Download For Chapter 16 on November 26, 2012

Detection Methods in Astronomy
• Much of the incoming solar radiation does not
make it to the Earth’s surface – due to
atmospheric absorption
• Electromagnetic radiation that will pass through
the Earth’s atmosphere can be studied using
ground-based detectors
• Other regions of the electromagnetic spectrum
must be detected by space-based instruments
• The Hubble Space Telescope is a good example of
an instrument outside Earth’s atmosphere
Intro
Earth Motions
• Daily Rotation on its axis (daily cycle)
• Rotation – spin on an internal axis
• Proofs: Foucault pendulum
• Annual revolution around the sun (annual cycle)
• Revolution – movement of one mass around another
• Proofs: stellar parallax and aberration of starlight
• Precession – the slow change of the earth’s
rotational axis (now at 23.5o) – see more info in
chapter 15
Section 16.3
Experimental Proof of the Earth’s Rotation:
Foucault Pendulum
The pendulum does not
rotate with reference to
the fixed stars.
Section 16.3
Parallax
• Parallax – the apparent motion, or shift, that
occurs between two fixed objects when the
observer changes position
• Parallax can be seen with outstretched hand
• The motion of Earth as it revolves around the
Sun leads to an apparent shift in the positions of
the nearby stars with respect to more distant
stars
Section 16.3
Stellar Parallax
• The observation of
parallax is
indisputable proof that
the Earth revolves
around the Sun.
• In addition, the
measurement of the
parallax angle is the
best method we have
of determining the
distance to nearby
stars
Section 16.3
Aberration of Starlight
• A 2nd proof of Earth’s orbital motion
• Telescopic observations of a systematic change
in the position of all stars annually
• Due to the motion of the Earth around the Sun
• Angular discrepancy between the apparent
position of a star and its true position, arising
from the motion of an observer relative to the
path of the beam of light observed
• This is similar to what you see when driving in the
rain
Section 16.3
Aberration of Starlight
• This discrepancy is very small and is measured
in a -- parsec
• Parsec  parallax + second
• Recall that a circle contains 360o. Each degree
is divided into 60 minutes, and each minute into
60 seconds
• Therefore 1 second = 1/3600 degree
• Parsec = the distance to a star when the star
exhibits a parallax of 1 second.
Section 16.3
Terrestrial Planets
• The terrestrial planets include: Mercury, Venus,
Earth, Mars
• Due to physical/chemical characteristics they
resemble Earth
• All four terrestrial planets are
• Relatively small in size and composed of rocky
material and metals
• Relatively close together and close to Sun
• Have no rings
• Only Earth and Mars have moons
• Only Earth has surface water and oxygen
Section 16.4
Mercury
• Mercury is the closest planet to the Sun
• Mercury has the shortest period of revolution (88
days), and is the fastest moving
• Temperatures on Mercury range from about
473oC on the side facing the Sun to about 173oC on the dark side
• Due to its small size and closeness to the Sun,
Mercury has practically no atmosphere
• It has a density close to that of Earth.
Section 16.4
Mercury, a Terrestrial Planet
The Messenger
spacecraft mapped the
surface of Mercury. It
was the first
spacecraft to orbit the
planet.
Rotates 3 times while circling the Sun twice
Section 16.4
Venus
• Venus is the closest planet to
Earth
• Venus is the third brightest
object in the sky
• The surface of Venus cannot
be seen from Earth, due to
dense, thick clouds that cover
the planet
• Magellan radar images indicate
that the surface of Venus is
composed of black, hot rock
• Most surface rocks appear
to be volcanic
Section 16.4
The Atmosphere of Venus
• Venus’ atmosphere is composed of 96% CO2
• It is so dense that the surface of Venus has a pressure
of 90 atm
• Atmosphere rotates faster than solid planet.
