neptunian Objects (TNO’s) that orbit the Sun at a greater
distance than Neptune
- the Minor Planet Center (MPC; based at the Smithsonian Astrophysical Laboratory) has
registered over 619,000 Minor Planets and over 16,000 have received official names
1. Dwarf Planets
- celestial objects orbiting the Sun that are massive enough to be spherical due to their gravity but
not massive enough to clear the neighboring region of planetesimals
- Dwarf Planets are part of “Larger Populations”; Ceres is part of the Asteroid Belt (between
Mars and Jupiter); Pluto, Haumea and Makemake are part of the Kuiper Belt (situated between
Neptune at 30 AU to about 55 AU from the Sun); Eris is a member of the Scattered Disc (the
region dominated by icy minor planets, overlapping with the Kuiper Belt and extending beyond
100 AU)
2. Asteroids
- small rocky-icy or metallic bodies with diameters from a few tens of meters up to
approximately 1000 kilometers across
- most asteroids orbit the Sun within the Asteroid Belt between Mars and Jupiter
3. Centaurs
- minor planets with characteristics of both comets and asteroids
- centaurs orbit the Sun between Jupiter and Neptune and cross the orbital paths of one or more
of the Jovian planets
F. Interplanetary Dust
- space particles ranging in size from a few molecules to about 0.1 micron
- include comet dust, asteroid dust, dust from the Kuiper Belt, and interstellar dust passing
through the Solar System
VII. The Earth
A. General Features of the Earth
Astronomy Notes, Page 28
1. Size
- radius approximately 4,000 miles (6400 kms)
2. Shape
- an oblate spheroid
3. Composition of the Earth’s Crust
- Oxygen (47%), Silicon (28%), Aluminum (8%), Iron (5%), Calcium (4%), Sodium (3%),
Potassium (3%), Magnesium (2%), Titanium (1%), Other Elements (1%)
4. Density
- mass of an object divided by its volume
water density = 1 gm/cm3; crustal rocks = 3 gm/cm3; Earth Average Density = 5.5 gm/cm3
5. Age of the Earth
- about 4.6 billion years old based on radiometric dates on Moon rocks and meteorites
6. Minerals
- naturally-occurring, inorganic, homogeneous, crystalline solids
- many minerals on Earth are silicates, in which silicon and oxygen are combined with other
elements
7. Rocks
- naturally occurring substances made of minerals or mineraloids (mineraloids have a definite
chemical composition but are atomically structureless)
- rocks have a high variability in composition and texture (particle size, shape and arrangement)
a. Igneous Rocks
- rocks that solidify from molten material (magma)
- include most rocks found in the Solar System
- examples include basalts, granites, anorthosites
b. Metamorphic rocks
- rocks formed from pre-existing rocks by solid state transformation in response to change in the
physical or chemical environment
Types of Metamorphism Include:
Contact Metamorphism - occurs in country rock bordering igneous intrusions
Regional metamorphism - occurs on a regional scale due to tectonism (movement and
deformation of the Earth’s crust)
Impact Metamorphism - due to the impact of bolides (such as meteorites and asteroids) on a
Astronomy Notes, Page 29
planet's surface
c. Sedimentary Rocks
- rocks formed from consolidation of loose sediment, formed by chemical precipitation, or rocks
consisting of secretions or remains of plants and animals
8. Earth Layers
a. Crust
- thin, relatively brittle outer shell of the Earth ranging from 7 to 70 kms thick
- consists primarily of relatively low-density silicate rocks
b. Mantle
- constitutes about 80% of the Earth's volume
- the mantle is sandwiched between the crust and the core; it extends to a depth of about 2900 km
- the mantle includes a portion of the Lithosphere (brittle material including the crust and upper
mantle; extends to a depth of 100 km), the Asthenosphere (ductile material extending from about
100 to 350 kms) and the Mesosphere (relatively solid, rigid material that extends from 350 to
2900 kms)
c. Core
- composed mostly of an iron-nickel alloy
- includes the molten outer core and the solid inner core
9. The Interior Heat of Earth is due to:
a. Original heat remaining from impacts during Earth formation
b. heating due to decay of radioactive elements such as uranium
B. Structure of the Earth
- determined largely by Seismology (earthquake study)
- Earthquake Waves include:
1. P (Primary) Waves
- earthquake body waves that travel fastest, moving the particles forward and backward
- P Waves provide evidence for the presence of the Mohorovicic Discontinuity (the boundary
between the crust and mantle)
2. S (Shear) Waves
- waves that advance by shearing displacement of the rock
- S Waves provide evidence for the Gutenberg discontinuity (boundary between the core and
mantle at 2900 kilometers depth)
- S Waves indicate that the outer part of the Earth’s Core is liquid
Astronomy Notes, Page 30
C. Earth Magnetism
- probably due to to a combination of convection of the molten core and the Earth's rotation
(termed the Dynamo Effect)
- the degree and position of Earth's magnetism and magnetic polarity changes through time
- as a plasma (ionized gas) moves through a magnetic field the charged particles are trapped in
spiral paths along the magnetic field lines; this interaction forms the Magnetosphere, which
deflects the solar wind and forms the Van Allen Radiation Belt
D. Plate Tectonics
- theory that the Earth's crust is divided into a series of large lithospheric plates
1. Lithosphere
- brittle material forming the plates
- consists of the Earth's crust and upper portion of mantle
- the lithosphere rides on the moving asthenosphere (ductile material separating the lithosphere
from the lower mantle); rate of plate movement averages 2 to 9 centimeters per year
2. Types of lithospheric plates
a. Oceanic (Simatic) Plates
- formed from basalt being produced along rift zones at mid-oceanic ridges
- have high specific gravity and therefore form basin areas
b. Continental (Sialic) Plates
- granitic composition and form continental areas
3. Plate Boundaries
a. Divergent boundaries
- new simatic oceanic plate material is produced at mid-oceanic ridges through seafloor
spreading; this results in the production of basaltic magma along rift zones
b. Convergent boundaries
- lithospheric plates collide
- usually results in Subduction (where an oceanic plate "dives" beneath a continental plate)
- produce mountain ranges, volcanoes and faults at the edges of the lithospheric plates
c. Transform Plate Boundaries
- shear boundaries in which plates slide past one another (Ex. = San Andreas Fault, California)
- transform faults offset mid-oceanic ridges at perpendicular angles; they develop due to different
rates of seafloor spreading and due to the fracturing of a round object (the Earth's surface)
4. Hot Spots and Mantle Plumes
Astronomy Notes, Page 31
- chains of seamounts and volcanic islands are often formed by lithospheric plates moving over
"fixed" mantle plumes or hot spots
- hot spots may show the direction of plate movement (Example = Hawaiian Islands)
F. The Atmosphere and Oceans
1. Atmospheric Features
a. Composition of the Earth’s Atmosphere
- mostly nitrogen (78%) and oxygen (21%); also argon (0.93%), carbon dioxide (0.035%) and
aerosols (minute liquid and solid particles)
b. Air Density and Pressure
- approximately one-half the mass of the atmosphere is found in the lowermost 4 kilometers
- at sea level atmospheric pressure is about 14.7 pounds per square inch
- at 16 km atmospheric pressure is approximately 10% sea level pressure
2. Formation of the Atmosphere
a. Degassing of Earth's interior
- during differentiation and by volcanic activity
- produced water vapor, nitrogen, carbon dioxide
b. Comet Impacts
- frozen water and gases from comets may also supply atmospheric gases
c. Plant Photosynthesis
- provides oxygen
- there was little oxygen present in the atmosphere during early Earth history
3. Ozone Layer
- most O3 is found at about 25 kilometers altitude (the Ozone Layer)
- ozone forms due to splitting and recombination of O2 molecules
4. Greenhouse Effect
- visible sunlight passes freely through a planet's atmosphere but infrared radiation produced by
warm surfaces cannot escape readily into space; this acts as a "heat blanket" and raises
temperature
- "greenhouse gases", such as carbon dioxide, intensify this effect
5. The Oceans
- oceans were formed from water probably outgassed from the Earth's interior, and water vapor
condensed during Earth cooling
Astronomy Notes, Page 32
- cometary bodies may also have contributed to formation of the Earth’s oceans
- "modern" ocean salinity (averaging 35 parts per thousand dissolved solids) was obtained early
in Earth history
F. Earth Motion
1. The Seasons
- are due to Earth's rotation axis tilted about 23.5 degrees from the perpendicular to the Earth's
orbit around the Sun
2. Coriolis Effect
- deflection of a moving object due to its motion across the surface of a rotating body
- deflected to right in Northern Hemisphere on Earth
- the Coriolis Effect makes storms on Earth spin, generates large scale wind systems, and creates
cloud belts on many planets
3. Precession
- "wobbling" of the Earth on its axis through time
- a complete cycle of Earth Precession is about 26,000 years
VIII. The Earth’s Moon
- the Moon is the Earth’s only natural satellite
- it is about 380,000 kilometers from the Earth
- the Earth’s Moon is the fifth largest satellite in the Solar System (it is about one-fourth the
diameter of the Earth and about 1/81 its mass)
- the temperature on the moon ranges from -387° F to 253°F (-233°C to 123°C)
A. Moon Features
1. Highlands (Terrae)
- heavily cratered, light-colored areas
2. Maria
- smooth, dark-colored areas
3. Lunar Mountains
- were formed by impact events (not by plate tectonics)
4. Rilles
- lunar "valleys" that may extend 100's of kilometers
- rilles may be due to ancient lava flows and crustal cracking
Astronomy Notes, Page 33
5. Craters
- from fraction of millimeter to 360 kilometers (200 miles) across (note that Maria are probably
lava-filled craters and are often much larger than this); craters typically have raised rims as much
as several kilometers high
- most were formed from bolide impacts
- many have light streaks of pulverized rock (rays) radiating from the craters
B. Moon Rotation and Revolution
- the Moon rotates on its axis at the same rate as revolves around Earth (Synchronous Rotation)
due to Earth gravity locking the spin to it's orbital motion
- we see the same face but 5/8 of its surface due to slight turnings (librations)
- the Moon's orbit is tilted approximately 5 degrees with respect to Earth's orbit
1. Sidereal Revolution Period
- the Moon makes a complete orbit in 27.3 days with respect to the stars
2. Synodic Revolution Period
- interval between successive New Moons (29 1/2 days; this is the Synodic Month)
C. Moon Phases
- defined by the amount of the Moon's surface that is illuminated
1. New Moon
- the Moon is not visible in the sky
- angular distance of the Sun and Moon is small
Crescent Moon - occurs a few days after the New Moon; the Moon reappears in the west about
the time the sun sets
2. First Quarter
- the moon lies 90° east of the Sun
- the right half of the moon is illuminated (it is termed first quarter because it has completed onefourth of its revolution)
Waxing Gibbous - more than half of the Moon's surface is illuminated, and the area of
illumination is increasing in size
3. Full Moon
- the moon rises in the east at sunset, and is fully illuminated
- occurs approximately two weeks after the New Moon
- the Moon is 180° from the Sun
Waning Gibbous - more than half of the Moon's surface is illuminated, and the area of
illumination is decreasing in size
Astronomy Notes, Page 34
4. Last (Third) Quarter
- the left half of the moon is illuminated
D. Moon Composition
- all rocks are igneous (formed from magma) or impact metamorphic
1. Basalts
- fine-grained, dark, extrusive igneous rocks; form the Maria
2. Anorthosites
- calcium-rich plagioclase feldspars that form the Highlands
- there are also minor ultramafic igneous rocks (dunites and gabbros) present in the Highlands
3. Lunar Breccias
- fragments welded together by impacts; especially common in the Highlands
4. Lunar Soil (Regolith)
- includes dust and larger fragments pulverized by impacts, and material broken down by charged
particles bombarding the Moon’s surface
E. Moon Structure
1. Regolith
- loosely-consolidated material on the surface
2. Crust
- composed of relatively light material (primarily anorthosite)
- the Moon’s Crust is approximately 50 kilometers thick
3. Mantle
- silica-rich denser material; probably olivine and pyroxene
- the lower portion of the Mantle is often differentiated into a partially molten 500 kilometerthick “Boundary Layer”
4. Core
- with a molten iron rich outer core with a radius of about 300 kilometers and a solid iron-rich
inner core with a radius of about 240 kilometers
F. Moon Atmosphere
- the Moon’s atmosphere is almost a vacuum due to the absence of volcanic activity and its weak
gravitation
- there is some outgassing of the Moon’s interior and “sputtering” (the release of atoms into the
Moon’s “atmosphere” due to the bombardment of the regolith by the “Solar Wind”,
Astronomy Notes, Page 35
micrometeorites and sunlight)
G. Water on the Moon
- in permanently-shadowed parts of lunar craters around the polar areas there may be about 1000
ppm of water incorporated in the lunar regolith
H. Origin of the Moon
- it is believed that the Moon originated about 4.5 billion years ago by the collision of a Marssized body and the youthful Earth (the “Giant Impact Hypothesis”)
- the ejected debris collected in an orbit around the Earth, and coalesced to form the Moon
- most of the Moon’s composition came from the impacting body
I. Lunar Chronology
1. Formed about 4.5 billion years ago
2. the Moon’s surface was probably molten until 4.4 billion years ago
- highland rocks are approximately 4.4 billion years old; these may represent plagioclase feldspar
cumulates in the ancient “lunar magma ocean”
3. 4.1 - 3.8 billion years ago with the most intense bombardment (the Late Heavy
Bombardment, or LHB)
4. Approximately 3.5 to 3.3 billion years ago with radioactive heating, melting and lava fills
basins (forming the Maria)
5. all volcanism ends by approximately 1 billion years ago; moon stabilizes
- bolide impacts continue to affect Moon geology and topography
J. Eclipses
- created when one astronomical body casts its shadow on another
1. Types
- eclipses can occur only near new or full phases of the Moon and only when the Moon is near
the ecliptic plane
a. Lunar Eclipse
- Earth's shadow falls on the Moon
- occurs when the Moon is Full and the Earth is between the Sun and the Moon
b. Solar Eclipse
- Moon's shadow falls on the Earth
- occurs when the Moon is New and is between the Earth and the Sun
Astronomy Notes, Page 36
2. Characteristics
- the Earth and Moon have shadows that consists of the Penumbra (where the sunlight is partially
blocked) and the Umbra (where the sunlight is totally blocked; the Umbra is the region of total
eclipses)
3. Rarity of Eclipses
- eclipses are rare because Moon's orbit around Earth is tilted with respect to Earth's orbit around
the Sun; shadows usually fall above or below the Moon or Earth
- Eclipse Seasons occur twice yearly when the Moon crosses the Earth's orbital plane (the
ecliptic); often a solar eclipse follows 14 days after a lunar eclipse or vice versa
K. Tides
- distortions in a body's shape resulting from tidal forces
1. Tidal Forces
- the differences in gravity that occur because the parts of a body are at different distances and in
different directions from the object attracting them
- the strongest tidal forces are felt by sizable bodies located close to massive attractors
2. Tides
a. Solid Earth Tides
- maximum tidal distortion of the solid Earth is about 20 centimeters
- tidal forces raise, lower, and tilt the Earth's surface
b. Ocean Tides
- periodic rise and fall of oceans due to gravitational attraction of the Moon and Sun on oceans
and centrifugal forces due to revolutions of the Earth-Moon-Sun
- the Earth and Moon revolve around a common center of mass approximately every 27 days
- in theory most places on earth should have 2 equal high tides (Bulges) and 2 equal low tides
during 24 hrs, 50 min (= 1 Lunar Day)
- Spring Tides form when the Sun, Moon, and Earth are in line (NewMoon/Full Moon) and tidal
bulges are largest (and smallest); Neap Tides form when the Sun, Moon, and Earth are at right
angles (1st/3rd quarter of moon) and the tidal bulge is intermediate
3. Tidal Braking
- Earth's rotation is slowing due to Moon's gravitation; lengthens the day 0.002 seconds per
century
- the Moon is accelerating in its orbit and therefore is moving away from the Earth at 3
centimeters per year
- Tidal braking of Earth on the Moon caused the Moon synchronous rotation and deformation of
the Moon's crust
Astronomy Notes, Page 37
IX. Other Terrestrial Planets
A. Mercury
- the smallest planet and the closest planet to the Sun
1. Revolution
- Mercury revolves around the Sun in 88 days
- Mercury has the most eccentric orbit of any planet
- Mercury exhibits phases as seen from the Earth
2. Rotation
- Mercury rotates in approximately 59 days
- it slowly rotates due to Sun-Mercury tides (termed Resonance)
- Mercury has the smallest axial tilt of any planet (0.027°)
3. General Features
- small (4878 kilometers diameter)
- with a relatively large iron-rich core (about 42% of total volume) as indicated by Mercury's high
density; the core is probably molten; with a weak magnetic field probably due to the movement
of the core and with a weak magnetosphere that deflects the “Solar Wind”
- temperature = -297° to +806°F (-183° to +427°C)
- Mercury’s “atmosphere” (its Exosphere) is almost a vacuum (Mercury is too close to the Sun,
too small, no tectonism) but traces of hydrogen and helium (captured from the Solar Wind) as
well as oxygen, sodium, calcium and potassium have been detected
- there is probably water ice present in the floors of deep craters within the polar areas (probably
due to outgassing and/or comet impacts)
- Mercury has no Natural Satellites/Moons
4. Geology
- consists of about 70% metallic material and 30% silicates; the silicate-rich crust is about 100300 kilometers thick
- Mercury does not generate much internal heat (too small, not much radioactivity) and therefore
not much geologic activity
- heavily cratered
- with "lobate scarps" up to 3 kilometers high (due to planet shrinking?)
