Download Teachers` Manual - Amundsen High School

INTRODUCTION
Powers of Ten
 A way to express big and large numbers concisely
Forces Important in Astronomy
 Gravity - every piece of matter attracts every other piece
 Depends on masses and separation - Expressed in math shorthand as
F = GMm/r2
 Electricity and Magnetism
Depends on charge (+ or -) - Like charges repel; unlike attract
 The nuclear or strong force
THE COSMIC LANDSCAPE
The Earth, Our Home
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A sphere (Known to ancient Greeks) - R about 6000 km
Rotates - causes day and night
Has an atmosphere - held on by Earth's gravity
Has a magnetic field - deflects compass needle
The Moon
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Orbits Earth - held in place by gravity
A sphere - about 1/4 radius of Earth
Mass is much smaller than Earth's Mass - consequently weaker gravity
No atmosphere - result of that weak gravity
The Sun
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a star
Gaseous - mix of mostly H and He
Held together by gravity
Holds Earth and other planets in orbit by its gravity
Has a magnetic field - sunspots
Hot - heated by nuclear reactions (Fusion of H into He)
Tsurf - 6000 K
Tcore - 15 million K
The Solar System
 flattened system - held together by gravity
 The Nine Planets
 Asteroids and comets
 The Astronomical Unit - distance between Earth and Sun
Other stars
 Nearest is 4.3 light years
 If pinheads, separated by about 35 miles
The Milky Way Galaxy
 Assemblage of some 100 billion stars - held together by gravity
 100,000 ly across (approx)
 Rotates - approx once every 220 million years
Galaxy Clusters and the Universe
 The Local Group - small cluster of about 30 galaxies
A few million ly across - held together by gravity
 The Local Supercluster - a group of galaxy clusters- held together by gravity
Dark Matter
 Universe contains large amounts of material that has not yet been directly
detected
 We know it is there from its gravitational attraction on matter (stars and
galaxies) that we can see
 Nature of this material is perhaps the major mystery in astronomy today.
 Amount of dark matter is huge - roughly 100 times the amount of luminous
matter
The Universe
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The largest astronomical structure we know of
HISTORY OF ASTRONOMY
Why Important?
 To see how astronomers have used observation and hypothesis to
understand the heavens
 Also introduces importance of astronomy to everyday life
All our time-keeping based on astronomical motion
Prehistoric Astronomy
Classical Astronomy
Astronomy in the Renaissance
Newton and Astrophysics
The Modern Astronomy
Early Ideas — Prehistoric Astronomy
Why?
Time keeper
Harvest crops
Study the future
God!!
Out comes
 Stars, Sun, and Moon are mounted on the Celestial Sphere - a sphere that
surrounds Earth( its not the real model)
 Star patterns are fixed – Constellations ( pattern of Stars, use imagination)
 Celestial sphere rotates around Earth and causes rising and setting of stars
and other objects
 Motion of the Sun and Stars
 Daily Motion and Annual Motion
 Saesons
 Sun's path across the sky changes with seasons
 The Ecliptic: The sun path across the background sky.
 High in Summer, low in winter
 Stars visible near Sun before dawn and after dusk change during year.
Sun's shifts its position on celestial sphere with respect to the stars
Sun's path = Ecliptic
Solstices and Equinoxes
The Direction of the rising and sitting change through the year
 Sun rises due east and sets due west on Equinoxes
 Equinoxes (approx. equal hours of day and night)
Marks start of spring and autumn
 Sun rises farthest north along horizon at June solstice
Rises farthest south at December solstice
Marks start of summer and winter
The Planets and the Zodiac
 Some star-like objects change position with respect to stars
Called Planets, from ancient Greek for "wanderer"
 Their path always near the ecliptic.
Band they are found within = Zodiac
The Moon
 Cycle of phases - caused by changing illumination as Moon orbits
Earth (rises in the east and sets in the west).
 Moon shifts position against stars - approx follows Zodiac
 Eclipses - caused by Moon's shadow falling on Earth, or vice-versa
Early Ideas of the Heavens: Classical Astronomy
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The Shape of the Earth
The Size of the Earth
Distance and Size of Sun and Moon
Models for the Motion of the Planets
Earth at center
Other objects move around it affixed to "crystalline spheres"
Planets sometimes reverse direction with respect to stars
Retrograde motion
Explained by "epicycles" - small spheres attached to larger ones
Ptolemaic Model
Astronomy in the Renaissance
 Copernicus - revives Sun-centered model of Heavens
 Tycho - made detailed observations of planet positions
 Kepler - used Tycho's positional data to derive "Laws of Planetary Motion"
Orbits are ellipses - Sun off-center at focus
Equal area law
Orbital period related to orbital size (P2 = a3)
 Galileo - applied telescope to study of heavens
Laws of motion
Discovered moons orbiting Jupiter - Earth not center of all
motion
Discovered phases of Venus - proof that Venus orbits Sun, not
Earth
Isaac Newton and the Birth of Astrophysics
 Law of Gravity
 Mathematical expression of laws of motion
GRAVITY AND MOTION
Why Important?
 Solving the Problem of Astronomical Motion
Inertia
 Tendency of an object at rest to remain at rest and an object in motion to
keep moving
Extended by Newton to First Law of Motion
 An object at rest remains at rest. An object in motion continues to move in
a straight line at a constant speed unless a net force acts on it.
 Such motion is called "uniform"
 Non-uniform motion is called an acceleration - a change of either speed or
direction
Orbital Motion is not uniform
 Object follows a curved path
 Therefore, according to First Law, a force must act on it
 For most astronomical objects that force is Gravity
Law of Gravity
 Described by Newton - F = GMm/r2
Newton's Second Law of Motion
 F = ma
 Relates acceleration of an object to the force acting on it
 Allows shape of orbit and details of motion to be worked out
Mass measures amount of material
 more technically, its inertia
Newton's Third Law
 action = reaction
Measuring a Body's Mass Using Orbital Motion
 Newton's laws of gravity and motion allow mass of object to be deduced
from orbital motion of object moving around it
Surface Gravity
 Determines gravitational force of planet on object = weight of object
Escape Velocity
 Speed needed to move away from an object and not fall back
 Gravitational force of planet, star, and so forth makes it difficult for a mass
to move away from object.
 Higher velocity allows a mass to rise further from object.
 Important in determining whether planet has atmosphere and
for black holes (vescape = c)
LIGHT AND ATOMS
Why Important?
