Instructor:
Ted Eltzroth
Office: Room 231
ATC
Phone #
847-214-7324
e-mail:teltzroth@elgin.edu
Office hours
are posted outside ATC 231
Reference:
Textbook: The Essential
Cosmic Perspective Bennett et al. 4th ed.
Field Reference: Starry
Night Enthusiast Digital Download 5.7
Online Reference: NASA/JPL Planets
Online Reference: Several Sites
Online Reference: Review of the
Universe
Class Lecture Material:
Powerpoint
Files
Grades
are based on:
Four Hour Exams: 20%
per exam
Laboratory/Online Work:
20%
Course
Objectives:
Introduction to
Astronomy.
Development of
observational skills in Astronomy.
Development of
laboratory techniques and scientific methods.
Student
Responsibilities:
Attendance and
participation in class discussion is required.
Five unexcused absences
will reduce your points total by 5%.
Disruptive behavior
will result in dismissal from the class and/or course.
Withdrawal from the
class if necessary needs to be completed by week 10.
Prior to the test date
notice is required to schedule a makeup exam, emergency situations are
exceptions.
Classroom and
laboratory protocol is safety, respect and fun in that order.
Student
with Disabilities:
ECC welcomes students
with disabilities and is committed to supporting them as they attend college.
If a student has
a disability (visual, aural, speech, emotional/psychiatric, orthopedic, health,
or learning), s/he may be entitled to some accommodation, service,
or support. While the College will not compromise or waive essential skill
requirements in any course or degree, students
with disabilities may be supported with accommodations to help meet these
requirements. The laws in effect at college level
state that a person does not have to reveal a disability, but if support is
needed, documentation of the disability must be
provided. If none is provided, the college does not have to make any exceptions
to standard procedures. All
students are expected to comply with the Student Code of Conduct and all other
college procedures as stated in the
current College Catalog.
Procedure For
Requesting Accommodations:
1. Go to SRC108 and
sign release to have documentation sent to the college, or bring in
documentation.
2. Attend an
appointment that will be arranged for you with the ADA coordinator.
3. If you
have questions, call 847-214-7220 (TTY 847-214-7392) or e-mail Annabelle Rhoades
arhoades@elgin.edu
Link to
Course Outcomes List
Chapter
Concepts
The
universe is comprehensible. Planets
in Detail
The scientific method
is a procedure for formulating theories that correctly predict how the universe
behaves.
A
scientific theory must be testable, that is, capable of being
disproved.
Theories are tested and
verified by observation or experimentation and result in a process that often
leads to their refinement or replacement and to the progress of
science.
Observations of the
heavens have led astronomers to discover some fundamental physical laws of the
universe.
Space debris collected
on Earth, as well as particles from space, such as neutrinos and cosmic rays,
and gravitational radiation are providing astronomers with new insights into the
universe.
Origins of a
Sun-Centered Universe
The ancient Greeks laid
the groundwork for progress in science.
Early Greek astronomers
devised a geocentric cosmology, which placed the Earth at the center of the
universe.
Copernicus's
heliocentric (Sun-centered) theory simplified the general explanation of
planetary motions compared to the geocentric theory.
In a heliocentric
cosmology, the Earth is but one of several planets that orbit the Sun.
The sidereal orbital
period of a planet is measured with respect to the stars.It determines the
length of the planet's year.
Its synodic period is measured with respect to
the Sun as seen from the moving Earth (for example, from one opposition to the
next).
Kepler's and
Newton's Laws
Ellipses describe the
paths of the planets around the Sun much more accurately than do
circles.
Kepler's three laws give important details about elliptical
orbits.
The
invention of the telescope led Galileo to new discoveries, such as the phases of
Venus and the moons of Jupiter, that supported a heliocentric view of the
universe.
Newton based his
explanation of the universe on three assumptions now called Newton's laws of
motion.
These laws and his law of universal gravitation can be used to deduce
Kepler's laws and to describe planetary motions with extreme accuracy.
The mass of an object
is a measure of the amount of matter in the object; its weight is a measure of
the force with which the gravity of some other object pulls on the object's
mass.
In
general, the path of one astronomical object about another, such as that of a
comet about the Sun, is an ellipse, a parabola, or a
hyperbola.
The Nature of
Light
Photons, compact units
of vibrating electric and magnetic fields, all carry energy through space at the
same speed, "the speed of light" (3x108 m/s in a vacuum).