- with intercrater plains and smooth plains (that represent old lava flows)
5. History
a. Metallic core and silica-rich crust develops
- the presence of a large core may be due to early impact by a large planetesimal; this impact is
believed to have stripped off most of the original crust and mantle
b. Intense meteorite bombardment about 4.1-3.8 billion years ago (during the “Late Heavy
Astronomy Notes, Page 38
Bombardment”, or LHB)
- the impact of a large plantesimal created the Caloris Basin, with development of "Weird
Terrain" on the opposite side
c. Lava flows produce plains
d. Surface slightly modified by minor impacts and “space weathering” processes due to the
Solar Wind and micrometeorite impacts
B. Venus
- second planet from the Sun
- Venus is the second-brightest object in the night sky (after the Moon); often forms the
“Morning Star” and “Evening Star” situated relatively low in the eastern or western sky
immediately before dawn and after dusk
- similar to Earth in size and composition (Venus is sometimes referred to as “Earth’s Sister
Planet”
- essentially no magnetic field (probably due to the absence of core convection)
- Venus has no Natural Satellites/Moons
1. Revolution
- Venus revolves around the Sun in 225 days and has an almost circular orbit
- Venus displays phases like those of the Earth’s Moon as seen from the Earth
2. Rotation
- Venus rotates "backward"/clockwise (retrograde rotation) in 243 days
- the slow rotation may be due to Sun/Venus gravitation and/or “atmospheric tides” slowing the
rotational period
3. Atmosphere
- the atmospheric pressure of Venus is about 92 times that of Earth (this is the densest
atmosphere of all terrestrial planets)
- 96.5% carbon dioxide, 3.5% nitrogen, with some sulfuric, hydrochloric and hydrofluoric acid
- with high clouds of sulfuric acid between 30 and 60 kilometers from surface
- dense lower atmosphere at approximately 900°F (due to the greenhouse effect; sunlight can
travel through the atmosphere but infrared radiation is trapped from escaping back into space)
4. Geology/Topography
- Venus probably has an Earth-like Crust, Mantle and Core
- although Venus is about 4.6 billion years old, no surface features appear to be older than about
500 million years (and most are probably less than 50 million years old)
- the surface of Venus is dominated by volcanism (especially basalt); there is no evidence of plate
tectonics (with "hot spot plumes"?) and there is no granite
- about 80% of Venus consists of smooth volcanic plains
- two major “continents” cover the rest of the surface; the northern Ishtar Terra and the southern
Astronomy Notes, Page 39
Aphrodite Terra
- the highest mountain is Maxwell Montes (about 11 kilometers high; on Ishtar Terra); it may
have formed due to a mantle plume and/or through crustal compression
- Venus is dominated by many strange volcanic features including Canali (rille-like channels),
pancake-like Farra, spider web-like Arachnoids, ring-like Coronae and mosaic chip-like Tesserae
5. History
a. Probably early history like Earth
b. Less cratering than Moon/Mars; the crust averages less than 0.5 billion years old
c. Flooding by lava (and water?)
d. Surface changes dominated by volcanism and effects of the “runaway greenhouse effect”
C. Mars
- Fourth planet from the Sun
- often termed the “Red Planet” due to iron oxide on its surface
- approximately 1/2 Earth diameter (6796 kilometers) and is about 11% of Earth’s mass
1. General Features
a. Revolution
- Mars revolves around the Sun in about 687 days
- the orbit of Mars is relatively eccentric
b. Rotation
- Mars rotates in 24 hrs., 40 min.
- the axis of rotation is inclined at about 25° (therefore Mars has seasonal changes and
“weather”)
c. Magnetism
- Mars has little magnetism (although there is some evidence of paleomagnetic stripes which may
indicate plate tectonics about 4 billion years ago)
2. Atmosphere
- atmosphere approximately 1% density of Earth’s
- temperature from approximately -125°F (-87°C) in the winter to +23°F (-5°C) during the
summer
- gases include 95% carbon dioxide, 3% nitrogen, 1.6% argon, with minor oxygen and water
vapor outgassed from the interior; also with clouds of dry ice and water
- polar caps consist primarily of water ice (the South Pole has a frozen carbon dioxide cap on the
Astronomy Notes, Page 40
water ice; the North Pole has a seasonal thin layer of this “dry ice”); the ice caps greatly
increase/decrease in size seasonally
- there is currently no liquid water on the surface (because of the low atmospheric pressure),
although the polar permafrost extends to about 60° latitude; it is believed that there are large
amounts of water ice beneath the surface
3. Geology/Topography
- Mars structure consists of an iron sulfide partially molten Core; a silicate Mantle (probably now
geologically inactive) and a relatively thick (up to 125 km) Crust
- there are no moving lithospheric plates (therefore no current plate tectonic activity)
- surface igneous rocks on Mars are primarily basalt, with some andesite also present; much of
the surface is covered with iron oxide dust
- the Borealis Basin, in the Northern Hemisphere, is a relatively smooth region that covers about
40% of Mars; it may represent a giant impact structure
- at the mid-latitudes there is a large upland region (the Tharsis Bulge); on this feature there is a
27 kilometer-high volcanic peak (Olympus Mons; this extinct volcano is the largest in the Solar
System); the largest canyon in the Solar System is also on the Tharsis Bulge (The Valles
Marineris; 4,000 kilometers long and 7 kms deep; this canyon was probably formed by
“swelling” of the Tharsis Bulge)
- there is abundant evidence for running water on Mars 4 billion years ago including outflow
channels, dendritic channels, gulleys, deltas and alluvial fans; the ancient presence of larger
“oceans” has been proposed but is controversial
- Mars has the largest dust storms in the Solar System; sand dunes are present on the planet
4. History
a. Formation of the Crust, Mantle and Core at approximately 4.6 billion years ago
- after formation there were effects from the “Late Heavy Bombardment”/LHB
b. Volcanism begins
- the Tharsis Bulge probably formed about 4.1 to 3.7 billion years ago
- although there is no plate tectonics now, Mars paleomagnetism may indicate plate activity at
about 4 billion years ago
- during this period surface water was also present, and may have included Mars “lakes” and
“oceans”
c. Formation of Lava Plains and Volcanoes
- between about 3.7 and 3.0 billion years ago large lava plains were formed; Olympus Mons
probably formed during this time
- some large outflow channels may have formed, as well as short-lived “lakes” and “seas”
d. Late History
- after 3 billion years ago with some bolide impacts, some volcanism, as well as minor releases of
liquid water
Astronomy Notes, Page 41
5. Moons of Mars
- Deimos and Phobos are about 10 kilometers across
- they are probably captured asteroids
6. Life on Mars
- during the early Twentieth Century astronomers believed there were "canals" on Mars built by
intelligent life forms; these are not present
- examination of a presumed Mars meteorite has caused scientists to speculate that life could
have developed on Mars, but there is no evidence that Mars currently supports life
X. The Jovian Planets
A. Jupiter
- largest planet in the Solar System, with about 2 ½ times more mass than all other planets
combined
1. Composition
- average density = 1.3 gm/cm3; pressure and density increase steadily toward the core
- composition is mostly hydrogen and helium with a small percentage of ammonia (NH3),
methane (CH4) and water
- relatively small core (probably rocky, but precise composition is not known); core is surrounded
by metallic hydrogen layer (constitutes about 78% of Jupiter’s radius); above metallic layer is a
transparent atmosphere of about 75% hydrogen and 24% helium (it is gaseous in the upper
approx. thousand kilometers, and liquid below that)
2. Characteristics
- oblate shape; with axial tilt of about 3°; orbital period = 11.867 years; period of rotation = 9h
50m 30s (with differential rotation)
- equatorial diameter = 142,984 kilometers
- emits more energy than receives from Sun, primarily due to gravitational/adiabatic heating
- strongest magnetosphere in the Solar System (generated by movement of liquid metallic
hydrogen beneath the surface of the planet) and intense radiation belt, aurora displays, and strong
radio emission; with powerful lightning discharges in the atmosphere
3. Atmosphere
- outer atmosphere with ammonia and water, inner with hydrogen
- cloud layer about 50 km thick, composed of ammonia crystals (under the ammonia layer there
may be a thin layer of water clouds, as indicated by the powerful lightning); clouds are arranged
into latitudinal bands consisting of belts (dark) and zones (light) with jet streams between (zonal
jet speeds up to about 360 kms/hour); temperature at cloud tops = -166°F
- with atmospheric vortices (Ex. = Great Red Spot; represents anticyclonic atmospheric flow)
Astronomy Notes, Page 42
4. Ring System
- small ring consists of tiny particles of rock dust
5. Natural Satellites / Moons
- Jupiter has 67 confirmed moons
a. Jupiter’s Regular Satellites
- eight satellites with prograde, nearly circular orbits that are not greatly inclined with respect to
Jupiter’s equatorial plane
- probably formed by accretion during Jupiter’s early history
a1. Inner Regular Satellites / Amalthea Group
- the four regular satellites that are smaller, closer to Jupiter, and provide dust for Jupiter’s rings
a2. Galilean Satellites (the “Main Group”)
- discovered by Galileo Galilei in 1610
- spheroidal in shape, with characteristics similar to Dwarf Planets
a2a. Io
- consists of silicate rocks surrounding a molten iron or iron sulfide core
- with about 100 mountains uplifted by compression at the base of the silicate crust
- Io is the most geologically-active object in the Solar System; volcanoes spew molten and
gaseous sulfur, also oxygen and sodium; Io’s volcanism is due to tidal heating by Jupiter
a2b. Europa
- slightly smaller than the Earth’s Moon
- with an icy crust a few kms to tens of kms thick; also with a silicate interior and probably with
an iron core
- cracks (lineae) in the icy crust are probably due to tidal heating by Jupiter; study of cracks in
Europa's surface ice may indicate the presence of liquid water beneath
a2c. Ganymede
- largest satellite in the Solar System (it is larger than Mercury)
- crust with equal amounts of silicate rock and water ice; this is underlain by an outer icy mantle
and inner silicate mantle; also with an iron sulfide or iron core
- surface with an older highly-cratered dark region and slightly younger lighter region with
grooves and ridges
- a saltwater ocean may be present at about 200 kms depth, sandwiched between layers of ice
a2d. Callisto
- third-largest moon in the Solar System (about the size of Mercury)
- Callisto’s surface is heavily-cratered; with an icy crust, interior of compressed rocks and ices
and with a small silicate core
- possibly with a subsurface ocean at about 50 to 200 kms beneath the crust
Astronomy Notes, Page 43
b. Jupiter’s Irregular Satellites
- include the remaining 59 moons of Jupiter, which are situated much farther from Jupiter versus
the Regular Satellites
- highly inclined and eccentric orbits; some with prograde orbits, others retrograde
- Jupiter’s Irregular Satellites probably represent “captured” planetesimals, etc.
B. Saturn
- second-largest planet in the Solar System
1. Characteristics
- orbital period = 29.461 years; period of rotation = 10h 13m 59s (with differential rotation)
- equatorial diameter = 120,660 kilometers; average density = 0.7 gm/cm3
2. Composition
- with a small iron-, nickel-, silicon- and oxygen-rich core; above this is a metallic hydrogen
layer, an intermediate layer of liquid hydrogen and helium, and an outer gaseous layer
- like Jupiter but smaller liquid metallic hydrogen region; thus small magnetosphere and less
radiation
3. Atmosphere
- like Jupiter but higher velocity jet stream and fainter cloud belts/zones due to ammonia crystals
hiding deeper layers; outer atmosphere with about 96% hydrogen and 3% helium
- produces more energy than receives from sun by condensed helium falling through the
atmosphere; Temperature at cloud tops = -292°F
- with wind speeds up to 1800 kms/hour
- a persistant hexagonal-shaped wave pattern is developed around the North Polar Vortex
4. Rings
- orbit in Saturn's equatorial plane
- rings consist mostly of frozen water a few centimeters to hundreds of meters across, with
smaller amounts of rock debris and dust; the rings are only about 20 meters thick but extend from
about 6,630 to 120,700 kms from Saturn
- with about 9 sets of rings; the main rings are labelled A-G, and there are many ringlets due to
interaction with Saturn’s Moons
- the rings are probably temporary features (?)
5. Saturn’s Natural Satellites / Moons
- approximately 62 known, with hundreds of “Moonlets” imbedded within the rings
a. Saturn’s Regular Satellites
- include approximately 24 Moons; with prograde orbits which are not greatly inclined to
Saturn’s equatorial plane
Astronomy Notes, Page 44
- include Saturn’s seven major satellites
- The most important (nearest to farthest from Saturn) are:
a1. Mimas
- the smallest of the icy satellites
- the gigantic crater Hershel covers about one-third the diameter of Mimas, indicating that Mimas
was nearly destroyed by an impact
a2. Enceladus
- probably with the most reflective surface of any Solar System object
- is lightly cratered and with ridges, scarps and fractures
- Enceladus is geologically active with water-rich plumes erupting (“water volcanism”) from a
subsurface body of liquid water
a3. Tethys
- with a very bright, highly-cratered surface
- mostly consists of water ice, with a small amount of rock
a4. Dione
- similar to Rhea in appearance; with a heavily-cratered leading hemisphere and a trailing
hemisphere with bright ice cliffs
- consists of water ice with about 46% denser silicate material in the interior
a5. Rhea
- with heavily-cratered and bright leading hemisphere and bright ice cliffs on the trailing side
- consists of about 75% ice and 25% rock
a6. Titan
- second-largest satellite in the Solar System (Titan is larger than Mercury)
- consists primarily of water ice and rocky material; there may be a liquid ocean beneath Titan’s
surface made of water and ammonia
- with a relatively smooth surface (therefore Titan is geologically young)
- surface temperature is about -179°C (-290°F)
- only moon in the Solar System with a significant atmosphere (mostly consists of nitrogen; with
wind; also with rain of liquid methane and other organic compounds)
- with sand dunes, rivers, lakes and seas (probably consisting of liquid methane and ethane);
possibly with “cryovolcanoes” of water- and ammonia-rich “lava”
a7. Iapetus
- with the most inclined orbit of Saturn’s regular satellites
- consists mostly of ice with about 20% rocky material; with a large equatorial ridge
- with dark organic compounds on the "leading" side, bright material on the "trailing" side of the
orbit; surface is heavily cratered
Astronomy Notes, Page 45
b. Saturn’s Irregular Satellites
- 38 moons whose orbits are farther from Saturn, with high inclinations, and include both
prograde and retrograde orbits
- probably represent captured Minor Planets, etc.; it has been suggested that Phoebe may be a
captured Centaur
C. Uranus
- Uranus and Neptune have a different chemical composition that Jupiter and Saturn, and are
sometimes placed within a separate category termed the “Ice Giants”
1. Characteristics
- orbital period = 84.013 years
- axis of rotation lies nearly in the plane of its orbit (the axial tilt is about 97.77°!)
- relatively strong magnetic field inclined 60° to the axis of rotation [orientation of rotational and
magnetic axes and surface features of satellites (especially Miranda) suggest Uranus has been
"knocked on its side" by collision with a planet-like object!]