 Nearly all our knowledge of heavens comes to us through the
"light" we receive from astronomical objects. Understanding nature of light
from these objects yields knowledge of their properties
Properties of Light
 The Nature of Light -Waves of electric/magnetic energy or Particles
(Photons)
 Light and Color - color set by wavelength (red long, blue short)
 White Light - mix of all wavelengths
The Electromagnetic Spectrum: Beyond Visible Light
 Ordering by decreasing wavelength gives Radio Waves, Infrared, Visible
light, Ultraviolet, X-rays, Gamma rays
Energy Carried by Electromagnetic Waves (shorter
wavelengths, more energy)
Wien's Law: A Color-Temperature Relation
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Red cool, Blue hot
Atoms
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Structure of Atoms (nucleus and orbiting electrons)
The Elements (number of protons in nucleus determines kind of element)
Atoms store energy by lifting electrons to higher orbits
Return of electron to lower orbit releases energy which appears as light
(or more generally, electromagnetic radiation)
 Each kind of atom has unique set of electron orbits - motion of electrons
between orbits leads to unique set of wavelengths of light
 Thus - Atoms can be identified by the light they emit or absorb
Gives composition of stars, gas clouds, and planets
Types of Spectra
 Hot low density gas - emission line spectrum
Light only at some wavelengths
 High density hot gas, liquid, or solid - continuous
spectrum
Light at all wavelengths
 Cool gas between observer and light source absorption line spectrum
Light missing or dimmer at some wavelengths
The Doppler Shift
 Observer sees the wavelengths emitted or absorbed
by a moving source shifted
If source and observer move apart, wavelengths
increase
(shifted to red - redshift)
If source and observer approach, wavelengths
decrease
(shifted to blue - blueshift)
Earth's atmosphere absorbs some wavelengths
 Especially in non-visible wavelengths
 Atmospheric absorption limits astronomers ability to
study heavens from ground
(IR, UV, etc)
THE EARTH - OUR HOME PLANET
The Earth as a Planet
 Shape is approx sphere - radius about 6400 km
 Rotation bulges equator slightly
Composition of the Earth
 Crustal rocks directly sampled
 Interior composition deduced from density = Mass/Volume
 Density is about 5.5 gm/cm3
Midway between rock (3 gm/cm3) and iron (8 gm/cm3)
The Earth's Interior
 Probed with Earthquake Waves
 Reveal thin crust, mantle, liquid core, and solid inner core
 Crust and Mantle are rock (silicates) Liquid and inner core are iron/nickel
Heating of the Earth's Core
 Some heat is probably residual from Earth' formation
 Some heat supplied by radioactivity in rock - like natural nuclear reactor
Age of the Earth
 Deduced from radioactivity in rock - about 4.5 billion years
Heat of interior "stirs" interior - convection
 Convective motions make large thin areas of crust slide over mantle rock
Plate Tectonics
 Collision of plates buckles crust - mountain ranges
 Separation of plates opens rifts - some ocean basins
 Plate motion also triggers volcanic eruptions along some plate edges
The Earth's Atmosphere
 Composition of the Atmosphere - 78% Nitrogen, 21% oxygen
smaller amounts of water, carbon dioxide and other gases
 Origin of the Atmosphere
Gas from volcanic eruptions has accumulated over time
Gases added include nitrogen, water, and carbon dioxide
Some of atmosphere may have come from comets that
struck Earth and evaporated
Oxygen in atmosphere formed (mostly) by photosynthesis by
plant-life
 The Ozone Layer - absorbs UV, protecting us
 The Greenhouse Effect
Carbon dioxide and water in atmosphere trap heat
Makes Earth warmer than if gases not present
Earth's Magnetic Field
 Rotation and convection in liquid iron/nickel core probably generate
 Earth's Magnetic Field
 Magnetic field affects motion of gases in upper atmosphere - leads to aurora
Motions of the Earth
 Earth spins on axis - cause of day and night
 Orbits Sun
 Sun orbits Milky Way Galaxy
The Seasons
 Earth's rotation axis is tilted about 23.5o with respect to its orbit
 Earth acts like gyroscope and keeps approx same orientation as it orbits Sun
 As result, Sun shines most directly on different hemispheres at different
times of year - leads to more heating and more hours of daylight
Air and Ocean Circulation
 Rotation of Earth creates the Coriolis Effect
 A deflection of the path of objects moving over Earth's surface
Deflection drives winds and ocean currents
Precession
 Earth's rotation angle does not keep exactly same orientation
 Slowly "wobbles" - once every 26,000 years
 Leads to different "North Stars"
 May cause climate change
THE MOON
Description of the Moon
 Barren ball of rock orbiting Earth
 R about 1/4 REarth
 M about 1/80 MEarth
Surface Features
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Highlands - low density, light colored rock - heavily cratered
Maria - higher density, dark colored rock - relatively smooth
Most Craters are impact features - circular pit, raised rim of ejected material
Rays - light colored streaks of pulverized rock radiate from some craters
Rilles - lunar canyons
No folded mountains, few if any volcanic peaks
No signs of tectonic activity as on Earth
Structure of the Moon
 Crust and Interior - Outer layers rigid rock
Low density implies much smaller iron core than Earth
Inner core may be hot, but
Small size of Moon let it cool far more than Earth
Absence of a Lunar Atmosphere
 Low gravity means hard to retain an atmosphere - gases readily escape
 Lack of appreciable inner heat means little or no volcanic activity
to generate atmosphere
Orbit and Motions of the Moon
 Moon rotates although it keeps same face to Earth
 Rotation period = orbital period - rotation synchronous
 Orbit tilted about 5o with respect to Earth's orbit around Sun
Origin and History of the Moon
 Moon probably formed by giant impact that splashed matter from Earth's
crust
 Material orbited Earth, gradually accumulated to form Moon
 Bombardment by debris late in formation left craters
Huge impacts blasted basins that later flooded with lava to form Maria
 Moon's small size led to rapid cooling and little subsequent heating
 Since then Moon essentially inactive
Eclipses
 Rarity of Eclipses results from tilt of Moon's orbit
Shadows of Earth and Moon therefore generally miss one another
 Appearance of Eclipses
Tides
 Moon's gravitation attraction creates two tidal bulges on Earth's oceans
 Earth rotates under bulges, so at a given location on ocean, water rises twice
and falls twice each day.
 Tidal Braking - rotation of Earth under the tidal bulges creates friction like a
brake slows Earth's rotation
BASIC FEATURES AND ORIGIN OF THE SOLAR
SYSTEM
Solar System
 Consists of 9 planets (MVEMJSUNP), their moons, the Sun, and
smaller objects: asteroids and comets. All held in single system by Sun's
gravity
 Sun's mass about 1000 greater than all other objects put together
 Solar System is essentially a disk
 All planets orbit Sun in same direction and in nearly same plane.