• Retrograde rotation of planet
• The large percent of CO2 in the atmosphere results in
high surface temperatures (477o C) due the
“greenhouse effect”
• Radar images have revealed relatively few impact
craters
• Most of these craters are fairly large, because the
smaller incoming objects are consumed by Venus’s
thick atmosphere
Section 16.4
Mars
• Mars has a red color, as viewed from the Earth,
and was named for the Roman god of war
• The surface of Mars has two outstanding
features that have intrigued scientists for
decades; polar ice caps and extinct
volcanoes
• The ice caps are composed of frozen CO2 in
the winter and CO2 vapor with frozen water in
the summer
• The red color is thought to be due to fine grain
iron oxide minerals.
Section 16.4
Factoids on Mars
Mount Olympus, the largest known
volcano in the solar system, at 24 km
in height, it is about three times that of
Mauna Loa
Mars at closest approach
Mars – Valles Marineris
This canyon on Mars is
4000 km in length and
6 km deep
Geologists think that it
is a crustal fracture
caused by internal
forces
Section 16.4
In 2006 the Opportunity rover reached the edge
of Victoria crater
Section 16.4
The Jovian Planets
• Jupiter, Saturn, Uranus, Neptune
• Much larger than the terrestrial planets
• Composed mainly of hydrogen and helium
• The four Jovian planets have a very low average
density (approximately 1.2 g/cm3)
• All four are thought to have a rocky core
composed of iron and silicates
• Thick layers of frozen methane, ammonia, and
water are found above the core
Section 16.5
Formation of the Terrestrial Planets
• The two least massive elements – H & He –
were the most abundant when the planets
started to coalesce about 5 billion years ago
• Due to the heat from the Sun most of these less
massive elements escaped the gravitational pull
of the inner planets
• Leaving behind more of the massive elements and
resulting in thick rocky cores and higher densities for
the inner planets
Section 16.5
Formation of the Jovian Planets
• The four large outer planets were much farther
from the Sun and therefore much colder
• The Jovian planets retained most of their H and
He which now surround their ice layers and
innermost rocky cores
• As a consequence the Jovian planets have a
much lower average density
Section 16.5
Jupiter
• Largest planet of the solar system, in both volume and
total mass
• Named after the supreme Roman god of heaven
because of its brightness and giant size
• Diameter is 11 times Earth’s -- 318 times more mass
than Earth
• The average density of Jupiter approximately 1.3 g/cm3
• Jupiter is covered with a thin layer of clouds composed
of hydrogen, helium, methane, ammonia, and several
other substances
• It is a fast rotator – taking about 10 hours to rotate.
Section 16.5
Jupiter
Section 16.5
Jupiter’s Great Red Spot (“eye”)
• The Great Red Spot
has erratic movement,
shape, color, and size –
sometimes even
disappearing
• Likely a huge
counterclockwise
“hurricane-like” storm,
lasting hundreds of
years
Section 16.5
Saturn
• Distinctive system of three prominent rings
• Rings are inclined 27o to orbital plane
• The rings are thought to be composed of ice and
ice-coated rocks (micrometers  10 m)
• Most spectacular sight that can be viewed from
Earth with a small telescope
• Diameter is 9 times Earth’s -- 95 times more
mass than Earth
• Average density of only 0.7 g/cm3
Section 16.5
Saturn and its rings
Section 16.5
Uranus
•
•
•
•
Discovered in 1781 by William Herschel (1738-1822), an English
Astronomer
Thin ring system composed of boulder-size particles (>1m), with very little
dust-size
Average density of only 1.3 g/cm3
Unlike the other planets Uranus revolves around the Sun on its side and
rotates in a retrograde fashion
Section 16.5
Neptune
• Discovered in 1846 by Johann Galle, a German
astronomer
• Englishman John Couch Adams and Frenchman U.J.J.
Leverrier were mathematicians using Newton’s law of
gravitation
• They noted that Uranus’ motion was disturbed and
predicted the location of another planet – this is how
Galle eventually discovered Neptune
• Neptune also has a large dark spot similar to Jupiter’s
and thought to be the result of large wind systems
• Neptune and Uranus are similar in size and in the
composition of their atmospheres
• In many respects these two planets can be considered
twins
Section 16.5
Neptune
Section 16.5
Designations of Celestial Bodies
International Astronomical Union (IAU)
• In 2006 the IAU adopted the following criteria for
a solar system body to be a planet:
•
•
•
•
•
• (1) It must be in orbit about the Sun.