- period of rotation = 17h 14 m
- equatorial diameter = 51,118 kilometers
2. Composition
- with a silicate/iron-nickel core, an icy mantle made of water, ammonia and other volatiles, and
an outer gaseous layer of hydrogen and helium
3. Atmosphere
- Uranus has the coldest planetary atmosphere in the Solar System (49°K; -224°C; -371°F)
- predominantly helium and hydrogen; also with “ices” such as water, ammonia, and methane
and with traces of hydrocarbons
- the presence of methane in the upper atmosphere gives Uranus a blue coloration
- clouds form close to the surface (water clouds lowest, ammonia next, then methane) and
overlain by hydrogen
- extreme seasons (due to inclined orbit); each pole gets 42 years of continuous sunlight followed
by 42 years of darkness
- with west to east atmospheric circulation (due to rotation rather than position of the Sun)
4. Rings
- with at least 13 narrow rings, each usually a few kilometers wide; the inner rings are gray (the
darkness is probably due to radiation-damaged methane ice) and the outermost rings are blue
(due to water ice?); there is little rock dust; most ring particles are probably at least several
centimeters across
- orbits probably controlled by shepherd satellites
- the rings were probably made by fragmentation of some of Uranus’ moons
Astronomy Notes, Page 46
5. Uranus’ Natural Satellites / Moons
- approximately 27 known natural satellites; orbit approximately perpendicular to Uranus' orbit
a. Regular Satellites
- relatively large moons that are relatively dark and made of approximately equal amounts of
ammonia- and carbon dioxide-ice and rock (except Miranda, which is mostly ice)
- all major moons were probably formed from the accretion disk that surrounded Uranus just
after its formation
- The most important (from nearest to farthest from Uranus) are:
a1. Miranda
- smallest and innermost of Uranus’ five major moons
- surface mostly water ice; also probably with silicates and organic compounds in the interior
- part of Miranda’s surface consists of undulating cratered plains and part with regions of scarps
that show as dark bands, and with global faults; this “messed up” geology may be due to a major
collision early in Solar System history or the complex geology may be due to tidal heating caused
by prior orbital eccentricity (resulting in extensional graben-like features, diapir-like structures
resulting from upwelling of warm ice, and possible cryogenic “ice magma” eruptions)
a2. Ariel
- consists of approx. equal parts of rock and ice; with a rocky core surrounded by a mantle of ice
- Ariel has a geologically-young surface, probably due to a prior more eccentric orbit and tidal
heating
- surface features include impact craters, fault scarps, ridges, troughs and canyons
a3. Umbriel
- consists of water ice with about 40% darker material (possibly rock and carbonaceous material)
- darkest Uranian Moon, probably due to energetic particles “sputtering” the methane-rich water
ice surface
- with numerous impact craters
a4. Titania
- largest Uranian moon
- consists of approx. equal amounts of ice and rock/carbonaceous material; with a rocky core and
icy mantle (and possibly with a liquid water layer at the core/mantle boundary)
- surface is relatively dark and slightly red (possibly due to “space weathering”/sputtering), with
numerous impact craters; Titania had an early “resurfacing” event that obliterated the old cratered
surface
- expansion of the interior layers led to the formation of enormous canyons and scarps
a5. Oberon
- second-largest Uranian moon
- second-darkest moon of Uranus (surface is reddish except in fresh impact regions, which are
Astronomy Notes, Page 47
neutral or slightly blue)
- structure and composition similar to Titania, and with possible liquid water at the core/mantle
boundary
- with many large craters, high mountains, canyons/cracks and scarps
- dark patches in crater floors may be due to excavating dark material beneath the icy crust during
impacts or due to cryogenic “lava” flows
b. Uranus’ Irregular Satellites
- approximately nine moons, probably representing captured objects trapped by Uranus soon after
its formation
D. Neptune
- farthest planet from the Sun
1. Characteristics
- orbital period = 164.79 years
- period of rotation = 16h 3m; equatorial diameter = 49,500 kilometers
- axial tilt approx. 28°
- magnetic field like Uranus's (strongly inclined)
2. Composition
- with a similar composition to Uranus (mostly made of hydrogen and helium; with higher
proportions of “ices” such as water, ammonia and methane than the “gas giants” Jupiter and
Saturn)
- radiates more energy than it receives (therefore with internal heat source)
- density = 1.67 gm/cm3
- similar composition to Uranus with core made of iron, nickel and silicates and a mantle rich in
water, ammonia and methane
3. Atmosphere
- the upper atmosphere is dominated by hydrogen (80%), helium (19%), with a trace of methane
which gives the atmosphere a blue color
- Neptune has the highest wind speeds of any planet (up to 2100 kms/hr/1300 mph; these are
supersonic speeds!)
- discovered to have an Earth-size Great Dark Spot (a high-pressure system with nearly
supersonic winds); this spot later disappeared and has been replaced by other dark spots; also
with faint belts and zones
- Temperature at cloud tops = -218°C (-360°F; 55 K)
4. Rings
- consist of icy particles coated with silicates or carbonaceous material (which give the particles a
reddish color)
- with three main sets of rings
Astronomy Notes, Page 48
5. Neptune’s Natural Satellites / Moons
- fourteen known moons
a. Neptune’s Regular Satellites
- include the innermost seven moons
- with prograde orbits that lie along Neptune’s equatorial plane
- some of the regular satellites orbit within Neptune’s rings
b. Irregular Satellites
- include the outer seven moons
- the orbits are highly inclined and the orbits may be prograde or retrograde
- the two outermost satellites have the largest orbits on any moons in the Solar System
b1. Triton
- largest moon of Neptune
- with a retrograde orbit, and probably represents a Kuiper Belt object captured by Neptune
(Triton is slightly larger than Pluto and has about the same composition; therefore probably
Triton and Pluto had similar origins)
- with a rocky core, an icy mantle, and a crust made primarily of water ice
- approx. half the surface consists of “cantaloupe terrain”, which may be due to emplacement of
less dense dirty water ice as “diapirs”
- the rest of the frozen nitrogen surface is probably actively resurfaced due to eruptions of interior
liquids and with geyser-like plumes of nitrogen gas leaving streaks of dark sooty material more
than 100 km long
- with a thin nitrogen and methane atmosphere
b. Nereid
- has the most eccentric orbit of any satellite in the solar system
- Nereid may be a captured asteroid or Kuiper Belt object
XI. Dwarf Planets and Small Solar System Bodies
A. Minor Planets
- Solar System bodies that directly orbit around the Sun that are not major planets or comets
- Minor Planets include Dwarf Planets, asteroids, and centaurs
- most Minor Planets are Trans-Neptunian Objects (TNO’s) that orbit the Sun at a greater
distance than Neptune
1. Dwarf Planets
- celestial objects orbiting the Sun that are massive enough to be spherical due to their gravity but
not massive enough to clear the neighboring region of planetesimals
- Dwarf Planets are part of “Larger Populations”; they may be associated with the Asteroid Belt,
Astronomy Notes, Page 49
the Kuiper Belt, or the Scattered Disc
a. Kuiper Belt Objects (KBOs)
- the Kuiper Belt is the region dominated by small icy bodies made of methane, ammonia and
water that extends from beyond the orbit of Neptune (at 30 AU) to about 50 AU from the Sun
- the largest KBOs include Pluto, Haumea and Makemake (dwarf planets that extend beyond
Neptune’s orbit are often termed Plutoids)
a1. Pluto
- second-most massive dwarf planet; it is similar to Eris in size
- Pluto is probably the largest member of the Kuiper Belt; it probably represents a remnant
planetesimal that was created from the protoplanetary disc that formed the Solar System
a1a. General Characteristics of Pluto
- highly elliptical orbit steeply inclined to the plane of the ecliptic
- orbital period = 247.7 years
- period of rotation = 6d 9h 21m; Pluto rotates on it’s side, with an axial tilt of 120°
- equatorial diameter = 2306 kilometers (about two-thirds the size of the Earth's moon)
a1b. Composition of Pluto
- the surface appears to be about 98% nitrogen ice, with traces of methane and carbon monoxide
- surface coloration and brightness vary greatly, probably caused by seasonal changes due to the
axial tilt and orbital eccentricity of Pluto
- internal composition probably consists of an icy mantle and rocky core (astronomers speculate
that there may be a liquid water layer at the mantle/core boundary)
a1c. Pluto’s Atmosphere
- Pluto has a thin atmosphere of nitrogen, methane and carbon monoxide
- the atmosphere probably changes dramatically, with the gases turning to ice when Pluto moves
into its outer orbit
- average atmospheric temperature is about -230°C (-382°F; 43 K)
a1d. Pluto’s Moons
- Pluto has five known natural satellites
- Charon is the largest Moon of Pluto; 1280 kilometers diameter; the orbital period of Charon is
the same as Pluto's rotational period (they are tidally locked together); Pluto and Charon are
sometimes referred to as a “binary system” (are they “dwarf double planets”?); the presence of
water crystals and ammonia hydrates may indicate the presence of “geysers” on Charon’s surface
a2. Haumea
- the fourth largest dwarf planet in the Solar System (it is about one-third the mass of Pluto)
- it has a highly elliptical orbit with an orbital inclination of about 28°
- Haumea has an ellipsoidal shape probably due to its very rapid rotation; the rapid rotation and
its high density are probably a result of a massive impact early in its history (Haumea, its moons
Astronomy Notes, Page 50
and several other TNOs have been referred to as the “Collisional Family”; the remnants of a
larger body that was destroyed by a massive impact)
- the highly-reflective surface of Haumea consists of crystalline ice; a large dark reddish spot on
Haumea’s surface may represent an impact structure
- Haumea has two small moons
a3. Makemake
- the third-largest dwarf planet in the Solar System (it is about two-thirds the size of Pluto)
- it has an eccentric orbit that is inclined at 29°
- Makemake is the second-brightest KBO (after Pluto); Makemake’s reddish surface appears to
be rich in methane and possibly ethane and tholins (tholins are secondary organic compounds
created by irradiation of methane and ethane)
- Makemake has no known satellite
b. Scattered Disc Objects (SDOs)
- the Scattered Disc is an unstable portion of the Solar System where icy bodies were “scattered”
due to the gravitation of the Jovian Planets (and their orbital motions may even now be affected
by Neptune)
- the Scattered Disc region overlaps the Kuiper Belt and extends beyond 100 AU
- over 200 SDOs are known
b1. Eris
- the most massive dwarf planet in the Solar System [its size (about 2320 km) is about the same
as Pluto but Eris is 27% more massive than Pluto]
- Eris is about 3 times farther from the Sun than Pluto, with an orbital period of 557 years
- it has a highly-reflective gray-colored surface, probably due to the presence of methane ice
- Eris has one known moon, Dysnomia
2. Centaurs
- minor planets with characteristics of both comets and asteroids
- centaurs orbit the Sun between Jupiter and Neptune and cross the orbital paths of one or more
of the Jovian planets
- some astronomers have suggested that Centaurs may represent Scattered Disc Objects (SDOs)
that were slung into their current orbits by the gravitation of Neptune
- although no Centaur has been photographed up close, it has been suggested that Saturn’s moon
Phoebe is a “captured” Centaur
- Centaurs display a wide range of coloration; this may be due to composition differences and/or
change of coloration due to “space weathering”
3. Asteroids
- minor planets of icy-rocky or metallic composition that range from a few tens of meters up to
approximately 1000 kilometers in diameter; asteroids extend from the inner Solar System out to
the orbit of Jupiter
- most asteroids are found in the Asteroid Belt between Mars and Jupiter; these asteroids may
Astronomy Notes, Page 51
have formed from the original solar nebula or from planetesimals that smashed together due to
the gravitational influence of Jupiter
- asteroids are classified by the characteristics of their orbits or by their spectral characteristics
a. Spectral Classification of Asteroids
- based upon color, reflectance (albedo) and spectral characteristics of asteroids
- the major types are:
S Type = more common in inner orbit; usually bright and red; these are believed to be stony,
silica-rich asteroids (include about 17% of known asteroids)
C Type = more common in outer orbit; very dark; similar to carbonaceous chondrites; include
about 75% of known asteroids
X Type (or M Type) = probably iron-nickel cores of broken planetesimals; moderately bright but
not as red as S type
- most inner Asteroid Belt asteroids are silica-rich; outer Asteroid Belt asteroids tend to be more
carbon-rich
b. Orbital Classifications of Asteroids
b1. Effects of Jupiter
- Jupiter's gravitation depletes certain orbits in the asteroid belt (forms "Kirkwood Gaps")
- gravity also captures the Trojan asteroids in two locations (Lagrangian Points) along Jupiter's
orbit
b2. Ceres
- the largest asteroid (950 km diameter) and only Dwarf Planet in the inner portion of the Solar
System; it is the largest and most massive object in the Asteroid Belt
- Ceres structure probably consists of a rocky core, icy mantle and a surface consisting of a
mixture of water ice and hydrated minerals (including clays and carbonates); the surface
composition appears to be similar to C Type asteroids; the presence of a liquid water layer
beneath the surface has been hypothesized
b3. Amor and Apollo Asteroids
- move in elliptical orbits that cross the orbits of Mars and Earth
- Amor Asteroids are near-Earth Asteroids, but do not cross the orbit of Earth [most Amor
Asteroids cross the orbit of Mars; the moons of Mars (Deimos and Phobos) may be captured
Amor Asteroids]
- Apollo Asteroids are near-Earth Asteroids that cross the orbit of Earth (therefore they are
potentially dangerous asteroids)
c. Asteroids and Extinction
Astronomy Notes, Page 52
- asteroid may have struck the Earth at the end of Cretaceous Period (approximately 65 million
years ago), stirred up dust and created fires, initiated a "nuclear winter" and caused mass
extinctions (including dinosaurs)
- evidence includes the "iridium layer" and presence of "shocked quartz"
B. Comets
- small icy solar system bodies that generally move in a highly-elliptical orbit around the Sun
1. Structure and Composition
- comet nuclei range from about 100 meters to about 40 kilometers in diameter; they consist of
frozen gases (methane, ammonia, carbon dioxide and carbon monoxide), water ice, rock, dust,
minor amounts of organic compounds and possibly amino acids; the surface of the nuclei are
typically dark (“dirty snowballs”) due to the presence of a carbon- and silicate-rich crust
- in the outer Solar System comets remain frozen and are extremely difficult to detect
2. Morphology
a. Development of the Coma and Tail
- as a comet approaches the Sun, the icy nucleus develops a luminous “atmosphere-like” coma
(the combination of nucleus and coma are termed the “head”) surrounded by a hydrogen
envelope
- the coma forms within 3 AU of Sun, with an ion tail and dust tail extending from the comet
- the tails of comets always point away from the Sun
b. Comet "Tails"
Type I (Gas/Ion Tail) - the pressure of the solar wind pushes ionized gas away from the comet
head
Type II (Dust Tail) – sunlight pressure pushes dust particles away from the comet head
c. gas and dust vents from active areas on the comet
- a comet's light is due to sunlight reflection and ionization/fluorescence of gases by photons
from the Sun
3. Classification and Origin of Comets
a. Short-Period Comets
- have orbital periods less than about 200 years and usually orbit approximately in the ecliptic
plane
- short-period comets are believed to originate in the Kuiper Belt or (more likely) in the dynamic
region of the Scattered Disc
b. Long-period Comets
- highly-eccentric comets with orbital periods from 200 to thousands (or possibly even millions)
Astronomy Notes, Page 53
of years
- long-period comets may be derived from the hypothetical "Oort Cloud", which probably
contains several trillion icy bodies at a distance of approximately 2,000 to 50,000 AU (or
possibly up to 200,000 AU) from the Sun
- it is believed that tidal forces exerted by the Milky Way or gravitational attraction from nearby
stars or giant molecular clouds may sling some of these objects toward the Sun
- the Oort cloud was probably created approximately 4.6 billion years ago from the
protoplanetary disc that created the Solar System; gravitational disturbances from the Jovian
Planets probably catapulted the icy bodies to their current position in the Oort Cloud
4. Effects of Comets
a. Meteor Showers
- most meteor showers are produced when Earth passes through a "meteor swarm" of "burnedout" cometary fragments (meteor showers are typically named after the “background”
constellation from which they seem to originate)
b. Comet Impacts
- the "Tunguska Event" in Siberia in 1908 may have been due to the airburst of a comet
approximately 5 to 10 kilometers above the Earth’s surface (although some astronomers believe
the object was an asteroid)
- Comet Shoemaker-Levy 9 fragmented and impacted Jupiter in 1994; the impacts created dark
spots in Jupiter’s atmosphere up to 12,000 kms across
c. Comets caused the extinction of the dinosaurs(?)