Solar System contains two major types of planets
 Small rocky (rich in silicates and iron) objects like Earth - Terrestrial
planets
 Large gaseous/liquid objects (rich in hydrogen and its compounds) like
Jupiter- Jovian
Planets
 Sometimes Inner and Outer planets used to distinguish these two types
 Differences show strongly in density - about 5 gm/cm3 for terrestrial
about 1-2 gm/cm3 for Jovian
Solar System contains two major types of small objects
 Asteroids and comets
 Asteroids are rocky and orbit between Mars and Jupiter in asteroid belt
 Comets are icy and most spend most of time far from Sun in Oort Cloud or
Kuiper belt.
All major objects about same age
 At least to the extent we can determine.
(4.5 Billion yrs)
Bode's Law
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A curious method for generating the distances of the planets from the Sun
Start with 0, 3, 6, and so forth, by doubling
Add 4, then divide by 10.
Gives distance in AU
Works accurately for Mercury - Uranus. "Predicted" asteroids
Fails for Neptune, but "works" for Pluto
 Reason not understood
Model for Origin of System
 Solar System formed from collapse of Interstellar cloud
 Cloud was mostly H and He, about few % heavier elements
 H and He gaseous, heavy elements in form of tiny solid dust grains
rich in iron, silicates, carbon compound, water.
 Cloud was slowly spinning
 Collapse result of gravity
 As cloud shrinks, its rotation speed increased (Cons. of Ang. Mom.)
Faster rotation leads cloud to form disk shape = Solar Nebula.
 Lump forms in center of disk. Becomes young Sun
 Cloud heated by collapse, but once in form of disk, cools again.
Inner disk kept warm by young Sun
 Higher temperature in inner portion of disk allows only silicates and iron
compounds to condense there as new small grains
 Farther out, disk is cooler, and ices and some gases freeze out as well
 Disk therefore has two zone structure
Inner zone - iron/silicate grains
Outer zone - iron/silicate/carbon/water/frozen gases
 Grains "coagulate" and form pebble-size particles. These stick together
into boulder size objects
 Gravity begins to help clumps form and they rapidly grow to many
kilometer size objects = planetesimals
 Planetesimals of two types
Inner ones rocky (silicate/iron)
Outer ones water and hydrogen-rich as well
 Inner planetesimals collide and form inner (terrestrial planets)
 Ditto for outer planetesimals. They are in region cold enough that they can
capture hydrogen and helium (otherwise uncondensed). Turn into outer
planets.
 Some evidence that the 4 big outer planets may have grown directly from
"lumps" in the solar nebula.
 Also evidence that orbital position of outer planets may have drifted
 Some planetesimals survive: Captured by planets - form moons.
Others become asteroids and comets
 Smaller bodies bombard planets and moons and create numerous craters
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Sun blows left over gas and dust out of system
Extra-Solar Planets
 Many stars have planets orbiting them.
 Planets detected by their gravitational tug on parent star
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That tug moves star slightly, creating a Doppler shift
From Doppler shift can deduce (with reasonable assumptions) mass of
planet
All planets found so far are very massive (Jupiter's mass or more,
typically)
Many puzzling features in these other planetary systems
Orbits of many of these planets are very eccentric.
 Many of the Jupiter mass planets are very close to star ( some less than
an AU)
 Do these large planets form far out in preplanetary disk and migrate
inward?
THE TERRESTRIAL PLANETS
MERCURY (Dist. from Sun = .4 AU)
 Radius about .4 REarth
 Mass about 0.05 MEarth
 High density (about 5 gm/cm3)implies high iron content.
Possibly result of impact stripping much of its rocky mantle
 No atmosphere - escape velocity so small any gas readily escapes
 Surface heavily cratered. Unmodified by tectonic forces/volcanic activity.
 No moon
 Odd rotation. Locked into 3/2 resonance with its orbital motion around
Sun.
VENUS (Dist from Sun = .7 AU)
 Radius about 0.95 REarth
 Mass about 0.8 MEarth
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(Thus approximate twin of Earth in size and density [about 5.3 gm/cm3])
Clouds prevent our viewing its surface from Earth with optical telescopes.
Spectra show clouds to be sulfuric acid droplets.
Spectra also show atmosphere is very dense (approx 100 times density of
Earth's) and composed mostly (96%) of Carbon dioxide
Dense CO2 atmosphere traps heat by greenhouse effect. Planet very hot as
result (T about 900 F)
Radar maps show surface is complex
Craters, volcanic peaks, lava flows, lava flood plains,
Level of volcanic activity implies thin crust and active planet
Rotation very slow and retrograde
MARS (Dist from Sun = 1.5 AU)
 Radius about 0.5 REarth
 Mass about 0.1 MEarth
 Density about 4 gm/cm3
 Atmosphere is thin and contains clouds. Spectra show gas is mostly CO 2.
Clouds are water-ice and dry ice
 Surface has reddish tint from high iron concentration in crustal rocks.
 Surface features include
Craters
Canyons
Old River beds, sand-bars, islands
Inactive volcanic peaks.
Polar caps of water-ice and frozen CO2.
"Deserts" with sand dunes.
 Mars has two small moons - Phobos and Deimos. Probably captured
asteroids.
COMPARISON OF THE TERRESTRIAL PLANETS
 All have a density of about 5 gm/cm3. Thus, all probably have iron core
 Small size of Mercury allowed it to lose heat quickly - thus inactive now.
 Mars too is small, so its interior is probably relatively inactive compared to
that of larger Venus and Earth
 Venus and Earth large enough to have
a) retained heat from their formation or
b) generated it from radioactive decay
 Higher core temperature of Earth and Venus make them tectonically active
and creates internal motions
 On Earth, motions lead to plate tectonics.
 On Venus, for still unknown reasons, little plate motion.
Internal activity more like that seen near "hot-spots" on Earth
 Difference between Earth and Venus atmospheric composition attributed to
Lack of liquid water on Venus and in its atmosphere.
Liquid H2O absorbs CO2 to form carbonic acid
Combines with rock to form carbonates
Removes much CO2 from atmosphere.
(Amount of CO2 in Earth rock is comparable to that in Venus's
atmosphere)
OUTER PLANETS (Jupiter, Saturn, Uranus, Neptune,
and Pluto)
JUPITER (Dist from Sun = 5.2 AU)
 Radius about 11 REarth
 Mass about 315 MEarth
Huge mass makes Jupiter more massive than all other planets combined
 Density = Mass/Volume = about 1.3 gm/cm3
This low value compared to the approximately 5 gm/cm3 of terrestrial
planets implies very different composition.