• (2) It must have sufficient mass for self-gravity to
form a nearly round shape.
• (3) It must be the dominant body within its orbit.
The last statement disqualifies Pluto
The IAU established two new categories for objects that
orbit the Sun.
Dwarf planets is one of the categories
Pluto, Ceres, Eris, Haumea and Makemake are now
designated dwarf planets
So far, there are known more than 200 TNOs (Trans
Neptunian Objects) inside the solar system
Section 16.6
Outermost reaches of the Solar
System
• The Kuiper Belt extends just beyond the orbit of
Neptune and into the space of Eris.
• Consists of comet and cometary material and other
small objects – Trans Neptunian Objects
• Many astronomers put the edge of the solar system to
be at about 100 AU.
• Voyager 1, launched in 1977, and in 2004 reached 100
AU. In 2010 it crosses the boundary of zero solar wind
velocity.
• Sedna- the coldest, most distant object in the solar
system: Dr. Mike Brown- CalTech scientist- used data
from the Palomar Observatory to identify it.
Section 16.6
Origin of the Solar System
• Any theory that purports to explain the origin and
development of the solar system must account
for its present form
• According to our best measurements, our solar
system has been in its present state for about
4.5 billion years
• A valid theory for solar system formation – must
be able to explain a number of major properties
of our solar system
Section 16.7
Major Questions
Concerning Solar System
•
•
•
•
•
•
•
•
Origin of material?
Forces that formed the solar system?
Isolated planets, circular orbits, in same plane?
Revolution (orbit) in the same direction?
Most Rotate in same direction (except two)?
Terrestrial versus Jovian planets?
Origin of the asteroids?
Origin of comets and meteoroids?
Section 16.7
The Formation of the Solar System
Condensation Theory
•Began with a large, swirling
volume of cold gases and dust –
a rotating solar nebula
•Contracted under the influence
of its own gravity – into
a flattened, rotating disk
•Further contraction produced
the protosun and eventually
accreted the planets
•As particles moved inward,
the rotation of the mass had to
increase to conserve angular
momentum (refer to the ice
skater analogy)
Section 16.7
Other Planetary Systems
• Are there other planetary systems in the
universe?
• If so, we would expect to find some of these
systems in different stages of formation
• In other words, we should be able to find clouds of
gas and dust, primordial nebula, and protosuns, etc.
• We should also be able to use gravitational
effects to detect small wobbles due to rotational
objects in space
• These are called exoplanets or extra-solar
planets
Section 16.8
Gravitational Effects
• A star with a large planet orbiting about it will
have a small wobble superimposed on its motion
as a result of gravitational effects
• This change in motion (the wobble) is likely to be
very slight, but in some cases may be detected
as a Doppler shift of the star’s spectrum
• As the star approaches the observer, the
wavelengths are compressed (‘blue shift’)
• As the star move away from the observer, the
wavelengths are lengthened (‘redshift’)
Section 16.8
Gravitational Effects
• The amount of wobble can be used to determine
the planet’s mass (related to gravitational pull)
• The wobble’s cycle time can be used to
determine the orbital period
• Once the orbital period is known, Kepler’s third
law (T2= kR3) can be used to determine the
planet’s average distance from the star
Section 16.8
Star Wobble
Due to the gravitational pull of an orbiting planet.
Section 16.6
The wobbling in this illustration is greatly exaggerated!
Section 16.8
Transit method
• A planet passing in front of its star as seen from
Earth
• The star’s light will temporarily dim
• The Kepler mission uses the transit method.
Section 16.8
First Planets
Discovered Beyond our Solar System
• In 1992, using the Arecibo Observatory in
Puerto Rico, astronomers reported the discovery
of two objects revolving about a pulsar
• Pulsars are very dense, rapidly rotating stars
• Pulsars have a very precise rotation period
• If the rotation period is disrupted, this would indicate
the presence of an object rotating about the pulsar
• These two objects are the first planets detected
beyond our solar system
Section 16.8