- this controversial theory proposes that the Sun's hypothetical companion star ("Nemesis")
caused comets (or asteroids) to be thrown at the Earth (?); other theories propose that the
comet(s) impacted Earth due to orbital change related to gravitational disturbances by the Milky
Way or Giant Molecular Clouds
C. Meteors and Meteorites
1. Definitions
Meteoroid = sand- to boulder-sized rock or ice particle in space
Meteor = meteoroids that enter the atmosphere (a very bright meteor is often termed a “Fireball”)
Meteorite = meteoroid that strikes the Earth’s surface and remains intact
2. Origins
- provide information about the origin and age of the Solar System
- most meteors are made of small particles from comets
- most meteorites probably are asteroid fragments or material from the Asteroid Belt that never
Astronomy Notes, Page 54
formed large objects
- some arrive in "showers", others are sporadic
3. Classification
- although modern meteorite classifications are complex, meteorites have traditionally been
divided into three major classes (iron, stony iron and stony)
a. Iron Meteorites
- consist of an iron-nickel alloy; constitute approx. 5% of meteorites
- probably represent the shattered cores of asteroids (they appear to be very similar to M-type
asteroids)
- polished sections of iron meteorites have Widmanstätten patterns
b. Stony Meteorites
- are made of silicate minerals; they are the most common meteorites
- most stony meteorites are Chondrites (contain round silica-rich grains or Chondrules;
chondrites include about 86% of meteorites); chondrites appear to have been formed from
silicates that were melted in space (they may represent material from the Asteroid Belt that never
formed larger objects)
Carbonaceous Chondrites - rare stony meteorites that may be relatively unmodified material
from the Solar Nebula; made of silicates, oxides and sulfides and often contain volatile organic
compounds and water; some may have played a role in the origin of life on Earth (some
carbonaceous chondrites contain amino acids)
Achondrites – do not have chondrules; constitute about 8% of meteorites; most are believed to
represent asteroid crustal material (they resemble mafic igneous rocks)
c. Stony-Iron Meteorites
- consist of a combination of silicates and metal; they may have formed in the “boundary zone”
within asteroids (immediately above the asteroid core)
- constitute about 1% of meteorites
4. Meteorite Impacts
- the fastest meteoroids move at about 26 kilometers per second (since the Earth moves at 18
kilometers per second, a collision may produce a meteor speed of 44 kilometers per second
- the collision of the meteor with the Earth’s surface may produce an Impact Crater (the
characteristics of this crater depend upon the composition, velocity, degree of fragmentation and
angle of the incoming meteor)
- the largest impact structures appear to be created by iron meteorites [including the Barringer
Crater (“Meteor Crater”) and the Odessa Meteor Crater]
- the impact of the meteorite creates a supersonic shock wave and huge compression that results
in “Impact Metamorpism” (including production of “Shatter Cones” and impact breccias); during
this process “shocked quartz” and Tektites (glassy spherical or elongate blobs up to several
Astronomy Notes, Page 55
centimeters in diameter probably caused by melting rocks) may be created; this is followed by
explosive decompression of the rocks (the rocks are pushed downward, then upward and
outward, which results in elevation of the crater rim); Ejecta is thrown outside the crater
C. Formation of the Solar System
- the Solar System formed about 4.567 billion years ago
1. Solar Nebula Hypothesis
- the Solar System formed from a portion of a giant molecular cloud; isotopic studies indicate
that a shock wave from a supernova may have caused the density of gases and dust to increase,
accompanied by gravitational collapse and the formation of star clusters (a similar situation
seems to presently occur in the Orion Nebula)
- conservation of angular momentum caused the nebula to spin faster during collapse, with
increased molecular collisions and conversion of kinetic energy to heat (the center of the disc
became hotter due to its higher mass)
- as the nebula rotated it flattened into a protoplanetary disc with the “protostar” Sun forming in
the center of the disc (it consisted of dense, hot gases but there was no hydrogen fusion; the Sun
at this time was probably a T Tauri star); within about 50 million years the Sun began to fuse
hydrogen to helium (and therefore entered the “Main Sequence”)
- planets began as dust grains that grew first by condensation (adding matter from the
surrounding gas) and then by accretion (in which the solid particles began to stick together);
these particles accumulated to produce planetesimals, which ranged in size from a few
millimeters to hundreds of kilometers, and moved in rotating plane; protoplanets formed from
the planetesimals
- the inner Solar System was too hot for the condensation of volatiles, but silicates and metals
could form (this would form the terrestrial planets); terrestrial planets are small because these
materials are rare (only about 0.6% of the composition of the Solar Nebula); there may have been
50 to 100 Moon- to Mars-size “planetary embryos” that collided and coalesced to form the
terrestrial planets
- beyond the orbit of Mars the Solar System was cool enough for ices to form; the gravitational
attraction of these icy cores allowed the collection of vast amounts of ices and gases, resulting in
the formation of the Jovian Planets (these contain about 99% of Solar System mass surrounding
the Sun)
- Uranus and Neptune are believed to have formed later than Jupiter and Saturn (after the early
“T Tauri” Sun developed a strong stellar wind, which accounts for the lower amounts of
hydrogen and helium in these “icy giants”)
- planetary embryos probably developed around the asteroid belt, but the gravitational attraction
of Jupiter and Saturn probably caused numerous collisions and fragmentation; the inner
migration of Jupiter caused the scattering of these planetesimals (some moved toward the Sun to
add to the Terrestrial Planets), while others were ejected outward
- it is believed that the “icy giants” Uranus and Neptune developed somewhat closer to the Sun
but migrated outward to their current position, scattering the small icy planetesimals to help
create the Kuiper Belt, Scattered Disc and Oort Cloud
Astronomy Notes, Page 56
- the gravitational disruption of the Jovian Planets is believed to have sent numerous asteroids
into the inner Solar System, resulting in the “Late Heavy Bombardment”
- “regular” satellite systems of the outer planets probably formed in the same way that planets
formed; “irregular” satellites probably represent planetesimals, etc. that were scattered by the
Jovian Planets which were later captured by the planets’ gravity
XII. The Sun: Our Star
A. General Features
- the Sun is the star at the center of our Solar System
- the Sun’s diameter is about 1,392,000 kms (about 109 times the diameter of the Earth) and has
about 99.86% of the known mass of the Solar System
- the Sun rotates counterclockwise (as seen from the ecliptic North Pole); it has differential
rotation due to its plasma composition (the poles rotate in about 35 days; the equator rotates in
approx. 25 days)
B. Internal Structure
1. Central Temperature and Pressure
- the Sun is in hydrostatic equilibrium (gravity balances the internal pressures produced)
- central temperature of the Sun is about 15.7 million K
- gas in the Sun's core is compressed to about 150,000 kg/m3 (about 150 times the density of
water)
- almost all of the Sun's energy is produced in the inner 25% of the Sun's radius (about 1.5% of
the Sun's volume)
2. Sun's Surface
- the Sun’s surface has about 75% hydrogen and 24% helium (at the Sun's center hydrogen is
only about 40%)
- hydrogen has been used as fuel by the Sun for most of the 4.6 billion years the Sun has existed,
and slightly more than half of the hydrogen at the Sun's center has been consumed
C. The Sun's Energy
1. The Proton-Proton Chain
- a series of fusion nuclear reactions through which stars like the Sun produce energy by
converting hydrogen to helium
- named because the first reaction in the series is the fusion of two protons to produce deuterium
(2H); then fuses a deuteron with a proton to produce 3He, and then fuse two 3He to produce a 4He
nucleus
2. Flow of Energy to the Surface
- Radiative Diffusion (outward flow of photons) carries the Sun's energy outward from the core
Astronomy Notes, Page 57
to about 70% of the Sun's radius (within the Radiative Zone); at that point convection begins to
occur and carries most of the energy to the surface
- it is estimated that it may take up to 200,000 years between the time solar energy is produced
and the time it reaches the Sun’s surface
- Solar Neutrinos (particles with no charge and with very small mass that are produced in nuclear
reactions) pass unimpeded through the Sun's gases and escape into space; neutrinos are very
useful for obtaining an immediate view of conditions at the Sun's core
D. The Outer Layers of the Sun
- the outer layers are the only ones that emit radiation that reaches Earth
- the deepest layer we see is the photosphere, above which is the chromosphere and corona
(which extends into interplanetary space to form the solar wind)
- the parts of the Sun above the photosphere are termed the “Solar Atmosphere”
1. The Photosphere
- the layer that we see in visible images of the Sun
a. The Solar Limb
- the sharp edge of the solar disc (the Limb) shows that density decreases rapidly in the Sun's
outer layers; the relative faintness of the Sun's limb (Limb Darkening) shows that temperature
decreases outward in the visible layers of the Sun (the Sun's surface temperature is about 6000 K)
b. Granulation
- the Sun's surface shows a pattern of bright and dark markings (Granules), which are rising
columns of hot gas about 1000 kilometers across that last about 15 minutes; a much larger
convective pattern (Supergranulation), that lasts about a day, is harder to see and carries little
energy outward
c. Helioseismology
- the Sun vibrates in a complex pattern of oscillations similar to seismic waves in the Earth
- helioseismology is the study of the patterns of oscillations, which have led to discoveries of
how temperature, composition, and rotation vary with depth in the Sun
d. Sunspots and Solar Magnetism
Sunspot - region of the Sun's photosphere that appears darker than its surroundings because it is
cooler; the inner darkest part of the sunspot is the Umbra, the outer part is the Penumbra;
sunspots tend to occur in groups (Sunspot Groups) and the area around the sunspot group (the
Active Region) has many solar flares and other indications of activity
- regions of strong magnetic fields are concentrated in sunspot groups; the large magnetic fields
in sunspots inhibit convection and reduce the flow of energy to the Sun's surface
- the rate at which sunspots cross the disk of the Sun varies with latitude because the rotation rate
of the Sun decreases as the distance from the equator increases
Astronomy Notes, Page 58
The Sunspot Cycle- the number of sunspots increases and decreases during an 11-year Sunspot
Cycle (although there have been long periods of time in which very few spots have been seen on
the Sun); the Sunspot Cycle and the pattern of magnetic polarities in sunspots are probably due to
the differential rotation of the Sun, that stetches and twists magnetic field lines
2. The Chromosphere
- a tenuous region of gas that lies just above the photosphere; the temperature at the top of the
chromosphere is about 20,000 K
- most of the chromospheric gas is in the form of rapidly rising jets of gas (Spicules), which
shoot upward at about 25 kilometers per second (50,000 miles per hour) and last for about 5
minutes
3. The Corona
- outermost layer of the Sun's atmosphere, within which the temperature rises to 1 to 2 million
degrees kelvin; the high temperature of the corona is probably due to heating by waves that move
into the corona along magnetic field lines
- the corona is densest and brightest above active regions, where coronal gas is trapped by
magnetic fields; away from active regions the Sun's magnetic field can't trap coronal gas, which
accelerates away from the Sun, forming dark coronal holes
- Prominences are clouds of relatively cool gas that extend upward into the corona; as
prominences are cooler than the photosphere they appear as dark Filaments when seen against the
bright disc of the Sun [when viewed against the background of space, many prominences are
loop-shaped (termed Loop Prominences)]; large prominences erupt about once or twice a day,
blasting gas outward into space
- Solar Flares are sudden brightenings of the Sun’s surface due to explosive releases of the Sun's
magnetic energy (they are found in the Sun’s active region around sunspots); high-energy
radiation and energetic particles are both produced by flares
- Coronal Mass Ejections (CME’s) are massive bursts of plasma and electromagnetic radiation
that are associated with flares and prominences; when CME’s reach the Earth geomagnetic
storms may result, with deformation of the magnetosphere and release of huge amounts of energy
in the Earth’s upper atmosphere (CME’s may damage satellites, disrupt radio communications,
create strong auroral displays, cause power outages, and the radiation could be harmful to
astronauts)
4. The Solar Wind
- the flow of coronal gas into interplanetary space; the magnetic field lines in the solar wind
remain attached to the Sun, so the rotation of the Sun (approx. once every 27 days) stretches the
field lines into a spiral pattern (creating the Heliospheric Current Sheet); the solar wind reaches
the Earth in about 4 days
- the Heliosphere is the region of space affected by the solar wind; at a distance of about 100 AU
the solar wind begins to merge into interstellar gas (at the Heliopause), which compresses the
heliosphere in the direction that the solar wind is moving and pushes the heliosphere back in the
opposite direction (creating the Bow Shock)
Astronomy Notes, Page 59
XIII. Stellar Astronomy
A. Star Names
Stars are named by:
1. Ancient Sources
- typically use names from Latin, Greek or Arabic
- Arabic names often have prefix “al”
2. Using Constellation Names
- for brighter stars astronomers often pair a Greek letter with the name of the constellation in
which the star is found; stars are designated in roughly descending order of brightess with α
brighter than β brighter than γ, etc. (Ex. = Sirius, the brightest star in the constellation Canis
Major, is designated α Canis Major )
3. Star Catalogs
- the existence of numerous star catalogs has led to the same star given several names, sometimes
a confusing situation
B. Determining Star Distances
1. Parallax
- the apparent change in the direction of a star resulting from viewing it from different places on
the Earth’s orbit around the Sun
- all stellar parallaxes are less than one second of arc, so in order to get precise measurements
astronomers measure positions of a nearby star relative to more distant stars that lie in the same
direction
2. Units of Measurement
- distances of stars are often measured in light years (the distance light travels in a year; 9.46
trillion kilometers / 5.88 trillion miles) OR Parsecs (pc; the distance at which a star has a parallax
of 1 second of arc; one parsec equals 30.9 trillion kilometers, 19.2 trillion miles, or 3.2616 light
years)
3. Standard Candles
- an astronomical body in which the luminosity is known, allowing its distance to be determined
by measuring its apparent brightness and applying the inverse square law (Exs. = Cepheid
variable stars, RR Lyrae variables, Supernovas)
C. Motions of Stars
1. Proper Motion
Astronomy Notes, Page 60
- the rate at which a star appears to move across the celestial sphere with respect to very distant
objects
- proper motions are caused by the movement of stars through space relative to the Sun, and
therefore are very slow (typically less than 1 second per year)
2. Solar Motion
- the motion of the Sun with respect to nearby stars; this causes the stars toward which the Sun is
moving appear to diverge and the stars away from which the Sun is moving appear to converge
- the Apex is the direction toward which the Sun is moving; the Antapex is the direction from
which the Sun is moving
D. Brightness of Stars
Apparent Brightness - observed brightness of a celestial body as viewed from Earth
1. Stellar Magnitude
a. Apparent Magnitude
- system devised by Ptolemy to describe the apparent brightness of stars
- in the magnitude system objects brighter than the brightest stars are given negative magnitudes,
whereas faint objects have large positive magnitudes; each additional magnitude corresponds to a
decrease in brightness by a factor of 2.512
b. Absolute Magnitude
- the apparent magnitude a star would have if it were at a distance of ten parsecs; absolute
magnitude measures the star’s intrinsic brightness (i.e. the luminosity)
2. Luminosity
- the rate of total radiant energy output of a celestial object; luminosity is the intrinsic brightness
of a star summed over the entire electromagnetic spectrum
3. Luminosity Function
- the relative number of stars for each value of absolute magnitude
- a plot of the relative number of stars versus absolute magnitude shows that fainter stars are
much more numerous than brighter stars
E. Stellar Spectra
- are often used to obtain information about a star’s surface temperature, chemical composition,
and speeds at which they approach or recede from the Earth
1. Atoms and Spectral Lines
a. Kirchhoff’s Laws
- formulated by Gustav Kirchhoff (German physicist; 1824-1887)
Astronomy Notes, Page 61
a1. A hot solid, liquid, or dense gas produces a Continuous Spectrum in which emission
appears at all wavelengths
a2. A thin gas, seen against a cooler background, produces a Bright Line, or Emission Line,
Spectrum; an Emission Line Spectrum consists of narrow, bright regions separated by dark
regions
a3. A thin gas in front of a hotter source of continuous radiation produces a Dark Line, or
Absorption Line, Spectrum; the Sun (and other stars) produce absorption line spectra
- each chemical element or compound gives its own unique pattern of dark or bright lines, so
absorption line or emission line spectra can be used to identify particular elements or chemical
compounds in a cloud of gas
b. Atomic Structure
- the electrons in an atom can exist only at specific distances from the nucleus and each allowed
electron distance has a different energy (Energy Level is the term for this allowed electron
distance); electrons can jump from one energy level to another
- when an atom absorbs a photon the energy carried by the photon causes an electron to jump to a
higher energy level; when an atom emits a photon one of its electrons jumps to a lower energy
level; the absorption and emission of radiation by atoms produces dark line and bright line
spectra; a given element or compound can only absorb or emit photons of certain energies
(therefore the pattern of dark or bright lines in the spectrum of an element or compound is
characteristic of that element)
- all absorptions that begin with the electron in the first (lowest) energy level or emissions that
end with the electron in the first energy level are called Lyman lines (designated α, β, γ, Δ, etc.);
absorptions from or emissions to the second, third, and fourth energy levels are called Balmer
(second level), Paschen (third level), and Brackett (fourth level) Lines; only Balmer lines fall in
the visible part of the spectrum
c. Spectral Classification
- system for classifying stellar spectra is based mostly on the strengths of the Balmer lines of
hydrogen; in descending order of temperature, the classes are O, B, A, F, G, K, M (O stars are
blue, A stars are white, G stars are yellow, and M stars are red); each spectral class is divided into
subclasses 0 to 9 (Ex. = the Sun is a G2 star, with a spectrum 20% from G0 to K0)
2. Temperature versus Atomic Absorption
- temperature affects the appearance of stellar spectra more than anything else; it determines the
energy level in which the electrons of atoms are likely to be found