Radio waves, infrared,
visible light, ultraviolet radiation, X rays, and gamma rays
are all forms of electromagnetic radiation.
Visible light occupies
only a small portion of the electromagnetic spectrum.
The wavelength of a
visible light photon is associated with its color.
Wavelengths of visible
light range from about 400 nm for violet
light to 700 nm for red light.
Infrared radiation and
radio waves have wavelengths longer than those of visible light.
Ultraviolet
radiation, X
rays, and gamma rays have wavelengths that are shorter.
Optics and
Telescopes
A telescope's most
important function is to gather as much light as
possible.
Its second function is to reveal the observed object in as much
detail as possible.
Often the least important function of a telescope is to
magnify objects.
Refracting telescopes,
or refractors, produce images by bending light rays
as they pass through glass lenses.
Glass impurity, opacity
to certain wavelengths, and structural difficulties make it inadvisable to build
extremely large refractors.
Reflecting telescopes,
or reflectors, produce images by reflecting light rays from concave mirrors to a
focal point or focal plane.
Reflectors are not subject to many of the
problems that limit the usefulness of refractors.
Telescopes that employ
advanced technologies, such as active or adaptive optics, produce extremely
sharp images.
Charge-coupled devices
(CCDs) are used at a telescope's focal point to record images.
Earth-based telescopes
are being built with active and adaptive optics.
These advanced
technologies yield resolving power comparable to the Hubble Space
Telescope.
Radio
Astronomy-and Beyond
Radio telescopes have
large reflecting antennas (dishes) that are used to focus radio waves.
Very sharp radio images
are produced with arrays of radio telescopes linked together in a technique
called interferometry.
The Earth's atmosphere
is transparent to most visible light and radio waves, along with some infrared
and ultraviolet, but it absorbs much of the electromagnetic radiation at other
wavelengths.
For observations at
other wavelengths, astronomers depend upon telescopes carried above the
atmosphere by rockets and satellites.
Satellite-based observatories are
giving us a wealth of new information about the universe and permit coordinated
observation of the sky at all wavelengths.
The Solar
System
Hydrogen, helium, and
traces of lithium, the three lightest elements, were formed shortly after the
creation of the universe.
The heavier elements were produced much later by
stars and cast into space when the stars died.
By mass, 98% of the matter in
the universe is hydrogen and helium.
The solar system formed
4.6 billion years ago from a swirling, disk-shaped cloud of gas, ice, and dust
called the solar nebula.
The four inner planets
formed through the accretion of dust particles into planetesimals and then into
larger protoplanets.
The four large outer planets probably formed through the
runaway accretion of gas onto rocky protoplanetary cores.
The Sun formed at the
center of the solar nebula.
After about 100 million
years, the temperature at the protosun's center was high enough to ignite
thermonuclear reactions.
For 800 million years after the Sun formed, impacts
of asteroid like objects on the young planets dominated the history of the solar
system.
The
four inner planets of the solar system share many characteristics and are
distinctly different from the four giant outer planets and from Pluto.
The four inner,
terrestrial planets are relatively small, have high average densities, and are
composed primarily of rock and metal.
Jupiter and Saturn have
large diameters and low densities and are composed primarily of hydrogen and
helium.
Uranus and Neptune have large quantities of water as well as much
hydrogen and helium.
Pluto, the smallest of the nine planets, is of
intermediate density owing to its rock-ice composition.
Asteroids are rocky and
metallic debris in the solar system larger than about a kilometer in
diameter.
Meteoroids are smaller pieces of such debris.
Comets are debris
that contain both ice and rock.
Astronomers have
observed disks of gas and dust orbiting young stars.
At least 80 extrasolar
planets have been discovered orbiting other stars.
Earth: A Dynamic,
Vital World
The Earth's atmosphere
is about four-fifths nitrogen and one-fifth oxygen.
This abundance of oxygen
is due to the biological processes of life-forms on the planet.
The Earth's atmosphere
is divided into layers named the troposphere, stratosphere, mesosphere, and
thermosphere.
Ozone molecules in the stratosphere absorb ultraviolet light
rays.
The
outermost layer, or crust, of the Earth offers clues to the history of our
planet.
The
Earth's surface is divided into huge plates that move over the upper
mantle.
Movements of these plates, a process called plate tectonics, are
caused by convection in the mantle, upwelling of molten material along cracks in
the ocean floor produces seafloor spreading.