Must be large quantities of low density elements such as H and
its compounds such as water, ammonia, and methane
(H2O, NH3, CH4)
 Spectra support this difference and show presence of helium as well.
 Atmosphere presumably thickens with depth, changing from gas to liquid as
depth increases and compresses material.
 Below atmosphere a "sea" of liquid hydrogen. Below that, a zone of
metallic hydrogen
 Core of rock and iron compounds likely on the basis of
gravitational field and shape of planet
Abundances of elements in Sun
 Theoretical models and measurements of heat loss from atmosphere imply
interior is very hot. Heat drives convective motions
 Planet exhibits strong cloud belts - result of convection inside and Coriolis
Effect
 Cloud motions show rapid rotation of planet. Approx 10 hours
 Other atmospheric features included Great Red Spot, (a huge atmospheric
vortex) and smaller, fainter vortices.
 Faint, thin, ring system
 Extensive satellite system
Most lie in equatorial plane and orbit in same direction:
form mini-Solar System
Four largest moons are Galilean satellites: Io, Europa, Ganymede, and
Callisto - All rich in silicates (rocky material). All but Io rich in ice
Io is heated by strong tidal forces exerted on it by Jupiter.
Has erupting sulfur volcanoes
Europa also appears to have been heated and partially resurfaced
(few impact craters compared to Ganymede and Callisto)
 Strong magnetic field presumably caused by rapid rotation and convection
in deep interior
SATURN (Dist from Sun = 9.5 AU)
 Radius about 9.5 REarth
 Mass about 95 MEarth
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Density = Mass/Volume = about 0.7 gm/cm3 - lowest of any planet
Low density implies composition similar to Jupiter's
Spectra show rich in hydrogen, helium, and hydrogen compounds.
Bright, wide ring system
Extensive satellite system - similar to Jupiter's
Most interesting moon is Titan, nearly size of Mercury. Has an
atmosphere rich in nitrogen and perhaps seas of the hydrocarbon
ethane
URANUS (Dist from Sun = 19 AU) NEPTUNE (Dist from Sun = 30 AU)
Radius about 4 REarth
Mass about 15 MEarth
Radius about 4 REarth
Mass about 17 MEarth
 Uranus and Neptune were discovered in the 18th and 19th centuries
respectively.
 They are very similar in size, mass, and composition.
 Spectra show their atmospheres to be mostly hydrogen and its compounds
 Both appear blue because the methane in their atmosphere absorbs red light
strongly
 Both probably have a lot of water in their interior.
 Uranus's atmosphere shows few features (although infrared observations
suggest features may appear and disappear)
 Neptune's atmosphere shows cloud bands and a Great Dark Spot,
reminiscent of Jupiter's Red spot
 Both have faint rings.
 Both have extensive satellite systems
 Uranus has very large tilt of its rotation axis - result of ancient impact?
PLUTO (Dist from Sun = 40 AU)
 Radius about 0.2 REarth
 Mass about 0.003 MEarth
 Density is about 2 gm/cm3, implying a rocky/icy structure
 Spectra show a thin methane atmosphere and methane frost on surface.
 Orbits the Sun exactly twice in the time Neptune orbits three times –
suggesting Pluto has been "captured" into its orbit by Neptune.
 Many dozen other icy objects orbit at about the same distance.
 Many astronomers now consider Pluto just the largest of the Kuiper belt
objects.
METEORS, ASTEROIDS, AND COMETS
Meteors
 We see as very brief bright streak of light in sky
 Caused by small (typically raisin) size piece of solid material entering
atmosphere from space
 Friction of air molecules with object heats it and makes it glow
 Most burn up in atmosphere
 Those that survive to land on ground are called meteorites
 Source - asteroid and comet fragments
Asteroids
 Rocky and or metallic objects
 Most orbit in Asteroid belt - between orbits of Mars and Jupiter.
 Largest is Ceres - Diameter about 1000 km.
Most are much smaller (meter to kilometer size) and irregularly
shaped
 Origin of Asteroids - probably left over planetesimals and their fragments
Prevented from forming planet by Jupiter's gravitational influence
 A few cross Earth's orbit - Apollo object
 Other unusual asteroids
Comets
 Appear as pale, fuzzy, elongated object in night sky
 Structure of Comets - bright head and long tail
 Head consists of small (about 10 km) size nucleus - a solid icy and dusty
core
 When near Sun, ices evaporate and frozen gases thaw to create coma
Coma is a large diffuse "atmosphere" around nucleus
 Tail forms near the Sun when Sun's radiation pressure and solar wind
blow material from coma out into long plume
 Light from the Comet comes from reflected sunlight and fluorescence
solar UV radiation absorbed and re-emitted as visible light
 Comet nuclei reside in Oort cloud at extreme outer part of Solar system
40,000 -100,000 AU from Sun.
 Some nuclei may come from Kuiper belt (flattened region beyond orbit of
Neptune)
 Short Period Comets - such as Halley's
Appear periodically (Halley's every 76 years)
 Fate of Short Period Comets
Repeated passage by Sun evaporates ices and drives off gases
Leaves trail of rocky and metallic dust and small particles
Solids don't evaporate
 Meteor Showers occur when Earth passes through trail
Asteroids and Comets
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Occasionally strike Earth and other objects in Solar System.
Energy released can be huge if impacting body is large
Craters found from such events many places
A huge impact about 65 million years ago may have exterminated the
dinosaurs
THE SUN - OUR STAR
Basic Properties
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Incandescent ball of gas
Distance 1 AU - determined by radar or triangulation
Radius about 100 REarth - determined by measuring angular size
Mass - about 300,000 MEarth - determined from modified form
of Kepler's Third Law
 Density - about 1.4 gm/cm3 - suggests composition rich in light elements
 Composition - about 71% H, 27% He - deduced from density and spectra
 Tsurface - 6000 K - measured from color with Wien's Law
The Solar Interior
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Studied by calculations (Solar models) and observations of surface
Energy is generated in the central core where T is large enough for fusion
Energy flows from core by radiation - photons carry energy
Near the surface, gas is cooler and absorbs photons Impedes flow of energy
Energy carried instead by convection - Convection zone
Visible "surface" of Sun = photosphere
The Solar Atmosphere
 The hot solar gases above the photosphere
 Lower atmosphere = Chromosphere - hot (50,000 K) low density gas
 Upper atmosphere = Corona - extremely hot - 106 K, very low density gas
How the Sun Works
 Sun is held together by gravity of its matter - Sun is supported by pressure
of it gas
Hydrostatic Equilibrium = balance of gravitational and pressure
forces
Pressure in the Sun
 Pressure in normal gases depends on gas temperature and density
Perfect gas law - P = constant x density x Temperature
Powering the Sun
 Sun is powered by nuclear fusion
The bonding of light nuclei together to form heavier nuclei
Total mass of final nuclei is less than total mass of initial nuclei
Mass is converted to energy in this process (E = mc2)
 Fusion in Sun combines 4 hydrogen nuclei together to form a single helium
nucleus
Probing the Sun's Core
 Solar Neutrinos - emitted during hydrogen burning
Have immense penetrating power - roughly a light year through lead.