- as temperature increases, the number of atoms in the ground state decreases and the number of
atoms in the excited level increases
3. Luminosity Class versus Atomic Absorption
- density of the gas in the star’s outer layers (where the spectral lines are produced) depends on
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the gravity there (gravity of the star increases as mass increases but decreases as the square of its
radius increases); large luminous stars have weaker lines of atoms but stronger lines of ionized
elements than do smaller, less luminous stars
Luminosity Class - classification of a star’s spectrum according to luminosity for a given spectral
type; luminosity class is indicated by a Roman numeral that ranges from I for the largest, most
luminous stars (Supergiants), to III for large luminous stars (Giants) to V [for Main Sequence, or
“Dwarf” Stars, where hydrogen is fusing to helium (Main Sequence stars are the most common
stars); the Sun’s complete spectral classification is G2V]
4. Chemical Abundances
- strengths of individual spectral lines can by used to determine chemical composition of a star
- individual stars vary somewhat in chemical makeup, but their compositions are similar; atomic
abundances generally decrease as their atomic numbers increase (almost all of the gas in most
stars is hydrogen and helium)
5. Doppler Effect and Stellar Spectra
- the wavelengths of the spectral lines of a star depend on how rapidly the star is moving toward
or away from the Earth (Radial Velocity); the Doppler shift of a star’s spectral lines makes it
possible to determine the speed with which the star moves toward or away from us and possibly
how fast the star rotates
F. Hertzsprung-Russell (H-R) Diagrams
- plots of the luminosities of stars versus their temperatures (absolute magnitude is often
substituted for luminosity, and spectral type may be substituded for temperature)
- the regions within which most stars are clustered represent long-lived evolutionary stages of
stars; as stars evolve their position on the H-R diagram change
- most stars are clustered within four areas of the H-R Diagram; These include:
1. Main Sequence
- contains the most stars; runs diagonally on the H-R diagram from hot, luminous stars to cool,
dim stars
2. Giant Region
- contains cool, luminous stars; near center right of H-R diagram
3. Supergiant Region
- contains the most luminous stars; runs along the top of the H-R diagram
4. White Dwarf Region
- contains hot, dim stars; in lower left of H-R diagram
G. Stellar Masses
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- have been found for only a few stars; masses of stars may be found by applying Kepler’s Third
Law to the motions of stars in binary systems
1. Range of Stellar Masses
- binary stars range from about 0.1 to 20 solar masses; stars as much (or more) than 50 solar
masses probably exist but seem to be rare; objects less than 0.08 solar masses are not genuine
stars (they have no nuclear fusion)
Brown Dwarfs - objects that resemble stars in some respects and planets in others
2. Mass-Luminosity Relation
- the mass of a star strongly controls the rate of energy production in a star (the larger the mass,
the greater the luminosity)
H. Binary Star Systems
1. Classification of Binary Stars
a. Classification of Binary Stars Based on Whether the Stars affect each Other's Evolution
Wide Pair - binary stars are far enough apart that they evolved independently
Close Pair - binary stars are close enough together that they can transfer matter to one another at
some stages of their evolution
b. Classification of Binary Stars Based on how the Binary Star Systems were Detected:
b1. Visual Binaries
- binary systems for which the images of the two stars can be distinguished
- most visual binaries are widely separated and have long orbital periods
- the masses of the stars in a visual binary can be determined if the distance to the binary can be
found (most of the stars whose masses have been found have been visual binaries)
b2. Spectroscopic Binaries
- binary systems that are recognized by the shifting wavelengths of their spectral lines over time;
the shifting is due to variations in the Doppler shift that result from changes in the radial
velocities of the stars as they orbit each other
- most spectroscopic binaries have small separations and short orbital periods, resulting in high
orbital speeds and large Doppler shifts
b3. Eclipsing Binaries
- binary systems that show periodic drops in brightness as the two stars alternately eclipse each
other
- most eclipsing binaries have small separations and short periods; the temperature and sizes of
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the two stars can be determined from the shape of their light curve
2. Multiple Star Systems
- consist of close binary pairs orbited, at a much greater distance, by other binary pairs or single
stars
- it is believed that the majority of stars may be members of binary or multiple star systems
3. Formation of Binary and Multiple Star Systems
- most binary and multiple star systems were probably created due to the fragmentation of the
molecular cloud during initial protostar formation
XIV. The Birth of Stars
A. General Theory
- on average about 3 to 5 solar masses (Mʘ) of interstellar gas are converted to stars each year in
our galaxy (the Milky Way Galaxy)
- because the rate at which stars form declines rapidly with increasing stellar mass, star formation
favors the production of less massive stars
- most stars are probably members of binary or multiple systems and many stars form at the same
time in the same localized region
B. Star Formation
- stars form inside relatively dense concentrations of interstellar gas and dust (Molecular Clouds);
they are termed "molecular" because they are cool and dense enough such that most of the gas
they contain consist of molecules rather than atoms or ions [gases such as molecular hydrogen
(H2; about 71% ), Helium (He; about 27%) ammonia (NH3), and carbon monoxide (CO) can be
found in molecular clouds]
- stars are created due to gravitational instability within the molecular cloud; this may be due to
shock waves created by Supernovae (huge stellar explosions) or due to the collision and merging
of galaxies
1. Giant Molecular Clouds (GMCs)
- an unusually large molecular cloud that may contain as much as one million solar masses of gas
and dust; their structure is very complex, consisting of irregular clumps, bubbles, sheets and
filaments of gas and dust (dark clouds of dense cosmic gas and dust where star formation often
takes place are termed “Bok Globules”)
Cloud Cores - dense part of molecular cloud where star formation takes place; warm, massive
cores seem to be where massive stars form, whereas low mass stars seem to form in cool, less
massive cores (the increase in temperature of the gases in these cores is caused by the conversion
of gravitational energy to thermal kinetic energy during collapse)
2. Pre-Main Sequence/PMS Stars
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- stars whose energy is due to gravitational contraction (as opposed to Main Sequence/Dwarf
Stars, whose energy is due to the fusion of hydrogen to helium)
- Protostars are stars in the process of formation; the rotation of a cloud core causes infalling
matter to accumulate in a rotating, Protoplanetary Disc (the period of gravitational collapse and
formation of the protoplanetary disc seems to last about 10-15 million years)
- when the protostar’s core reaches about one million degrees Kelvin hydrogen begins to fuse to
helium, the internal pressure of the star builds, and the core stops collapsing (but surrounding
material continues to be pulled onto the star)
- stars become visible when they blow away the surrounding gas and dust
- most stars first become visible as T Tauri or Herbig Ae/Be stars, which have vigorous surface
activity and orbiting disks of gas and dust (T Tauri stars are less than about 2 solar masses;
Herbig Ae/Be have about 2-8 solar masses)
- massive stars reach the main sequence before they become visible (they can be detected as
highly luminous infrared objects within molecular clouds)
3. Stellar Winds
- young stars develop energetic outflows of gas (“Stellar Winds”) that blow away the surrounding
infalling matter, although it is not certain as to how such strong “winds” develop
- at first, the stellar wind from a young star can only blow outward along the polar axis of the star
(these “Bipolar Jets”, termed Herbig-Haro/HH Objects, may help to drive away the gases from
the surrounding molecular cloud); later it can blow away the disk of matter as well (if a planetary
system or binary companion star doesn't form in the disk before it is dispersed)
XV. Star Lives
A. Substellar Objects
- Substellar Objects are intermediate in size between planets and stars; they are created from
protostars with less than 0.08 stellar masses that do not reach temperatures where hydrogen will
fuse to helium
- above about 13 Jupiter masses protostars fuse deuterium; these Substellar Objects are termed
Brown Dwarfs
- Brown Dwarfs and other Sub-Stellar Objects shine dimly, cooling gradually over several
hundred million years
B. Red Dwarfs
- stars with masses between approximately 0.08 and 0.5 stellar masses; it has been estimated that
70% of the stars in the Milky Way are Red Dwarfs
- because of their very low mass, fusion by the proton-proton chain occurs very slowly; this
results in low core temperatures and low luminosity
- Red Dwarfs transfer their energy from their cores primarily by convection, and therefore can
burn most of their hydrogen before leaving the Main Sequence (because of their very slow
evolution, it is believed that no Red Dwarf has ever left the Main Sequence in the history of the
Universe)
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C. Star Evolution
- changes in the appearance and internal structure of a star
- star evolution takes place due to changes in the internal composition of the star by means of
fusion reactions, transforming light elements into heavier ones
- the chemical composition of a star determines the rate at which the star produces energy, the
rate at which energy flows to the surface of the star, and the generation of pressure (hydrostatic
pressures resist gravity and prevents the collapse of the star); as these processes change the star
evolves
1. Energy Generation
- stars produce energy by fusion and gravitational contraction or collapse
a. Fusion
- the temperature at the center of a star rises to about 10 million K at the end of the star's
contraction from a fragment of a molecular cloud; then, fusion of hydrogen to helium becomes
the major source of energy for the star; in the Sun and less massive stars the fusion of hydrogen
into helium occurs by means of the proton-proton chain
- in stars more massive than the Sun hydrogen fusion proceeds by means of the Carbon Cycle, in
which carbon, nitrogen and oxygen nuclei act as catalysts for the production of helium from
hydrogen
- at temperatures of 100 million K, the fusion of helium into carbon becomes possible by means
of the Triple α Process (three helium nuclei, or α particles, make each carbon nucleus); if the
temperature of the star becomes 500 million to 1 billion K, the carbon resulting from the triple α
process becomes fuel
b. Contraction and Collapse
- after nuclear fuel has been used up, a star often begins contracting; energy released during
contraction continues until the core temperature rises enough for another kind of fuel (helium,
carbon, etc.) to begin fusing
2. Opacity
- the ability to impede the flow of radiation; opacity depends upon the local temperature, density,
and chemical composition of the gas within a star; opacity decreases as temperature increases and
increases as density increases
- when opacity is low, radiation flows easily out of a star; when opacity is very high, convection
replaces radiation as the mechanism that carries energy out of a star
D. Equation of State
- relates the pressure of the gas to its temperature and density; depends upon the chemical
composition of the gas
- as nuclear reactions change the chemical makeup of a star, they also change the amount of
pressure produced by a given temperature and density
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1. Normal Gases
- obey the Ideal Gas Law (pressure is proportional to the product of temperature and density);
internal pressure of a star will decrease if its temperature or density decreases and vice versa; as
long as nuclear energy sources last a star remains the same size
2. Degeneracy
- at high densities particles in a gas are packed so tightly that they strongly resist further
compression; this Degenerate Gas can cool without losing its pressure
- therefore most stars end up as small, dense spheres of degenerate gas (White Dwarfs and
Neutron Stars), slowly cooling and radiating their remaining heat into space
E. Vogt-Russell Theorem
- states that the entire evolution of a star, unless it has a close binary companion, is determined by
its initial mass and chemical composition
F. Evolutionary Track
- the path through an H-R diagram that a star follows as it evolves
- by comparing calculated evolutionary tracks for stars of different masses to H-R diagrams of
clusters of stars which have the same age and chemical composition but different masses,
astronomers have been able to check the validity of the calculated evolutionary tracks
1. Main Sequence Stars
- period of time in which the star is consuming hydrogen in its core; most visible stars are on the
main sequence
a. Variety of Main Sequence Stars
- although all main sequence stars generate energy by the fusion of hydrogen into helium in their
cores, they differ from one another in mass, size, temperature, luminosity, and internal structure
- massive main sequence stars are larger, hotter, and more luminous than the Sun, whereas lowmass main sequence stars are smaller, cooler and dimmer than the Sun
b. Main Sequence Lifetime
- length of time a star spends consuming hydrogen in its core
- the temperature and luminosity of a star change relatively little while it is on the main sequence
- the main sequence lifetime of a star is proportional to its mass and inversely proportional to its
luminosity (however, the luminosity increases as mass increases, so the most massive main
sequence stars have the briefest lives)
2. After the Main Sequence
a. Low to Intermediate-Mass Stars
- stars of about 0.5 to 10 stellar masses
a1. Red Giant Stars
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- stars between about 0.5 and 10 stellar masses expand to become red giants
- production of energy by fusion at the center of a star ceases when hydrogen fuses to helium,
gravitational contraction increases, and the region around the hydrogen-depleted core becomes
hot enough to ignite the fusion of hydrogen into helium in a thin shell; the helium "ash" then
settles into the core of the star (making it denser and more massive) and the surface of the star
expands and cools (it becomes a Red Giant)
- eventually the core becomes hot enough for helium to become a fuel; this happens explosively
during the Helium Flash for Sun-like stars
a2. Core Helium Burning
- after core helium burning begins, stars like the Sun move horizontally across the H-R diagram,
growing hotter at constant luminosity (Horizontal Branch Stars)
- stars more massive than the Sun contract and also move horizontally across the H-R diagram
and grow hotter at roughly constant luminosity, but also become unstable and begin to pulsate,
forming Yellow Giants (the surface of the star moves in and out, with changes in temperature and
luminosity); these pulsations are due to variations in the rate at which energy can escape from the
star
- pulsating stars are found in a band along the H-R diagram termed the Instability Strip; this strip
contains Cepheid variables and the RR Lyrae stars
Cepheid Variables – “classical cepheids” are about 4 to 20 stellar masses; they consist of yellow
pulsating stars that vary in brightness as they expand and contract; the period of a Cepheid is
related to its luminosity (luminosity increases as pulsation increases); this period-luminosity
relationship provides a way to find distances to Cepheid variables and star clusters or galaxies
that contain Cepheids (therefore Cepheids are “standard candles” that may be used to find
distances to remote galaxies)
RR Lyrae Stars - dimmest, hottest stars in the instability strip; they are about one solar mass and
pulsate as they pass through the instability strip
a3. Asymptotic Giant Branch (AGB)
- portion of the H-R diagram occupied by enormous, cool stars with helium-burning shells
- when the supply of helium in the core of a star is exhausted, the shell of helium surrounding the
core begins to be consumed; a star with a helium-burning shell once agains swells and cools,
becoming an asymptotic giant branch star; after a Thermal Pulse, in which the helium is
consumed, luminosity increases for about a century; then it resumes its former appearance until
enough helium is built up to create another pulse; at the same time the pulse occurs the star
develops a cool wind that carries its outer layers into space; dust that forms in the wind obscures
the star and converts its light to infrared radiation; most stars end this phase of their lives when
they have almost completely shed their outer layers
a4. Planetary Nebulae
- at the end of its AGB phase, a star quickly becomes hotter; when its surface temperature
reaches 30,000 K, it ionizes the layer that it had earlier shed as a cool wind; the shell of ionized
Astronomy Notes, Page 69
matter glows as a planetary nebula
b. Massive Stars
- stars with more than 10 stellar masses are hotter, bluer and more luminous than our Sun, with
lifetimes typically less than 100 million years old
- although hydrogen is fused to helium in both low- and high-mass stars, the primary way in
which high-mass stars produce helium is through the CNO Cycle (in which four protons fuse to
form helium, utilizing carbon-nitrogen-oxygen as catalysts)
- the early lifetime of massive stars is otherwise similar to low- and medium-mass stars, but high
mass stars pass through a pulsating Yellow Supergiant stage before they evolve into Red
Supergiants
- during the Red Supergiant phase the helium in the core contracts and heats up, causing the
“shell hydrogen” to “burn”, resulting in a massive expansion of the star’s surface
- once the helium in the core reaches about 170 million Kelvin, the helium is “ignited” (as a
“Helium Flash”) and carbon and oxygen begin building in the core (at this stage a “Blue
Supergiant” may form)
- the Carbon-Oxygen Core heats up, hydrogen and helium burns in the shell, and another Red
Supergiant forms
- the C-O core collapses and heats to about 600 million Kelvins, which ignites Carbon Burning in
the core
- after carbon is ignited in the core, the stars inner composition changes so fast that the outer
layers don’t have time to respond; core burning then involves a series of reactions that
sequentially ignite neon, oxygen and silicon (this results in an onion-like layering of elements in
the star’s core)
- elements as massive as iron can be made in energy-producing reactions in stars; once the
innermost core of iron forms no other fusion reactions occur (the final core collapse of massive
stars results in a Supernova)
XVI. Star Death
A. Compact Object
- star collapses to form a white dwarf, neutron star or black hole
B. White Dwarf
- Earth-size star supported against gravity by the degenerate pressure of its electrons
1. Formation of White Dwarfs
- white dwarfs evolve directly from the central stars of planetary nebulae (the former cores of
AGB stars); the AGB stars have lost most of their outer layers due to cool winds [the greatest
mass a white dwarf can have is about 1.4 solar masses (the Chandrasekhar Limit)]
- white dwarfs are estimated to be the final evolutionary stage of about 97% of the stars in our
galaxy
- electrons resist being pushed further together and stops shrinking (Electron Degeneracy); the
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more massive the white dwarf star, the smaller its size; white dwarfs become tremendously dense
(a sugar cube-sized piece of white dwarf on Earth would weigh about 2 tons!)