Plate tectonics is responsible
for most of the major features of the Earth's surface, including mountain
ranges, volcanoes, and the shapes of the continents and oceans.
Study of seismic waves
(vibrations produced by earthquakes) shows that the Earth has a small, solid
inner core surrounded by a liquid outer core.
The outer core is surrounded by
the dense mantle, which in turn is surrounded by the thin, low-density
crust.
The Earth's inner and outer cores are composed primarily of iron with
some nickel mixed in.
The mantle is composed of iron-rich minerals.
The Earth's magnetic
field produces a magnetosphere that surrounds the planet and blocks the solar
wind.
Some
charged particles from the solar wind are trapped in two huge, doughnut-shaped
rings called the Van Allen radiation belts.
A deluge of particles from a
coronal mass ejection by the Sun can initiate an auroral
display.
The Moon and
Tides
The Moon has
light-colored, heavily cratered highlands and dark-colored, smooth-surfaced
Maria.
Many
lunar rock samples are solidified lava formed largely of minerals also found in
Earth rocks.
Anorthositic rock in
the lunar highlands was formed between 4.0 and 4.3 billion years ago, whereas
the mare basalts solidified between 3.1 and 3.8 billion years ago.
The Moon's
surface has undergone very little geological change over the past 3 billion
years.
Impacts
have been the only significant "weathering" agent on the Moon; the Moon's
regolith (pulverized rock layer) was formed by meteoritic action.
Lunar rocks
brought back to Earth contain no water and are depleted of volatile
elements.
Frozen water has been
discovered at the Moon's poles.
The collision-ejection
theory of the Moon's origin holds that the young Earth was struck by a huge
asteroid, and debris from this collision coalesced to form the Moon.
The Moon was molten in
its early stages, and the anorthositic crust solidified from low-density magma
that floated to the lunar surface.
The mare basins were created later by the
impact of planetesimals and were then filled with lava from the lunar
interior.
Gravitational and
centrifugal interactions between the Earth and the Moon produce tides in the
oceans of the Earth and set the Moon in synchronous rotation.
The Moon is
moving away from the Earth, and the Earth's rotation rate is
decreasing.
Outer planets composed
primarily of gasses and metal are classified as Gas Giants or
Jovian.
Mercury
Even at its
greatest orbital elongations, Mercury can be seen from Earth only briefly after
sunset or before sunrise.
The Mariner 10
spacecraft passed near Mercury in the mid-1970s, providing pictures of its
surface.
The Mercurian surface is pocked with craters like the Moon's, but
extensive, smooth plains lie between these craters.
Long cliffs meander
across the surface of Mercury.
These scarps probably formed as the planet
cooled, solidified, and shrank.
The long-ago impact of
a large object formed the huge Caloris Basin on Mercury and shoved up jumbled
hills on the opposite side of the planet.
Mercury has an iron
core much like that of the Earth.
Venus
Venus is similar to the
Earth in size, mass, and average density, but it is covered by unbroken, highly
reflective clouds that conceal its other features from Earth-based
observers.
While most of Venus's
atmosphere is carbon dioxide, its dense clouds contain droplets of concentrated
sulfuric acid mixed with yellowish sulfur dust.
Active volcanoes on Venus may
be a constant source of this sulfurous veil.
Venus's exceptionally
high temperature is caused by the greenhouse effect, as the dense carbon dioxide
atmosphere traps and retains heat emitted by the planet.
The surface pressure
on Venus is 90 atm, and the surface temperature is 750 K.
Both temperature
and pressure decrease as altitude increases.
The surface of Venus is
surprisingly flat, mostly covered with gently rolling hills.
There are two
major "continents" and several large volcanoes.
The surface of Venus shows
evidence of local tectonic activity but not the large-scale motions that play a
major role in continually reshaping the Earth's surface.
Mars
Earth-based
observers found that the Martian solar day is nearly the same as that of the
Earth, that Mars has polar caps that expand and shrink with the seasons, and
that the surface undergoes seasonal color changes.
A century ago observers
reported networks of linear features that many perceived as canals.
These
observations led to speculation about self-aware life on Mars.
The Martian surface has
many flat-bottomed craters, several huge volcanoes, a vast equatorial canyon,
and dried-up riverbeds-but no canals formed by intelligent life.
Flash-flood
features and dry riverbeds on the Martian surface indicate that large amounts of
water once flowed there.