Leave Sun freely and thus give us info on core conditions
 Solar Seismology
Just as earthquakes give info about interior of Earth, so waves
traveling through Sun give info on its interior
Waves create surface disturbances that can be observed.
Solar Magnetic Activity
 Sun has magnetic field whose level of activity changes over time
 Sunspots - dark region on face of Sun (photosphere) - often huge compared
to Earth
Regions are dark because cool
Cool because magnetic field inhibits heat rising freely in convection
zone
Magnetic effects directly measurable with Zeeman effect
Spots often in pairs or groupings
Number of spots changes with time. Reaches peak approx. every 11
years - The Solar Cycle
 Prominences and Flares
Prominences are plumes, arches, or other shapes of luminous gas.
Relatively long-lived, days to weeks
Prominence supported by magnetic field
Flares are brief intense brightenings of gas, generally near sunspots
Flares triggered by rearrangement of local magnetic field
 Heating of the Chromosphere and Corona
Magnetic activity probably heats chromosphere and corona - recall
hotter than photosphere
Waves travel out along magnetic field and agitate gas - heat it
 The Solar Wind
The hot coronal gas has such high pressure that Sun's gravity cannot
confine.
(recall P increases as T increases)
Coronal gas expands into space as solar wind
The Solar Cycle
 The varying level of magnetic activity (spots, prominences, flares)
 Cause of the Solar Cycle - winding up of the Sun's magnetic field below its
surface
Field begins weak but grows stronger
Field breaks through surface, creating spot pairs
 Links between the Solar Cycle and Terrestrial Climate
 Solar cycle not always same level of activity - many fewer spots at some
maxima than at others
 Some evidence that low levels correspond to climatic cooling on Earth
Maunder minimum - period of few spots 1645 - 1715
Coincides with "Little Ice age" - period of abnormal cold in
Europe North America.
Reason for such a link - if true - not understood
MEASURING THE PROPERTIES OF STARS
Measuring a Star's Distance
 Measuring distance by Triangulation and Parallax
Distance that cannot be measured directly may be measured using
triangulation
Method involves
Construct a base-line of known length
Measure angles from ends of base-line to object whose distant
you seek
Construct scale model of triangle with same angles
Measure unknown distance on scale model (or use Trig)
 Astronomers use similar method to find distance to stars
Use Earth's motion around Sun to observe star from two different
locations
Radius of Earth's orbit then becomes base-line
Star appears to lie in different positions as seen from ends of baseline
That change in position is called parallax
More remote stars show smaller change in position - smaller parallax
Distance is thus inversely related to parallax
By choosing units of distance cleverly, d = 1/p where d is measured in
parsecs and p in arc seconds. 1 parsec (pc) = 3.26 light years
Even nearby stars have only very tiny parallax, implying large
distance
 Measuring distance by the Standard Candle Method
Parallax method fails for very distant stars
Their change in position is too tiny to measure
 Standard candle method uses fact that the distance to a light source of
known brightness can be deduced from its apparent brightness
For example, headlights of on-coming cars give clues to their distance
Measuring the Properties of Stars from their Light
 Temperature - use Wien's Law - relates star's temperature to its color
 Luminosity - defined as power output of star the amount of energy it emits
per second
Luminosity (L) is related to star's distance (d) and
its apparent brightness (B) - how bright it looks to us - by
Inverse-square law - B = L/(4d2) .
Procedure - Measure apparent brightness (B).
Measure distance (d) using parallax
Solve inverse square law for L = 4d2B and
insert values for B and d
 Radius - Use Stefan-Boltzmann Law - L = 4R2T4
L = total energy emitted by star = energy emitted per square meter
from star's surface (T4) multiplied by the number of square meters
of star's surface, its surface area 4R2
 Procedure - Measure T - by Wien's law for example
Measure L as described above
Solve for R
The Magnitude System
 Astronomers sometimes use magnitude system to describe star brightness
 System based historically
Stars that look brightest called magnitude 1
Stars just barely visible called magnitude 6
(Note this give scale working backwards: brighter stars have smaller
magnitudes)
Scale is set so stars of magnitude 1 are 100 times brighter
than stars of magnitude 6.
Thus 5 magnitudes corresponds to a brightness ratio of 100
Spectra of Stars
 Used to measure MANY properties of stars
 Composition - from which lines appear in spectrum
 Temperature - Affects a Star's Spectrum
Different Temperatures lead to different appearing spectra
Spectra classified on basis of appearance - which lines are strong
Primary Classes are O, B, A, F, G, K, M
O stars are hot and blue, M stars are cooler and red
Sun is a G star
Subclasses denoted by number.
(G5 has T about mid-way between G0 star and A0 star)
Motion - use Doppler shift of spectrum lines
Binary Stars
 Some stars have companions held in orbit about them by their mutual
gravity
 Percentage of stars that are binary is unknown for sure.
Probably at least 40% - Maybe 80%
 Binary stars used to find masses and radii of stars
 Types of Binary Stars
Visual binaries - two separate stars visible telescopically
Spectroscopic Binaries - stars too close to see as separate stars
Detected by doubling of spectrum lines
Eclipsing Binaries - stars' orbits oriented so one passes between us
and the other - light is blocked - eclipsed
We see brightness of pair change
Sometimes we see light from both stars, sometimes only one
Measuring Stellar Masses with Binary Stars
 Use modified form of Kepler's third law m+M = a3/P2
m and M are in solar mass units, a, their separation, is in AU,
and P, their orbital period, is in years
 Method works because
stars held together by gravitational force and
gravitational force depends on mass
The H-R Diagram
 The Hertzsprung-Russell or H-R diagram is a graphical way
to depict the properties of stars
 Stars are plotted in the diagram according to their
Luminosity and surface temperature
 The temperature scale runs backwards - that is,
hotter stars are found on the left, cooler stars on the right
Equivalently, bluer stars are on the left, redder stars are on the right
 The plot shows
Most stars (about 90%) lie within a band from upper right to lower
left
(From luminous and blue to dim and red)
This band is called the MAIN-SEQUENCE
A few stars lie in the upper right (luminous and cool)
A few stars lie in the lower center (dim and hot)
Thus, stars form THREE MAIN CLASSES
 Analyzing the H-R Diagram
The Stefan-Boltzmann law allows us to interpret these classes
A star can be luminous by being either hot or big or both
A star can be dim by being either cool or small or both
Thus, luminous cool stars must be big - GIANTS
and dim hot stars must be tiny - DWARFS
The Mass-Luminosity Relation
 Measurement of main-sequence stars in binary systems shows that
the more massive a star, the more luminous it is
 L is approx M3, if we measure both L and M in solar units
 Thus, the main-sequence is a mass-sequence.