- once a star becomes a white dwarf its size remains constant
- as the white dwarf cools it grows dimmer and evolves down and to the right in the H-R
diagram; after billions of years it is thousands of times less luminous than the Sun and is hard to
detect
2. Close Binaries and the Evolution of White Dwarfs
- gravitational interaction of close binaries greatly influence their evolution
a. The Algol Paradox
- in Algol, and many other binary systems the currently less massive star (ex., β Persei) seems to
have evolved more quickly to become a giant before its more massive companion (ex., α Persei)
b. Equipotentials
- connect places that have the same potential energy (they are analogous to contour lines on a
topographic map)
- equipotentials indicate the directions in which matter can flow in a binary system and the
shapes of the stars in each binary
- a very important equipotential marks the surface of the Roche Lobe of each star; the Roche
Lobe of a star is the region in which its gravity dominates (matter outside either star's Roche lobe
belongs to the binary system rather than either star alone)
c. Binaries with Compact Objects
- binary systems that contain compact objects (white dwarfs, neutron stars, black holes) produce
some of the most energetic stellar phenomena; these phenomena are often caused or triggered by
transfer of mass from a star to its compact companion
- gas falling toward a compact object releases gravitational energy that makes the gas hot enough
to emit X-rays and ultraviolet radiation; the angular momentum of the infalling matter often
causes the matter to go into orbit about the compact object, forming an accretion disk; friction in
the accretion disk causes gas to spiral inward, releasing energy that heats the disk
- binary systems in which a normal star sheds its mass onto a white dwarf companion undergo
large, temporary increases in brightness
c1. Novas
- temporary brightenings of binary systems in which a normal star transfers mass to its white
dwarf companion
- the white dwarf accumulates an outer layer of hydrogen-rich gas that burns explosively in a
thermonuclear runaway in the surface layers of the white dwarf; the nuclear energy that is
released blasts the outer layer of the white dwarf into space; the same white dwarf may produce a
nova explosion each time enough new matter has accumulated
c2. Type Ia Supernovas
- supernovas are explosions in which a star's brightness temporarily increases by as much as one
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billion times; they are divided into Type I and Type II supernovas, based on the types of emission
lines in their spectra (Type I Supernovas do not show hydrogen lines)
- if the rate of mass transfer is high, the outer layers of a white dwarf in a binary system are so
hot that they do not become degenerate; fusion of hydrogen to helium in the accreted gas goes on
relatively smoothly
- eventually the mass of the white dwarf increases so that its core becomes dense and hot enough
for the almost instantaneous burning of carbon and other elements to take place; the energy
released by the fusion reactions disrupts the white dwarf in a Type Ia Supernova
C. Type II Supernova
- extremely energetic explosion due to the collapse of the core of a massive star (more than about
10 stellar masses), probably producing a neutron star or black hole
- Oxygen, neon and magnesium "burn" to form iron; iron absorbs energy, heats up, breaks down,
and the star's core collapses
- as the collapse of the massive star proceeds electrons and protons combine to form neutrons,
producing a neutron star in the core of the star
- infalling material rebounds from the neutron star and is driven outward, creating a shock wave
that expands the star and greatly increases its brightness; the expanding gas moves at speeds up
to 30,000 km/sec
- the shell of gas ejected by the supernova sweeps up and heats interstellar gas around the
supernova, producing a luminous supernova remnant; energetic electrons trapped in the magnetic
field of the supernova remnant emits synchrotron radiation that can be detected with radio
telescopes; the supernova remnant fades into the interstellar gas after tens or hundreds of
thousands of years
D. Neutron Stars
- stars with about 10 to 15 kilometers radius; neutron stars are composed primarily of neutrons
and are supported by the degenerate pressure of the neutrons
1. Formation of Neutron Stars
a. Sequence of Events
- supernova explosion leaves behind mass of approximately 1.4 to 2 or 3 stellar masses
- electrons collapse (combine with protons to form more neutrons) and neutrons compact
(neutron degeneracy)
- forms a neutron star
b. Characteristics
- extremely dense (a sugar-cubed size lump of neutron star on Earth would weigh about 100
million tons!) and with a strong magnetic field, high temperature (106K) and may have rapid
rotation
X-ray Bursters - accretion of gas onto neutron stars can produce sporadic bursts of X-rays (every
few hours) when the neutron star's surface is heated by runaway fusion in the accreted gas
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2. Pulsars
- stars that emit very regularly-spaced bursts of radio radiation (most with periods of tenths of
seconds or seconds, but may have periods as much as 1000 times per second)
a. Formation
- probably caused by rapidly-rotating neutron stars (this is termed the "lighthouse model")
- radio radiation from pulsars is probably due to energetic electrons near the magnetic poles,
which are tipped with respect to the rotational axis
b. Characteristics
- pulsars lose rotational energy and slow down with time
- after tens of millions of years the rotation period reaches several seconds and the star stops
emitting radio pulses
Binary Pulsars = the companion is often a giant star; the giant star loses mass to the neutron star
and liberates tremendous energy (high temperature, with gamma- and x-rays)
- before the companions became compact objects they shed matter to the pulsars, increasing their
rate of spin so that many of them have rotation periods of only a few milliseconds (these are
termed Millisecond Pulsars)
X-ray Pulsars - pulses of X-rays (from less than a second to 15 minutes) are seen when an X-ray
emitting magnetic pole of the neutron star sweeps past
E. Black Holes
- a mass that has collapsed to such a small volume that its gravity prevents the escape of all
radiation or the volume of space from which radiation does not escape
1. Spacetime
- spacetime is the three spatial coordinates and one time coordinate that are used to locate events
in space and time; spacetime may be curved or flat
- a geodesic (the shortest distance between two points) is a straight line in flat spacetime but is a
curved path in curved spacetime; geometric quantities (such as the sum of angles of a triangle)
are different for flat and curved spacetime
- black holes are regions of spacetime from which nothing can emerge
2. Formation of Black Holes
- form from "heavyweight" stars that retain cores of more than 2-3 solar masses after supernova
explosions
- the core keeps collapsing (degenerate neutrons cannot support weight) and gets more dense
- the object shrinks to zero radius and density and gravity becomes infinite
- a Singularity is the theoretical point of zero volume and infinite density to which any mass that
becomes a black hole must collapse (according to Einstein's Theory of Relativity)
Astronomy Notes, Page 73
3. Characteristics of Black Holes
a. Event Horizon
- the boundary of a black hole from which no radiation can escape (i.e. no event occurring within
the event horizon is visible to a distant observer)
- the size of the event horizon equals the "Schwarzschild radius" and is given by 3 kilometers
times the mass of the black hole in solar masses
b. "Jumping In" a black hole
According to the Theory of General Relativity clocks slow down in curved space-time (for the
observer, but not to the person “jumping in”; this is termed Time Dilation); there is also a
Gravitational Redshift (observers need to look at longer and longer wavelengths to see you
“jumping into” a black hole)
Tidal Forces - tear you apart and heat you up to several million degrees or so
c. Looking for Black Holes
- black holes can't be seen directly; all we can know about a black hole is its mass, its angular
momentum, and its electrical charge
- to locate black holes we must look for x-ray binary systems (accretions discs from companion
stars that are emitting strong x-rays as they lose matter into the black hole)
XVII. The Evolution of our Galaxy
- galaxies are massive systems bound by gravity that consist of stars, stellar remants, an
interstellar medium of gas and dust, and dark matter
Galactic Astronomy – the study of the Milky Way Galaxy and its components
A. Interstellar Matter
- makes up about 20% of the Galaxies' observable mass
Nebulae - clouds of gas and dust seen in visible light; constitute important parts of the Milky
Way
1. Interstellar Gas
- about 99% of the matter in interstellar space is gaseous and about 1% is dust
- hydrogen forms about 89% of the gases, helium 9%, with 2% “metals” (elements heavier than
hydrogen and helium)
Emission nebula - gas glows in the visible part of the spectrum
Astronomy Notes, Page 74
a. HII Regions
- bright nebulae that surround hot stars; hydrogen gas in an HII Region is ionized
- gas in HII Regions are kept warm and ionized by ultraviolet radiation from the hot star
- when a hot star forms within a molecular cloud, its HII region expands and eventually breaks
out of the molecular cloud; when the hot star grows cooler and leaves the main sequence, its
production of ultraviolet radiation declines and the HII region fades out
b. Diffuse Interstellar Gas
- atomic and molecular absorption and emission lines reveal that there is interstellar gas
everywhere in the galaxy
- intercloud interstellar gas has a temperature of about 10,000 K, except where it has been heated
to 1 million K by shock waves from supernovas; when many supernovas occur in a cluster of
stars, a superbubble of hot, ionized gas is produced
2. Interstellar Dust
- consist of small bits of solid material, mostly less than a millionth of a meter in size
- most interstellar dust particles form in the atmospheres of cool giant and supergiant stars; the
core of the particle is made of silicate or carbon soot, around which a mantle of water, ammonia,
methane, and other icy material slowly grows
a. Dark (Absorption) Nebula
- dense interstellar cloud containing enough dust and gas to block out the light of background
stars (dust appears as a dark spot against the emission nebula)
- the dimming of background stars gives the appearance of a region with no stars
b. Reflection Nebulae
- a cloud of interstellar gas and dust that is luminous because the dust it contains reflects the light
of a nearby star; often appear blue
c. Diffuse Interstellar Dust
- dims and reddens the light from distant stars
- dimming is due to scattering of light and absorption
- scattered light appears as a general glow in the galaxy
- absorbed starlight heats interstellar particles, which then emit infrared radiation (this infrared
radiation can be seen in nearly all directions)
B. Milky Way Subdivisions
- the modern view of the Milky Way emerged from the study of Globular Clusters (a tightlypacked, spherically-shaped group of thousands to millions of old stars), which are distributed
around the center of the galaxy
- the Milky Way is estimated to contain about 100 to 400 billion stars
1. Nuclear/Galactic Bulge
- located in the center of the galaxy; it is obscured by interstellar dust, but can be mapped in the
Astronomy Notes, Page 75
infrared and radio parts of the spectrum
- radius approximately 6 kiloparsecs (20,000 light years) and about 4 kiloparsecs (12,000 light
years) perpendicular to the galactic plane
- consists of densely packed old stars (Population II) and young stars (Population I)
- there is a Galactic Nucleus in the center; approximately 4 parsecs across; this region produces a
tremendous amount of energy
- the region in the center has a great concentration of stars, a rotating ring of gas extending from
2 to 8 pc from the center, and streamers of gas that may be falling into a central black hole of
several million solar masses (this intense radio source has been termed Sagittarius A*)
2. Galactic Disc
- the thin, phonograph record-shaped part outside the Nuclear Bulge
- the Galactic Disc is estimated to be about 70,000 to 100,000 light years (20-30 kiloparsecs)
across and about 1,000 light years (300 parsecs) thick
- composed of Population II “old” stars and Population I “young” stars, as well as gas and dust
- the Galactic Disc is warped at the ends, probably due to the gravitational influence of local
galaxies (the Small and Large Magellanic Clouds)
- the disc probably has spiral arms
- outside the Stellar Disc there is believed to be a thick disc of gas (probably about 12,000 light
years thick)
3. Galactic Halo
- the flattened spheroid surrounding the stellar disc; the diameter of the Galactic Halo is believed
to be about 200,000 light years (60 kiloparsecs)
- made of scattered older stars (Globular Clusters), which contain only “old” Population II stars
4. Dark Halo
- a hypothetical region approximately 60-100 kpc (200,000 to 330,000 light years) across
- made of non-luminous material which constitutes approximately 80-95% of the mass of the
galaxy (this is termed “Dark Matter”)
C. Spiral Structure of the Milky Way
- the Milky Way is believed to be a Barred Spiral Galaxy
1. Evidence
- the structure was determined by plotting positions of galactic clusters, HII Regions (ionized
hydrogen) and young stars (O and B series)
2. Formation of Spiral Arms
a. Doppler Shifts of non-rotating globular clusters and distant galaxies indicates the Sun
revolves around the Milky Way at approximately 250 km/sec (1 galactic year equals 250 million
years)
- our Solar System is situated about two-thirds from the center of the Milky Way, on the inner
Astronomy Notes, Page 76
edge of the Orion-Cygnus Arm
- stars further out have longer galactic years (= Differential Rotation)
b. Spiral-arm pattern is caused by (?)
b1. Density Wave Model
- the spiral arms are caused by compressional waves that move through the stars and gas in the
galaxy (it is believed that the arms represent regions of greater density that move more slowly
than the galaxy’s stars and gas)
- the density waves compress the gas and dust; they heat up and form protostars
b2. Supernova-Chain-Reaction Model
- the spiral arms were created when high-mass stars become supernovae, their expanding shells
formed more stars, more supernovae, etc., etc.
- the stars spread out into spiral arms due to the differential rotation of the galaxy
D. Origin of the Milky Way
- the Milky Way is believed to have formed at least 13.2 billion years ago
1. Types of Stars
a. Population II Stars
- contain 0.1% "metal" (atoms heavier than helium)
- they are located in the halo (in globular clusters) or in the nuclear bulge
- Population II stars are old stars with random, elliptical orbits
b. Population I Stars
- contain 2-3% atoms heavier than helium ("Metal-rich"); located in the disc; with circular orbits
in the plane of the galaxy
- Population I stars have formed within the last few billion years (Ex. = Sun)
- "metals" came from the explosion of old supernovae
2. Galaxy Formation and Evolution
- the Milky Way began as a spherical structure over 13 billion years ago with contraction of
hydrogen and helium gas
- collisions between gas clouds flattened the galaxy (the disc is believed to have formed about 7
to 10 billion years ago); nuclear reactions in stars increased the heavy-element content
- it is believed that there is a small galaxy (the Sagittarius Dwarf Elliptical Galaxy) colliding with
the Milky Way Galaxy; this small galaxy is now being torn apart by the gravity of the Milky Way
XVIII. Galactic and Extragalactic Astronomy
Astronomy Notes, Page 77
- it is estimated that there are more than 170 billion galaxies in the observable Universe
Extragalactic Astronomy – the study of astronomical objects external to the Milky Way Galaxy
A. Types of Galaxies
Hubble Classification System - formulated by Edwin Hubble; divides galaxies into different
classes (Hubble Types) according to their shapes
- Hubble’s "tuning-fork diagram" shows transitional types from elliptical galaxies to normal
spirals and barred spirals
- but galaxies probably do not evolve from one to another (the different shapes and gas content
result from different conditions at the time of formation or the shapes may have been affected by
"tidal forces" between galaxies)
1. Elliptical Galaxies
- smooth-looking spheroidal galaxies; the most common galaxy type
- essentially no gas/dust between the stars and contain few if any young stars
a. Size Classification
Giant Ellipticals – contain about 10 trillion solar masses and are 100,000 parsecs across; they are
rare; it is believed that giant elliptical were formed when similar-sized galaxies collided and
merged
Dwarf Ellipticals - contain a few million solar masses and average about 2,000 parsecs across;
their characteristics are intermediate between globular clusters and regular elliptical galaxies;
dwarf ellipticals may have formed by coalescing primordial globs of gas, etc. or could have been
formed by the gravitational disruption of spiral galaxies
b. Shape Classification
- range from Type EO (circular) to Type E7 (very elongate)
2. Spiral Galaxies
- with spiral arms extending from the nucleus or from a bar that crosses the nucleus
- with a lot of gas/dust (interstellar medium) between the stars
- arms unwind, the nuclear bulge is less prominent, and an obscuring "dust lane" in the disc is
more prominent from Type Sa to Sc
- Barred Galaxies (SBa, SBc, SBc) unwind from a Bar of stars, gas and dust on both sides of the
nucleus; about two-thirds of all spiral galaxies are Barred
3. Irregular Galaxies
- show no regularity; classified as Irr
- Irregular Galaxies have abundant gas and dust between the stars
- Irregular Galaxies were probably once Elliptical or Spiral galaxies that were deformed by
Astronomy Notes, Page 78
interactions with other galaxies [the Magellanic Clouds are typically classified as Irregular
Galaxies but have been found to contain barred spiral structures that probably represent remains
of their disrupted ancestors; they are sometimes classified as Barred Spiral (SBm) Galaxies]
4. Peculiar Galaxies
a. Starburst Galaxies
- gas-rich galaxies that produce stars at exceptional rates
- Starburst Galaxies seem to have been common in the early history of the Universe, and also
probably form due to galactic collisions
- intense starburst formation probably only lasts about 10 million years, producing numerous
massive stars
- about 15% of current star formation seems to be associated with starbursts
b. Active Galaxies
- emit huge amounts of radio energy; also infrared, ultraviolet and X-rays
- this energy is produced in the galaxy cores (AGN = active galactic nuclei)
- active galaxies are believed to form as accretion discs around Supermassive Black Holes
(SMBH); the radiation produced by the AGN is probably due to gravitational energy as material
falls toward the SMBH
- it is believed that most galaxies may have initially gone through an Active Stage; these may no
longer be active because the Supermassive Black Holes consumed most of the surrounding
matter
- the following Active Galaxy types may actually refer to very similar (or the same) objects; their
supposed differences may result from the rate at which matter is supplied to the black hole and/or
our “viewing angle” to these objects
b1. Double-lobed Radio Sources associated with Active Galactic Nuclei (DRAGNs)
- radio lobes on either side of highly energetic galaxies
- probably made by sporadic hot gas jets erupting from galaxies
b2. Seyfert Galaxies
- galaxies with unusually bright, tiny cores that fluctuate in brightness (with high-velocity ionized
gas); almost all Seyfert Galaxies are Spiral Galaxies
- Seyfert Galaxies are probably due to unstable galaxies "throwing" matter into a massive black
hole and gas is erupting through a hole in the accretion disc
b3. Quasars
- quasi-stellar objects (QSOs)
- very powerful, very distant (with large redshifts) Active Galaxies
- the great distances of quasars (they are 600 million to 28 billion light years away) mean that we
are seeing them as they were in the remote past, when the universe was only a fraction of its
present age (so by observing quasars we can sample conditions in the early universe); studies
indicate that quasars have evolved and the universe has changed with time
Astronomy Notes, Page 79
- quasars are 10 to 1,000 times more luminous than large galaxies (with synchrotron radiation
and thermal radiation from gas and dust) but probably the energy is produced in regions of space
no larger than the size of our Solar System!