Liquid water would
quickly boil away in Mars's thin present-day atmosphere, but the planet's polar
caps contain some frozen water, and a layer of permafrost may exist beneath the
regolith.
The
Martian atmosphere is composed mostly of carbon dioxide.
The surface pressure
is less than 0.01 atm.
Chemical reactions in
the regolith together with ultraviolet radiation from the Sun apparently act to
sterilize the Martian surface.
Mars has no global
magnetic fields, but local fields pierce its surface in at least nine
places.
Features that may be
fossil remains of bacteria have been found in several meteorites that are
believed to have come from Mars.
Mars has two
potato-shaped moons, the captured planetesimals Phobos and Deimos.
Both are
in synchronous rotation with Mars.
Jupiter and
Saturn
Jupiter is by far the
largest and most massive planet in the solar system.
Jupiter and Saturn
probably have rocky cores surrounded by a thick layer of liquid metallic
hydrogen and an outer layer of ordinary liquid hydrogen.
Both planets have an
overall chemical composition very similar to that of the Sun.
The visible features of
Jupiter exist in the outermost 100 km of its
atmosphere.
Saturn has similar features, but they are much fainter.
Three
cloud layers exist in the upper atmospheres of both Jupiter and
Saturn.
Because Saturn's cloud layers extend through a greater range of
altitudes, the colors of the Saturnian atmosphere appear muted.
The colored ovals
visible in the Jovian atmosphere represent gigantic storms, some of which (such
as the Great Red Spot) are stable and persist for years or even
centuries.
Jupiter and Saturn have
strong magnetic fields created by electric currents in the metallic hydrogen
layer.
Four
large satellites orbit Jupiter.
The two inner Galilean moons, Io and Europa,
are roughly the same size as our Moon.
The two outer moons, Ganymede and
Callisto, are approximately the size of Mercury.
Io is covered with a
colorful layer of sulfur compounds deposited by frequent explosive eruptions
from volcanic vents.
Europa is covered with a smooth layer of frozen water
crisscrossed by an intricate pattern of long cracks.
The heavily cratered
surface of Ganymede is composed of frozen water with large polygons of dark,
ancient crust separated by regions of heavily grooved, lighter-colored, younger
terrain.
Callisto has a heavily cratered ancient crust of frozen
water.
Saturn
is circled by a system of thin, broad rings lying in the plane of the planet's
equator.
Each major ring is composed of a great many narrow ringlets
consisting of numerous fragments of ice and ice-coated rock.
Jupiter has a
much less substantial ring system.
Uranus and
Neptune
Uranus and Neptune are
quite similar in appearance, mass, size, and chemical composition.
Each has a
rocky core probably surrounded by a dense, watery mantle; the axes of their
magnetic fields are steeply inclined to their axes of rotation; and both planets
are surrounded by systems of thin, dark rings.
Uranus is unique in
that its axis of rotation lies nearly in the plane of its orbit, producing
greatly exaggerated seasons on the planet.
Uranus has five
moderate-sized satellites, the most bizarre of which is Miranda.
The largest satellite
of Neptune, Triton, is an icy world with a tenuous nitrogen
atmosphere.
Triton moves in a retrograde orbit that suggests it was captured
into orbit by Neptune's gravity, and its orbit is spiraling down toward
Neptune.
Pluto and
Beyond
Pluto, the smallest
planet in the solar system, and its satellite, Charon, are icy worlds that may
well resemble Triton.
Other objects orbit the
Sun beyond the orbit of Pluto.
Hundreds of
these Kuiper belt objects have been observed.
Asteroids
Tens of
thousands of belt asteroids with diameters larger than a kilometer are known to
orbit the Sun between the orbits of Mars and Jupiter.
The gravitational
attraction of Jupiter depletes certain orbits within the asteroid belt.
The
resulting gaps, called Kirkwood gaps, occur at simple fractions of Jupiter's
orbital period.
Jupiter's and the Sun's
gravity combine to capture Trojan asteroids in two locations, called stable
Lagrange points, along Jupiter's orbit.
The Apollo asteroids
move in highly elliptical orbits that cross the orbits of Mars and
Earth.
Many of these asteroids will eventually strike one of the inner
planets.
Comets
Comets are
fragments of ice and rock that generally move in highly elliptical orbits about
the Sun often at a great inclination to the plane of the ecliptic.
As a comet approaches
the Sun, its icy nucleus develops a luminous coma surrounded by a vast hydrogen
envelope.