Hot, luminous stars are more massive than cool, dim stars
Variable Stars
 Some stars change in luminosity - illustrated by plotting a light curve
(Brightness versus time)
 For many variable stars, the changes are repetitive, that is, the stars cycle
from bright to dim and back again repeatedly
 Many variables pulsate - that is, they change size, expanding and
contracting
Finding a Star's Distance by the method of Standard Candles
 Method uses inverse square law - B = L/(4d2)
 Choose a type of star whose luminosity (L) we know
Perhaps from spectrum, perhaps from light variability properties
 Measure its apparent brightness (B)
 Solve equation for d2 by cross-multiplying by 4d2 to get 4d2 B = L
Divide by 4B to get d2 = L/(4B). Take square root to get d.
 Evaluate d by plugging in measured numbers for L and B.
STELLAR EVOLUTION
Star Formation
 Stars form from Interstellar Gas Clouds
Clouds are big (several light years diameter)
Cold (T about 10 - 100 K), low density
Gravity draws material into clumps that turn
into protostars
 Protostars - earliest stage of a star's life - star not
yet stabilized - still shrinking
 Protostar surrounded by disk of gas and dust
Similar to disk that formed planets in Solar
System
 Gas flows away from star and disk in two
narrow cones - Bipolar Flows
Main Sequence Stars
 Star's size stabilized and burning hydrogen into
helium
in its core
 Star's Mass Determines its Core Temperature
More mass leads to more gravity
More gravity requires more pressure to
support
More pressure achieved by being hotter
Hotter leads to more luminosity
Thus, along main sequence in H-R
diagram,
hotter and more luminous stars must be
more massive
 Mass also determines speed with which star
burns up its core hydrogen
Limits star's lifetime - Main Sequence
Lifetime
Massive stars are more luminous
More luminosity implies faster burning
of fuel
Faster burning of fuel leads to shorter
lifetime
Thus, massive stars do not live as long
as low-mass stars
Stellar Mass Limits
 If mass too small, core not hot enough to burn
hydrogen
(M greater than about 0.08 MSun for hydrogen
to burn)
If M too large, star so hot as it forms that it
drives away gas near it
Mass cannot accumulate on star and so its mass
remains less than
about 100 MSun
 Consumption of all the hydrogen in a star's core
forces star to adjust structure
Core contracts and heats.
Hydrogen burns in zone around core - shell
source
Outer layers of star swell up (expand) and cool
Star becomes Red Giant - leaves main sequence
 Giant Stars - tiny, dense, hot core with huge cool
"envelope"
Nuclear Fuels Heavier than Hydrogen
 T in core may be hot enough to fuse helium into
carbon
(3 4He ---> 12C) - requires T about 108 K
 Ignition of helium in core of low-mass star heats
core and
eventually leads to core expansion and
envelope contraction
 Star changes from red giant to yellow giant
(In H-R diagram, shifts to left and down
toward main sequence)
Pulsating Stars
 Some stars swell and shrink in radius.
 This causes their luminosity to change with time
- pulsating variable stars.
Time to complete pulsation cycle = period
 Several important classes - Cepheids and RR
Lyrae variables - will be used later to measure
distances
Why Do Stars Pulsate?
 Atmosphere traps radiation. Star swells,
allowing radiation to escape
The Period-Luminosity Relation
 The more luminous the star, the slower it
pulsates (longer its period)
 Law allows determination of star's luminosity by
timing its variations
 Inverse-square law then allows distance to be
found
Measure apparent brightness; comparison
with luminosity
determined from period gives distance
Death of Stars Like the Sun
 Helium burning in core eventually consumes all
the helium and forms carbon core
 Carbon core shrinks and heats
 Outer layers of star expand to make star red
giant again
 Carbon and silicon atoms in cool outer layers
condense to form grains
 Grains "pushed-on" by radiation pressure from
luminous core of star
 Grains in turn push on gas, stripping off outer
layers and
forming huge shell around star
 Shell expands and thins out - hot core of star is
revealed
Ultraviolet radiation from core heats shell
and makes it glow
Planetary Nebula
Old Age of Massive Stars
 Formation of Heavy Elements: Nucleosynthesis
High-mass stars have hotter cores than lowmass stars
Tcore in very massive stars hot enough to
fuse elements heavier than
helium and carbon, such as O ---> Neon --->
Silicon ---> Iron
Reactions produce energy to keep star
supported
 Core eventually becomes iron, but iron cannot
fuse and release energy
Iron core thus is end for high-mass star
 Iron core collapses - density rises
 Protons and electrons in core forced so close
they "merge" ---> neutrons
 Core becomes tiny (10 km) ball of neutrons
 Outer parts of star fall in on tiny core - triggers
massive explosion
Supernova Explosions
 Explosion blows off outer layers at 10,000
km/sec
 Ejected debris called Supernova Remnant
 Remnant core becomes neutron star (if core less
than about 3 MSun)
or black hole (if mass of core is more than
about 3 MSun)
Testing Stellar Evolution Theory
 We can test correctness of basic theory of stellar
evolution by comparing
H-R diagram of a real star cluster with H-R
diagram generated by a
computer for group of model stars.
 The procedure is as follows:
Construct computer models for stars with a number of different
masses
Let each model star with its different mass "evolve" on the computer
Plot the resulting evolutionary track of each model star on an H-R
diagram
Draw a line that connects the points on each track that have the same
age
For example, at t = 0, all stars will lie on the main-sequence
At some later time, the high-mass stars will have evolved
slightly off the main-sequence
At a still later time, some of the most massive stars will have
become red giants
Lines so determined = "isochrones", that is, lines of equal age
 In a real star cluster, we assume all stars form at approximately the same
time
At birth, all stars lie on the main-sequence
As real stars age, plotted positions on the H-R diagram shift
 We test to see whether our calculated H-R diagrams look like real H-R
diagrams
 We find that they do, implying our theory is correct.