- quasars may have been produced by interacting galaxies throwing mass down a massive black
hole and ejecting a jet moving nearly at the speed of light
b3. Blazars (Blazing Quasi-Stellar Objects)
- a type of active galaxy named for BL Lacertae, the first of the type discovered
- small, bright quasar-like galaxies (but show emission lines) which show rapid, unpredictable
variations in brightness
- in Blazars it is believed that the jet is pointed toward the Earth
C. Distances to Galaxies
- has been determined in a step-wise fashion in which relatively nearby objects are compared
with the same kind of objects in more distant galaxies (distances to the farthest known galaxies
are determined by their “redshifts”)
- distances to galaxies as far away as 13 billion light years have been determined through these
methods
D. Groups and Clusters of Galaxies
- these are aggregates of galaxies held together by their mutual gravitational attraction
1. Groups of Galaxies
- the smallest aggregate of galaxies, typically including less than 50 galaxies held together by
their mutual gravity
- The Local Group includes approximately 50 galaxies nearest to, and including, the Milky Way;
the Milky Way, Andromeda Galaxy and Triangulum Galaxy are the largest members of the Local
Group (it also includes the Small and Large Magellanic Clouds); the Local Group is
approximately 10 million light years [3 megaparsecs (Mpc)] across
2. Clusters of Galaxies
- aggregates of galaxies that are larger than Groups (but there is no sharp division between
“Groups” and “Clusters”)
a. Classification
- clusters of galaxies are classified by the number of bright galaxies they contain and by the
regularity of their appearance
Classification Categories Include:
- rich clusters (many galaxies) versus poor clusters
- regular cluster (with a concentration of galaxies in the center and a spherical structure) versus
irregular cluster
- there are also clusters of clusters (Superclusters); the Milky Way belongs to the Local
Supercluster (also termed the Virgo Supercluster), which contains at least 100 galaxy clusters
Astronomy Notes, Page 80
b. Other Clusters
- beyond the Local Group other clusters of galaxies are spread through space with typical
separations of tens of megaparsecs
- the Virgo Cluster, at a distance of about 16.5 Mpc (54 million light years) has about 1300 to
2000 galaxies, and the Coma Cluster, at 99 Mpc (321 million light years) has about 1000
galaxies; these are examples of clusters of galaxies that are much larger and richer than the Local
Group
- strong x-ray radiation from many clusters indicate that those clusters are filled with hot, lowdensity gas; most of the hot gas must have been present as intergalactic gas since the time that the
cluster formed
c. Arrangement of Clusters and Superclusters
- clusters and superclusters are arranged in patterns of sheets that surround voids, within which
few galaxies can be found
- the pattern of motions of galaxies in our part of the universe suggests that there is a large
concentration of mass (called the Great Attractor) which pulls us in the direction of the
constellation Centaurus
d. Dark Matter in Clusters
- the velocities of galaxies in clusters is very large (and therefore they shouldn’t stay together);
this indicates that clusters must have much more dark matter than luminous matter (in a typical
galaxy there is probably about 85% dark matter, 10% hot x-ray emitting gases, and only about
5% galactic material)
- the masses of clusters of galaxies act as gravitational lenses that distort the images of
background galaxies; the degree of distortion is used to study dark matter in clusters of galaxies,
which supports the conclusion that most of the mass in clusters is invisible
5. Formation of Clusters of Galaxies
a. Theories of Formation
- currently there are three major theories on the origin of clusters:
Hierarchical Clustering Model - galaxies formed first and then collected to form clusters; seems
to be the best model
Pancake Model – a huge ellipsoid of gas collapses and fragments, forming a pancake-shaped
structure; the collapse would form shock waves, increasing the temperatures in the fragments to
form the individual galaxies
Turbulence/Primoridal Vorticity Model - galaxies and clusters formed in high density regions
produced by shocks/turbulence in the early universe; these individual “galactic eddies” would
each form a galaxy
Astronomy Notes, Page 81
b. Galaxy Evolution in Clusters
- rich clusters are dominated by elliptical and S0 galaxies, whereas most single galaxies and
those in small clusters are spirals (S0 galaxies have flattened discs, like spirals, but have no spiral
arms)
- this difference probably arose partly due to the different densities of the regions in which rich
clusters and other galaxies formed, and partly from collisions that produced giant ellipticals and
SO galaxies, destroying spirals in clusters
XIX. Cosmology
Cosmology – the study of the universe
A. Historic Assumptions about Cosmology
1. The Cosmological Principle
- viewed on a large enough scale, the Universe would “look the same” no matter where you are
(this does not necessarily mean the Universe would have exactly the same physical structures
everywhere, but the same physical laws would apply)
a. Homogeneity
- matter is uniformly spread throughout the Universe (this means that we believe that the portion
of the Universe that we can see is a good sample of the Universe in general)
b. Isotropy
- the Universe looks the same in every direction (the same physical laws apply everywhere in the
Universe)
2. The Universe has no edge (and therefore no center)
B. The Universe is Expanding
How do we know?
1. Olbers' Paradox
- if the Universe were infinite, static and filled with stars, then every line of sight should end in a
star (and the sky should be bright)
- So why is the sky dark at night?
a. Galaxies are receding at high velocities and light is Doppler shifted to longer wavelengths
b. the Universe is not infinitely old and light from distant stars has not reached us
Astronomy Notes, Page 82
- Olbers’ Paradox indicates that the Universe is non-static
2. Cosmological Observation
a. Relativity
- Albert Einstein (German; 1879-1955) realized that Newton's assumption that the universe had
the same geometric properties as a flat surface was false
a1. Special Theory of Relativity
- Einstein's theory describing the relations between measurements of physical phenomena as
viewed by observers who are in relative motion at constant velocities
- Einstein showed that mass and energy are different forms of the same thing and can be
exchanged back and forth according to the formula E = mc2, where E is energy, m is mass, and c
is the speed of light
- the special theory showed that all events in the universe involve space and time together
(Spacetime), which has four dimensions (three of space, one of time); when anything happens in
the universe (an Event) it takes place in both space and time, and when you look deeper into
space you are also looking deeper into time
a2. General Theory of Relativity
- Einstein's theory of gravity
- mass and energy determine the geometry of spacetime and any curvature of spacetime shows
itself by what we commonly call gravitational forces
- Inertial Mass is determined by subjecting an object to a known force (not gravity) and
measuring the acceleration that results; Gravitational Mass is the mass of an object as determined
by the gravitational force it exerts on another object
- Einstein's Principle of Equivalence is the fundamental idea in Einstein's General Theory of
Relativity; you cannot distinguish between gravitational accelerations and accelerations of other
kinds (inertial mass and gravitational mass are equal); therefore, gravitational forces can be made
to vanish in a small region of spacetime by choosing an appropriate accelerated frame of
reference (when you are Weightless, you are following your natural motion in spacetime)
a3. The Geometry of Spacetime
a3a. Euclidean Geometry
- Euclidean Geometry is Flat Geometry
- in flat spacetime a geodesic (the shortest distance between two points) is a straight line; parallel
lines remain the same distance apart, the sum of the angles of a triangle is 180°, etc.
a3b. Non-Euclidean Geometry
Hyperbolic Geometry - more than one parallel line can be drawn through a point near a straight
line; parallel lines diverge in hyperbolic geometry; the sum of the angles of a triangle drawn on a
hyperbolic surface is always less than 180°; hyperbolic geometry has a negative curvature
Astronomy Notes, Page 83
because it bends away from itself; it extends infinitely far; in both hyperbolic and flat geometries
extended parallel lines never meet (therefore they are termed Open Geometries)
Spherical (Closed) Geometry - the sum of the angles of a triangle drawn on a spherical surface is
always greater than 180°; a geodesic (the shortest distance between two points) is curved, and
parallel lines converge, and therefore spherical geometry curves in on itself; it is finite but
unbounded (it has a definite size, but no edge)
a4. The Curvature of Spacetime
- according to Einstein's General Theory of Relativity, the distribution of mass (and energy)
determines the geometry of spacetime; all objects produce a curvature of nearby spacetime and
that curvature shows itself by accelerated motion
- Einstein's explanation of gravity is that matter (mass and energy) causes curvature of spacetime;
according to the principle of equivalence, light and freely moving bodies move along geodesics
in curved spacetime (however, we fail to perceive the curvature of spacetime so we invent a
nonexistent force, gravity, to account for the curved motion that we see)
- the concept of gravity as the curvature of spacetime has been tested and found to predict such
effects as the bending of starlight near the Sun, the orbit of Mercury, etc.
b. Geometry of the Universe
b1. Cosmic Geometry
- three geometries of the universe are possible (Flat, Hyperbolic, or Spherical)
- the density of mass (and energy) determines the curvature of space; if the density of mass of the
universe is equal to or greater than the Critical Density [approximately (1 to 3) X 10-26kg/m3]
then the universe curves back on itself and is closed
b2. The Expanding Universe
- astronomers found that galaxies are moving apart
- astronomical observations can be used to determine the distances to galaxies and their relative
velocity to us (Radial Velocity is the component of relative velocity that lies along our line of
sight; for all distant galaxies the radial velocities are recessional, i. e. they are moving away from
us)
b2a. Hubble's Law (Law of Redshifts)
- formulated by Edwin P. Hubble (1889-1953)
- Hubble's Law describes the expansion of the universe; more distant galaxies have greater
spectral redshifts; therefore, the more distant a galaxy lies from us, the faster it is moving away,
or receding from us; the law is defined by v = Hd, where v is expansion velocity, d is distance of
a galaxy, and H is the Hubble Constant [the Hubble Constant is the proportionality constant that
relates radial velocity and distance; the current value has been calculated at about 71 km/s/Mpc
(20 km/s/Mly; but this value is often disputed), but changes with time as the universe expands];
what the value says is that for every million light years we look away from our galaxy, the radial
velocity of other galaxies is 20 km/s higher
Astronomy Notes, Page 84
b2b. Significance of Hubble's Law
- Hubble's law implies that there was a beginning to the universe; a unique moment when the
universe was compressed in a dense state
- there is no "center of the universe"; the expansion is uniform, so if we lived in another galaxy
the expansion would look the same
- the universe does not expand into empty space; space itself is expanding
- by determining the average density of the universe, it would be possible to calculate the
curvature of space
- hyperbolic, flat and spherical cosmic geometries correspond to cases of greater than, equal to
and less than escape speed
- if the observed cosmological density (including both mass and energy) is greater than the
critical density, the universe is spherical and closed; if the cosmological density is less, the
universe is hyperbolic and open; if they are exactly equal the universe is flat (therefore whether
the universe is open or closed depends upon the density of matter and energy in it)
- in a closed (spherical) universe the rate of expansion slows, stops and then the universe
contracts (this final collapse is sometimes called the Big Crunch); in an open (hyperbolic)
universe the rate of expansion doesn't decrease as rapidly and expansion never stops; in a flat
universe the expansion slows down just enough to permit the universe to come to a stop after
infinite time of infinite expansion
C. The Big Bang
- the theory that there was an explosive expansion of the universe from a dense state
- Hubble Time is the estimate of how long ago the Big Bang took place, and is calculated
assuming that expansion has always gone on at the same rate
- the Big Bang did not occur at a specific place (it filled the entire universe from the first
moment)
1. Curvature of Space
a. Three-Dimensional Models of the Universe
- the three possibilities for curvature of space in the universe are negative, flat and positive
- if the universe has positive curvature it is finite in volume (otherwise its volume is infinite); for
all three curvatures the universe has no boundary and no center
b. Determining Curvature
- geometrical tests, such as determining the volumes of large spheres, can in principle be used to
determine the curvature in space; in practice such tests have proven unworkable
- the curvature of space is closely related to the density of the universe; by determining the
average density of the universe it would be possible to determine the curvature of space
- if the average density of the universe is large, the universe has positive spatial curvature; if the
density is small the universe has negative curvature; flat spatial curvature occurs if the universe
has exactly the critical density (about 1 to 3 times 10-26 kg/m3; about 10 hydrogen atoms per
cubic meter)
Astronomy Notes, Page 85
2. Future of Expansion
- whether the expansion of the universe will continue forever depends upon the curvature of
space (and therefore the density of the universe); only if the density of the universe is low and
space is negatively curved or flat will expansion continue forever; if density is high, contraction
will eventually take place (however, see the discussion on “Dark Energy”)
- the accuracy with which the Hubble Time approximates the age of the universe also depends on
density; unless average density is low, the actual age of the universe will be significantly less
than the Hubble time
3. Age of the Universe
- how long ago the expansion of the universe began (at the Big Bang) is calculated in several
ways:
a. Hubble Time
- Hubble Time approaches the actual age of the universe only if the density of the universe is
much less than the critical density (therefore Hubble Time serves as an upper limit for
determining the age of the universe)
- Hubble's Constant (H; the rate at which recessions speeds of galaxies increase with distance) is
probably from 50 to 100 kilometers per second per megaparsec (km/s per Mpc); the latest data
from the Hubble Space Telescope indicate Hubble's Constant is about 71 km/s per Mpc
- using the formula for Hubble Time, t = d/v = d/Hd = 1/H (where t = time; d = distance; v =
speed of recession; and H = the Hubble Constant), many estimates have been 10 to 15 billion
years for the age of the universe (using the latest Hubble data a good estimate is 13.75 billion
years)
b. Dating the Universe by the Oldest Objects it Contains
- the oldest star in the Milky Way Galaxy has been dated at about 13.6 billion years old
- H-R diagrams of globular clusters (probably as old as the galaxy) contain main sequence stars
that require about 15 billion years to consume their central hydrogen and leave the main sequence
- it takes about 10 to 15 billion years for the radioactive elements in the solar system and nearby
stars to have decayed to their present abundance
4. Sequence of Events in the Big Bang
- most of the significant events in the Big Bang took place extremely rapidly (tremendous
changes occurred within the first second of expansion); the extreme densities and temperatures
that existed caused very rapid interactions among photons and particles of matter
- the time scale on which the universe was changing lengthened steadily as the expansion
proceeded
a. Time 0
- don't know
- the physics of matter and energy under these extreme conditions is unknown
- immediately thereafter high-energy photons (gamma rays with short wavelengths) filled the
Astronomy Notes, Page 86
universe and with temperatures as high as 1027K, then cooled to 1013K
- from about 10-37 until about 10-32 seconds Cosmic Inflation occurred, by which the Universe
expanded exponentially (see discussion below)
- after inflation ended, the Universe consisted of a Quark-Gluon Plasma (QGP, or “Quark Soup”)
and other elementary particles (such as leptons); Quarks and Gluons are the basic “building
blocks” of matter (the process by which these fundamental particles were formed is termed
“Symmetry Breaking”)
- the universe was so hot that there was enough energy to make matter and antimatter through
Pair Production (creation of matter from radiation); radiation converted to quarks and antiquarks
and vice versa; matter and antimatter destroyed each other through the process of Annihilation
(where a particle and antiparticle collide and disappear in a burst of radiation)
- at some point an unknown process occurred (Baryogenesis) in which there were slightly more
(one in 30 million!) quarks and leptons produced versus antiquarks and antileptons; this is the
reason why matter dominates in our Universe (rather than antimatter)
- at about 10-6 seconds quarks and gluons combined to form baryons (such as protons and
neutrons); the temperature was low enough such that new baryon/antibaryon pairs could not form
(so another Annhilation began between protons/antiprotons and neutrons/antineutrons, which left
only about one in 1010 of protons and neutrons left, and none of the antiparticles)
- at about one second the same process occurred which resulted in the formation of electrons and
positrons (at this stage energy levels had decreased enough such that the universe was dominated
by photons, with minor amounts of neutrinos)
b. After Several Minutes
- the Universe cools to about one billion degrees kelvin with neutrons combining with protons,
creating deuterium and helium nuclei (Big Bang Nucleosynthesis), but most protons remained
uncombined as hydrogen nuclei
c. The Radiation Era
- radiation dominated the universe during the first few minutes after expansion began and for a
long time afterward; the radiant energy would have a density much larger than that of matter
- the universe was still too hot (1 million K for the first few years) for atoms to be stable; as
temperature and density dropped the radiation gradually shifted from gamma rays and X rays
(high energy, short wavelengths) to ultraviolet and visible light (lower energy, longer
wavelengths)
d. The Clearing of the Universe
- a few hundred thousand years after expansion began the temperature had fallen to a few
thousand kelvins; matter began to dominate the universe and the universe suddenly became
transparent to radiation; all the radiation that existed before this time was destroyed
- most of the ions and electrons in the universe combined to form atoms (therefore this period is
often termed the Recombination Epoch or Decoupling Epoch); most of the radiation emitted
during and after recombination is still present in the universe
Cosmic Microwave Background Radiation (CMB Radiation or CBR) - light emitted at the time
Astronomy Notes, Page 87
when the universe became transparent; it is seen in all directions but comes from such a great
distance that it is redshifted to such an extent that it is seen as radio waves
e. Formation of Astronomical Structures
- over a long time period slightly-denser regions of the nearly uniformly-dense Universe began to
attract nearby matter, creating molecular clouds, stars, galaxies and other astronomical structures
f. Dark Energy
- data from Type Ia Supernova and the Cosmic Microwave Background Radiation indicates that
the Universe is dominated by Dark Energy, which consitutes about 74% of the total mass/energy
density in today’s Universe [other mass density sources in the Universe are Dark Matter (about
22%), Regular/Ordinary Matter (including about 3.6% intergalactic gas and 0.4% stars,etc.) and
minor mass from neutrinos]
- Dark Energy seems to be accelerating the Universe at an ever-increasing rate (“gravity” was
dominant in the early universe, but with increased expansion the effects of Dark Energy grew)
- we don’t know the composition of Dark Energy, and the mechanism of Dark Energy is
unknown
D. The Inflationary Universe Model
1. Problems with the Standard Big Bang Theory
a. The Horizon Problem
- the cosmic background radiation that comes to us from opposite parts of the universe indicates
that these regions have the same temperature and density (they are Isotropic); but if they were
widely separated when the universe was "cleared" how could this be true?