Measuring the Age of a Group of Stars
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
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Construct an H-R diagram for the group whose age we seek
Compare the real H-R diagram to the computer generated H-R diagrams
Look for a computer H-R diagram of the same shape as the real one
Read off its computer-determined age. That will be the age of the real star
group
STELLAR REMNANTS
 The end stages in a star's life
White Dwarfs
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Formed when stars like Sun reach end of life
R about 0.01 RSun = about REarth
M about MSun
No nuclear burning - shine by residual heat
Will cool and grow dimmer
Structure of White Dwarfs
 Density about 106 gm/cm3 - roughly 16 tons/cubic inch
 At such compressions, material does not behave like ordinary gas degenerate
Degenerate gas is much less compressible
Pressure does not depend on Temp
 Adding material raises gravitational attraction of body on itself - BUT
Interior pressure supporting star rises less.
 Thus P insufficient to support star if mass made too large
Maximum mass = Chandrasekhar Limit = 1.4 MSun
White Dwarfs in Binary Systems: Novas and Supernovas of
Type I
 White Dwarf with a companion star may attract matter from companion
New matter is rich in hydrogen
Matter accumulates on White Dwarf and eventually burns explosively
Nova Explosion
Nova outbursts may recur
 If too much mass accumulates, mass of white dwarf exceeds
Chandrasekhar Limit and star collapses
 Collapse compresses and heats interior of White dwarf Triggers nuclear burning of carbon and oxygen - star explodes
Supernova Explosion - Type I
Neutron Stars
 Form when massive star's iron core collapses and triggers supernova
explosion
 Radius - 10 km
 Mass about 1 to 3 MSun
 Collapse to tiny radius increases rotation rate
(by Conservation of Angular Momentum)
 Collapse also amplifies any magnetic field in star
 Combination of rotation and magnetic field - radiation beams from poles
 Beams sweep cross sky - if one points at Earth,
we see burst of radiation each time star spins - pulses
 Spinning neutron star detectable as pulsar
X-ray Binary Stars results if neutron star in binary system
 Mass captured from companion falls toward neutron star, spins
around it as accretion disk before falling onto star
 Gas falling on to star and gas in accretion disk are very hot - may emit xrays
Black Holes
 Core of very massive stars may collapse into black hole when star explodes
as Supernova if core mass greater than about 5 MSun
 Why "black?"
Escape velocity equals speed of light = c
No radiation or matter can escape from such object
 Size of "black hole" measured by its Schwarzschild radius
(about 3 km for star of 1 MSun)
Observing Black Holes
 Black holes in binary star systems may capture mass from companion
Gas forms accretion disk around black hole before falling in
Orbital motion heats gas to 10 million K - emits x-rays
Gravitational Waves from Double Compact Stars
Hawking Radiation
 Black holes may emit tiny amounts of radiation
 "Quantum" process may permit hole to convert some of its mass into
radiation
 Loss of mass leads to evaporation of hole
 Process extremely slow - 1067 years or so for solar mass black hole
THE MILKY WAY
 Refers to the band of light made up of millions of faint stars that we see
in the night sky
 Band is our galaxy
Basic Structure of Milky Way
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Disk and spiral arms winding out from central regions
Bulge of older stars clustered around center of disk
Halo - swarm of much older stars completely surrounding disk
Dark Matter Halo - vastly larger and more massive - composition unknown
Nucleus - the innermost core
Sun about 1/2 to 2/3 way out in disk between two spiral arms
Basic Properties of Milky Way
 Diameter - about 100,000 ly for visible disk
 Mass - greater than a few times 1011 MSun
 Disk rotates - near Sun takes about 240 million years
Velocity of rotation is about 220 km/sec
Contents
 Stars and Interstellar matter
Interstellar matter is gas (mostly H and He) plus tiny dust particles
 Entire assemblage held together by gravity - each star and gas cloud follows
own orbit
Age of the Milky Way
 About 15 billion years (deduced from age of oldest stars)
How do We Know the Shape, Size, and Mass of the Milky
Way?
 Shape - appearance on the sky as a narrow band of stars.
Band implies disk shape not sphere shape.
 Mass - Treat Milky Way as giant binary star with Sun as one star and rest
of Milky Way as other star. Use modified form of Kepler's Third
Law
 Size and Sun's Location
Worked out by Harlow Shapley in 1920's
He observed globular clusters: Noted they outline Milky Way on sky.
Measured distances to clusters using variable stars in them
Plotted Clusters in a scale drawing of system
Noted that Sun lies off center
Measured size using scale of drawing.
Two Stellar Populations: Pop I and Pop II
 Pop I tend to occur in disk and move along roughly circular orbits
 Pop II are very old, tend to be red, occur in halo, move along more
elongated orbits
 Pop I contain higher proportion of heavy elements than Pop II
Star Clusters
 Two main types - open and globular
Open - few hundred stars, loosely clumped - youngest stars found in
them
Globular - 105 - 106 - densely clumped - oldest stars found in them
Gas and Dust in the Milky Way
 Interstellar Dust: Obscuration and Reddening - limits view of galaxy
In disk can only see a few thousand light years
Clouds contain much dust - block light of background stars
(Dark nebulas)
 Interstellar Gas
Visible Emission from Interstellar Gas
Gas near young luminous, hot stars heated and glows
(Emission nebulas)
Radio Waves From Cold Interstellar Gas
Useful for mapping the structure of Milky Way because radio
waves penetrate dust
The Galactic Center - The Nucleus of Milky Way
 Invisible at optical wavelengths - Too much dust
 Study at radio, infrared, and gamma ray wavelengths
 Large number of stars packed densely - about 1/1000 separation near Sun
 Rapid orbital motion of stars and gas around core deduced from spectra
 Black hole? - If so, M about million solar masses
 How Formed?
Starts small - few solar masses
Grows by collecting gas
Eventually gets big enough to "eat" stars
History and Formation of Milky Way
 (Offers not only picture of galaxy's birth but origin of two stellar
populations)
 Intergalactic gas cloud - pure H and He, slow spin
 Collapses under its gravity - some stars of pure H and He form in collapsing
gas
 Collapse is slow; takes several 100 million years
 Massive stars that form evolve, generate heavy elements in core and die
Explode as supernova and add heavy elements to gas
 Additional stars form in collapsing gas, but now contain some heavy
elements
These become Pop II stars
 "Polluted" gas collects in disk - result of cloud's rotation
 Spiral arms form as "density wave"
 Clouds in arms collapse and form  Second generation of stars in disk - Pop I
 Model requires modification now because of evidence that Milky Way has
"swallowed" a number of smaller galaxies.
OTHER GALAXIES
 Galaxies are immense star systems held together by the mutual gravity of
the member stars
Galaxy Types
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Spirals (S) - like Milky Way - Disk systems with Spiral arms
Ellipticals (E) - no arms - smooth distribution of stars
Irregulars (Irr) - no obvious order or structural pattern
Barred Spirals (SB) - like S type, but with arms winding out from end of bar
S0 - disk systems with no spiral structure
Properties of Galaxy Types
 S - mix of stars (about 85%) and interstellar matter (about 15%)
Stars are mix of Pop I ("young") and Pop II (old)
 E - mostly Pop II - generally no "cold" interstellar matter.
Thus, nothing to make new stars from
Why different Types?
 Answer not known for sure
 Possibilities include
Amount of rotation of gas cloud from which galaxy formed
(rapid rotation leads to S's; slow rotation to E's)
Rate of star formation in parent gas cloud
(rapid formation leads to E's; slow to spirals)
Amount of random motion in gas
(Lots leads to S's; little to E's)
Rate of rotation of dark matter halo.
(rapid rotation leads to S's; slow rotation to E's)
Measuring Distances to Galaxies and the Hubble Law
 In early 1900's galaxies found to be receding from Milky Way
Deduced from spectra which show redshift,
implying motion away from observer
Redshift observed in all but a very few nearby galaxies
 Redshift larger for dimmer galaxies
Recession is faster for more distant galaxies
To derive law for velocity, need distance of galaxy
 To find distance to galaxy
For nearby galaxies - use inverse-square law and method of standard
candles
Pick an object of known brightness (Cepheid variable, planetary
nebula, supernova).
Measure how bright it looks.
Compare to known brightness to get distance
 Recession velocity related to distance by simple law
Vel = constant x distance
The Hubble law - now written as
V = HD
H determined from nearby galaxies with D and V measured
independently
Value of H is uncertain. If V measured in km/sec and D in Mpc,
H = 50 km/sec/Mpc or as much as 100 km/sec/Mpc
 Law implies
Universe is expanding
Distance to galaxy can be found from its velocity
 Ex: Suppose measured V = 3000 km/sec and H is 75 km/sec/Mpc
V=HD. Therefore, D= V/H = 3000/75 = 40 Mpc.
ACTIVE GALAXIES
 Small, luminous nucleus, often variable, spectra imply core filled
with hot gas moving at high velocity
Types
 Seyferts - spirals - small luminous nucleus
 Radio Galaxies - E's strong radio emission at core and outside galaxy
in "lobes"
Often narrow intense jet point from core to lobes
 Quasars - appear star like - hard to tell type of galaxy
Immense distance inferred from large redshift of spectrum
(some as much as 10 billion ly)
High luminosity (about 1000 L Milky Way) inferred from
visibility at their large distance
Model to Explain Activity

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Model must explain high energy output from small region
Hypothesis - Massive (108 solar mass) black hole in core
Gas collects around black hole in accretion disk and is heated
Gas "boils off" disk to form jets
Disk may help channel gas into narrow streams
Evidence supporting model
High velocity of gas and stars in core implies large mass
Mass so inferred too large to be anything but black hole
Galaxy Clusters
 Groups of galaxies held together by mutual gravity
Types
 Rich and poor
 Poor contain dozen to hundred or so members - members often spirals
Milky Way belongs to a Poor cluster - the Local Group
 Rich contain hundreds to thousands of member galaxies - typically
ellipticals
 Present evidence is that galaxies form in small groups which then are drawn
together by their mutual gravity to form larger, richer systems.
 Galaxy clusters show evidence of dark matter - member galaxies move
faster than explainable on the basis of their luminous material
 Galaxy Clusters contain huge amounts of hot (million K) gas - detected by
its x-ray emission - in fact, clusters contains more mass in the form of hot
gas than stars.
COSMOLOGY
 The Structure and Evolution of the Universe
Basic Observations of the Universe
 Distribution of Galaxies - spread approximately uniformly
 Motion of Galaxies
Receding from one another as expressed by the Hubble Law
Note: Galaxies are carried apart by the expansion of space itself. They
are NOT flying away from each other the way fragments of a bomb do.
 The Cosmic Background Radiation
Weak radiation coming from all directions in space
Temp of radiation approx 3K above absolute zero
 Uniformity of hydrogen and helium abundance (about 71%, 27%
respectively)
Conclusions Deduced from the Basic Observations of the
Universe
 Galaxies were once closer together than now. That is, Universe smaller
in past than now
 Universe was therefore denser and thus compressed
 Universe was once very hot - compression heats
T ≈ 10 Trillion K
 Whole Universe packed into region approx size of Solar System
 Universe expands from this hot dense state in Big Bang
 We are not at the Center of the Universe
All points move away from all other points
Big Bang occurred about 15 billion years ago
 Deduced from time for galaxies to move apart.
 Expansion cools Universe and leaves only low temperature radiation that we
see as the CMB.
Evolution of the Universe: Expansion forever or Recollapse?
 Fate of Universe determined by whether its total mass is sufficient to exert
enough gravity to stop expansion. Observations suggest too little mass to
stop expansion. Thus,
 Universe probably will expand forever.
 Observation of distant galaxies suggests they are receding more slowly for
their distance than predicted by the Hubble Law - implies expansion is
accelerating.
(Recall that looking out in space = looking back in time)
 Acceleration may be evidence for Cosmological constant - a term
introduced by Einstein into his equations for the structure of the Universe.
Energy contributed by the cosmological constant may dominate the
mass/energy content of the Universe.
 Observations of irregularities (lumps) in the CMB suggest Universe is flat,
as predicted by certain models of the Universe (see below).
The Origin of the Universe
 History of Matter and Radiation in the Early Universe
At Birth (Big Bang) Universe mainly radiation.
Some radiation changes to matter via E = mc2
Forms quarks, anti-quarks, electrons and positrons.
 Expansion cools and some matter stabilizes as quarks turn into protons and
neutrons.
 Cooling continues with expansion. Some protons fuse into helium, creating
uniformity of H/He ratio.
 More cooling allows electrons to attach to nuclei to form atoms.
 Still more cooling allows gas to form clouds which turn into galaxies.
 Expansion continues - NOW
The Inflationary Universe and Grand Unified Theories
 Universe formed from "false vacuum"
 All forces (gravity, electromagnetism, strong) act as single force
Forces "Unified" - thus Grand Unified Theory (GUT's)
 Young Universe expanded by enormous factor
 Evidence mentioned above about size distribution of lumps in the CMB
support a flat Universe, as predicted by inflationary models.