b. The Flatness Problem
- the universe is close to being flat (the dividing line between positive and negative curvature),
but it seems that the initial density of the universe could have been different from the critical
density (which means that the universe should have a high positive or negative curvature)
c. The Structure Problem
- the universe is uniform at large scales, but has a complicated and non-uniform structure on
smaller scale (clusters of galaxies, etc.); the structure developed due to irregularites in the
universe during the Decoupling Epoch
- according to the Big Bang theory, the density fluctuations must have been there all along (this
does not seem to be true)
2. How Inflation Solves the Problems with the Big Bang Theory
- the Inflationary Model proposes that at about 10-37 seconds after expansion began, the universe
experienced about 100 intervals of doubling in size until about 10-32 seconds; during this period
the universe grew by at least a factor of 1078
Astronomy Notes, Page 88
a. Solving the Horizon Problem
- before inflation began, the universe was still small enough for energy to flow from hotter places
to cooler places and make temperature throughout the universe uniform
b. Solving the Flatness Problem
- during inflation the radius of the curvature of the universe became so great that it appears flat to
us today
c. Solving the Structure Problem
- before inflation the portion of the universe that we can see today was so small that density
fluctuations appeared and disappeared randomly; during inflation these random fluctuations were
inflated to great sizes to become fluctuations in the Cosmic Microwave Background Radiation
and influenced the large-scale structure of the universe
3. Why Did Inflation Occur?
a. Forces in the Universe
Grand Unified Theories (GUTs) - state that there are not several independent forces in the
universe (these are different aspects of a single unified force); includes electroweak and strong
forces
Strong (Nuclear) Force - strongest; binds particles together into atomic nuclei; dominates in the
atomic nucleus
Electroweak Force - electromagnetic force (electomagnetic radiation in the form of photons) and
weak force (decay of elementary particles); dominates at the scale of atoms and molecules
Gravitational force - dominates at the cosmic scale
- if GUTs Theory is successfully coupled with an explanation for gravity, the potential resulting
theory has been termed the TOE (Theory of Everything)
b. Phase Change
- the most widely accepted explanation for inflation depends on a Phase Change in the early
universe at the time that the electromagnetic, weak and strong forces became distinguishable
from each other
- the universe became "supercooled" below the temperature of the phase change, such that
gravity became a repulsive rather than an attractive force; therefore, the rate of expansion
accelerated and the "inflation" took place
E. Fate of the Universe
- the fate of the universe is either expansion forever or expansion leading eventually to
contraction
Astronomy Notes, Page 89
1. Positive Curvature
- if space is positively curved (if the density of the universe exceeds critical density) the universe
will eventually reach a maximum size and then begin to contract
- the early stages of the Big Bang will run in reverse (the Big Crunch) until conditions exceed our
ability to predict what will happen
- a new cycle of expansion and contraction may occur (often termed the Oscillating Universe
Theory; but we cannot predict this)
2. Negative Curvature
- if there is a low-density universe the universe will continue to expand throughout eternity
- the matter in the universe will eventually decay and the radiation in the universe will redden
and dim forever; if dark energy is dominant it has been hypothesized that the universe will end in
the “Big Rip”
F. The Multiverse (Meta-Universe)
- the hypothetical concept that multiple types of Universes could exist beyond our Observable
Universe
1. Proposed Multiverse Theories
a. Level I Multiverse
- the concept that there are universes beyond our Cosmological Horizon (i. e., universes exist
beyond all galaxies, etc. that form our Universe)
- there may be an infinite number of Hubble Volumes (a Hubble Volume is the region of the
Universe where all objects are receding from the observer; the Hubble Volume defines the
volume of our Universe)
- all Hubble Volumes would have the same physical laws and constraints as our Universe
b. Level II Multiverse
- the Multiverse is expanding, and will forever; as it expands some regions stop stretching (and
form bubble-like structures like gas bubbles in a rising loaf of bread)
- different “Symmetry Breaking” may occur in different bubbles, leading to the formation of
fundamental particles that are different than those in our Universe (therefore those universes
would have different physical constraints versus our Universe)
c. Level III Multiverse
- in quantum mechanics (the mathematical description of the interaction of matter and energy),
certain observations cannot be predicted absolutely (there can be several “interpretations of
quantum physics”)
- what if each of these “interpretations” was a different universe? (this “Many World’s
Interpretation”/MWI suggests there is one universe, but several possibilities – like when you
throw a die in a dice game there are 6 possible results)
Astronomy Notes, Page 90
d. Membrane Theory (M-Theory or “Superstrings Theory”)
- is based upon the hypotheses of String Theory (String Theory proposed that electrons and
quarks in atoms consist of 1-dimensional “strings”; the way in which the strings oscillate
determine the properties of the particles)
- theorists propose the existence of “Membranes” with 11 dimensions; they believe that the Big
Bang occurred due to the collision of two extradimensional Membranes
- since the collision of Membranes would not be unique, there should be many Big Bangs and
many universes
2. Problems with Multiverse Theories
- in order to be a valid scientific theory, a hypothesis has to be testable
- is all this stuff really testable?
XX. Astrobiology and Exobiology
Astrobiology – the study of the origin, evolution, distribution and future of life in the Universe
Exobiology – the search for extraterrestrial life forms, and the study of the effects of
extraterrestrial environments on organisms
A. Organisms
- ordered (i.e. with cellular organization) living creatures
- "life" is a series of chemical reactions, using carbon-based molecules, by which matter is taken
into a system and used to assist the system's growth and reproduction, with waste products being
expelled
- life forms pass on their organized structure when they reproduce
B. Composition of Life on Earth
1. Depends upon the synthesis of Carbon
- once carbon is synthesized, all other biogenic molecules may be formed (Organic Molecules are
complex, carbon-based molecules)
- the elements most prominent in organic molecules are carbon, hydrogen, oxygen and nitrogen
2. Life Forms on Earth are made of:
a. Amino Acids
- primary structural units composed mostly of hydrogen, carbon, nitrogen and oxygen
- amino acids build proteins
b. Adenosine Triphosphate (ATP)
- supplies energy for action and growth
Astronomy Notes, Page 91
c. Nucleic Acids
- pass genetic material (examples = DNA, RNA)
- DNA is tough to replicate; the earliest organisms may have used RNA or other molecules for
genetic replication
3. DNA Molecule
- a ladder-like structure that stores genetic information as chemical bases; information is copied
by RNA and directs manufacture of proteins and enzymes
- changes to the DNA molecule (due to damaging or "mis-copying") may produce mutations
(mutations contain new DNA information and properties)
- evolutionary natural selection determines which properties will be perpetuated
C. Origin of Life on Earth
1. Theoretical Data
- the Miller-Urey Experiment ran electricity through a hydrogen/ammonia/methane/water
mixture and produced amino acids (the building blocks of proteins)
- but now most scientists believe that Earth's primordial atmosphere was carbon dioxide, nitrogen
and water vapor with minor hydrogen (which still produces amino acids under theoretical
conditions); you also can produce amino acids using ultraviolet radiation rather than electricity
- these chemical reactions must take place in an anaerobic (lacks oxygen) environment because
oxygen attacks organic molecules and breaks them down
- all life on Earth probably has a common origin
- organic molecules may also have been introduced during bombardment by planetesimals
2. Historical Data
- earliest life forms on Earth are dated at more than 3 billion years
- metazoans (with cells differentiated into tissues) had their first extensive evolutionary radiation
about 600 million years ago
- the earliest hominids (the Human family) did not evolve until about 5.5 million years ago; the
earliest stone tools date at about 2.6 million years
Types of Organisms:
a. Prokaryotes
- oldest known organisms (ex.= bacteria and blue-green algae)
- lack organelles and no membrane-bound nucleus; with asexual reproduction
- these were the only cell types initially present on Earth
- many prokaryotes are chemosynthetic [they convert carbon molecules and nutrients into organic
matter by oxidizing inorganic molecules (such as hydrogen gas, hydrogen sulfide or methane) as
a source of energy]
- astrobiologists are especially interested in Extremophiles (organisms that can withstand extreme
ecological conditions) such as within ice, boiling water, inside rocks, etc.
Astronomy Notes, Page 92
b. Eukaryotes
- with organelles (internal structures that probably developed as symbionts = mutually-beneficial
organisms) and with a nuclear membrane
- with sexual reproduction
- the earliest types of eukaryotes belonged to the Kingdom Protoctista (simple single-celled
organisms) at about 1.5 billion years before the present
3. The Gaia Hypothesis
- says that life has altered Earth’s ecosystems (examples include the atmosphere, temperature and
ocean chemistry) to make it more hospitable
- although this theory has been ridiculed by scientists, it appears that there are some selfregulating biological and physical systems on Earth
D. Looking for Extraterrestrial Life
Some basic assumptions may help the astrobiologist focus the search for life:
1. Carbon Chemistry
- it is assumed that life forms would be carbon-based
- carbon is the fourth most-abundant element in the Universe, and it’s structure allows the
construction of long-chained, complex molecules that are believed to be critical for building cells
2. Sun-like Stars
- large stars don’t last very long, so life would not have time to evolve on them
- small stars don’t produce enough energy to allow life forms to exist on most of their planets (if
the planets were very close to these stars they may receive enough heat, but they would also
probably be tidally-locked to their sun which would result in an inhospitable planet)
- in stellar spectral classifications habitable stars would probably be F, G or K stars
- it is estimated that about 10% of the stars in our Galaxy are Sun-like, so there should be about
1000 Habitable Stellar Systems (“HabStars”) within about 100 light years from our Sun
- HabStars are probably “metal-rich” (with elements heavier than helium); metal-poor stars
probably do not form planetary systems and any planet that would form would not have much
mass
3. Habitable Planetary Systems or Satellites
The Habitable Planet/Satellite would probably have these characteristics:
a. Distance from the HabStar
- in order to have life, it is assumed that the planet must be within the Habitable Zone (HZ); this
is the region of the solar system where liquid water would be present
- because of the tidal forces of Jovian planets, a habitable terrestrial planet/satellite would not be
situated close to a Jovian-type planet
Astronomy Notes, Page 93
b. Rotation and Revolution
- the planet or satellite would not have a highly-eccentric orbit (which would probably create
extremes in temperature)
- the planet or satellite would have enough axial tilt to create seasons, but not extreme axial tilt
which would create extreme seasonality (it is believed that our Moon has stabilized Earth’s axial
tilt, so the presence of a Moon-like satellite may increase the chances of life)
- the planet/satellite rotates fast enough to prevent temperature extremes and the rotation would
also help create a magnetosphere (see below)
c. Composition
- the planet or satellite would be terrestrial planet-like (Jovian-style planets would not have life
forms); outgassing of volatile compounds from the planets/satellites interior are needed to help
form the atmosphere and liquid water (although cometary impacts could provide substantial
amounts of gases and water to these habitable systems)
- the planet or satellite would have substantial amounts of carbon, hydrogen, oxygen and nitrogen
(these elements constitute over 96% of the biomass on Earth)
d. Size
- the planet would be massive enough to maintain a substantial atmosphere and for liquid water
to exist
- if it was not large enough geologic processes would not occur (volcanic outgassing helps form
the atmosphere and plate tectonics is important in recycling nutrients) and there would not be a
magnetosphere (which protects the planet from dangerous ultraviolet radiation and cosmic rays)
e. Atmosphere
- the habitable planet or satellite would have a substantial atmosphere
- if the atmosphere was too thin temperature extremes would occur and there would be no liquid
water
f. Liquid Water
- liquid water is an excellent environment for the construction of complex carbon molecules
- some scientists say that an alternative environment might be liquid ammonia or a liquid
ammonia-water combination
D. The Search for Extraterrestrial Life
1. Communication with Extraterrestrial Intelligence (CETI)
- concentrates on sending and deciphering messages that could be understood by an
extraterrestrial technological civilization
- the SETI Institute (Search for Extraterrestrial Intelligence) and later the SETI League, Inc. have
primarily utilized radio frequencies in their search; there is considerable uncertainty as to what
radio frequency to use and in which directions to search
- communication by laser has also been studied
- if the nearest civilizations are more than a few light years away, the light travel time of the
Astronomy Notes, Page 94
signals will make it difficult to communicate with distant interstellar civilizations
- although there have been numerous attempts to detect signals from other civilizations, none
have been successful
- although some scientists such as Carl Sagan (1934-1996) have promoted the transmission of
messages, the famous cosmologist Stephen Hawking (1942- ) has warned against it (for
example, when the “New World” was “discovered” by the Europeans it didn’t work out too well
for the Indians)
2. Search for Life in our Solar System
a. Mars
a1. The Martian Meteorite
- examination of a meteorite that is believed to have come from Mars has raised the possibility
that life developed there
- the meteorite has organic compounds concentrated in the vicinity of 3.6 billion year-old
carbonate minerals that were deposited in water-filled cracks; iron sulfides in the meteorite could
have been produced by bacteria, and tiny carbonate globules resemble bacterial fossils on Earth
- these discoveries do not prove the existence of ancient life on Mars
a2. Mars Exploration
- a series of Mars explorers have explored the surface of Mars including the Viking Program’s
Viking 1 and Viking 2 (1975-1982), the Phoenix Lander (2008) and the Mars Exploration Rovers
Spirit (2004-2010) and Opportunity (2004 to present).
- the Mars Science Laboratory Curiosity is a nuclear-powered, compact car-size laboratory that
arrived on Mars in August 2012
- although Mars had or has a possible potential for the origins of life, there is no direct evidence
that life has ever existed on Mars
b. Moons of Jovian Planets
b1. Europa
- the presence of cracks (lineae) on Jupiter's moon suggests that a layer of liquid water exists
beneath the surface
- if Europa’s ocean is not too cold or too salty, life forms could possibly exist similar to
chemosynthetic organisms in Earth’s deep oceans
b2. Enceladus
- although Saturn’s sixth-largest moon has an icy surface, the presence of cryovolcanic plumes
probably indicate the presence of liquid water beneath the surface (analysis of the E-ring system
of Saturn seems to suggest that these oceans are “salty”)
- a May, 2011 NASA conference concluded that Enceladus is probably the most habitable
extraterrestrial body in our Solar System
Astronomy Notes, Page 95
b3. Titan
- Saturn’s largest moon has been described as having an early Earth-like atmosphere
- it has liquid lakes and possibly oceans, but these are probably made of liquid methane and
ethane
- data from the Cassini mission published in 2008 suggests the presence of a liquid ocean beneath
Titan’s surface made of water and ammonia
3. Chances of Life on Other Worlds
- Extrasolar Planets (Exoplanets) are planets external to our Solar System
- the Kepler Space Telescope has identified 941 exoplanets in 727 planetary systems (146
multiple planetary systems have been identified); in addition, Kepler also detected over 18,000
“transit events”, many of which represent probable planets (and 262 of these may be “habitable
planets”)
- based on Kepler data, Caltech astronomers hypothesize that the Milky Way Galaxy may contain
as many planets as stars (100 to 400 billion exoplanets!)