A gas (or ion) tail and a dust tail extend from the comet, pushed
away from the Sun by the solar wind and radiation pressure.
Many comets orbit the
Sun in the Kuiper belt, a doughnut-shaped region beyond Pluto.
Billions of
cometary nuclei are also believed to exist in the spherical Oort cloud located
far beyond Pluto.
Meteoroids,
Meteors, and Meteorites
Boulders and smaller
rocks in space are called meteoroids.
When a meteoroid enters the Earth's
atmosphere, it produces a fiery trail, and it is then called a meteor.
If
part of the object survives the fall, the fragment that reaches the Earth's
surface is called a meteorite.
Meteorites are grouped
in three major classes according to their composition: iron, stony-iron, and
stony meteorites.
Rare stony meteorites called carbonaceous chondrites may be
relatively unmodified material from the primitive solar nebula.
These
meteorites often contain organic hydrocarbon compounds, including amino
acids.
Fragments of rock from
"burned-out" comets produce meteor showers.
An analysis of the
Allende meteorite suggests that a nearby supernova explosion may have been
involved in the
formation of the solar system some 4.6 billion years ago.
An asteroid that struck
the Earth 65 million years ago probably contributed to the extinction of the
dinosaurs and many other species.
Such devastating impacts occur on average
every 100 million years.
The Sun's
Atmosphere
The visible surface of
the Sun is a layer at the bottom of the solar atmosphere called the
photosphere.
The gases in this layer shine as a blackbody.
Convection of
gas from below the surface produces features there called granules.
Above the photosphere
is a layer of hotter but less dense gas called the chromosphere.
Jets of gas
called spicules rise up into the chromosphere along the boundaries of
supergranules.
The outermost layer of
thin gases in the solar atmosphere is called the corona, which extends outward
to become the solar wind at great distances from the Sun.
The gases of the
corona are very hot but at low density.
The Active
Sun
Some surface features
on the Sun vary periodically in an 11-year cycle, with the magnetic fields that
cause these changes actually varying over a 22-year cycle.
Sunspots are relatively
cool regions produced by local concentrations of the Sun's magnetic field
protruding through the photosphere.
The average number of sunspots and their
average latitude increases and decreases in an 11-year cycle.
A prominence is gas
lifted into the Sun's corona by magnetic fields.
A solar flare is a brief,
but violent, eruption of hot, ionized gases from a sunspot group.
Coronal
mass ejections send out large quantities of gas from the Sun.
Coronal mass
ejections and flares that head our way affect satellites, communication,
electrical power, and cause auroras.
The magnetic dynamo
model suggests that many transient features of the solar cycle are caused by the
effects of differential rotation and convection on the Sun's magnetic
field.
The Sun's
Interior
The Sun's energy is
produced by the thermonuclear process called hydrogen fusion, in which four
hydrogen nuclei release energy when they fuse to produce a single helium
nucleus.
The
energy released in a thermonuclear reaction comes from the conversion of matter
into energy according to Einstein's equation E = mc2.
A stellar model is a
theoretical description of a star's interior derived from calculations based on
the laws of physics.
The solar model suggests that hydrogen fusion occurs in
a core that extends from the Sun's center to about 0.25 solar radius.
Throughout most of the
Sun's interior, energy moves outward from the core by radiative diffusion.
In
the Sun's outer layers, energy is transported to the Sun's surface by
convection.
Neutrinos generated and
emitted by the Sun are detected at a lower rate than is predicted by our
standard model of thermonuclear fusion.
This occurs because neutrinos have
mass; therefore, some of them change into other forms of neutrinos before they
reach Earth.
These alternative forms are now being
detected.
Stellar
Characteristics
Stars differ in size,
luminosity, temperature, color, and chemical composition.
Determining stellar
distances from Earth is the first step to understanding the nature of the
stars.
Distances to the nearer stars can be determined by stellar parallax,
the apparent shift of a star's location against the background stars while the
Earth moves along its orbit around the Sun.
The apparent magnitude
of a star, denoted m, is a measure of how bright the star appears to Earth-based
observers.
The absolute magnitude of a star, denoted M, is a measure of the
star's true brightness and is directly related to the star's energy output, or
luminosity.
The absolute magnitude
of a star is the apparent magnitude it would have if viewed from a distance of
10
pc.
Absolute magnitudes can be calculated from the star's apparent magnitude
and distance.
The luminosity of a
star is the amount of energy emitted by it each second.
Stars vary in their
physical properties-color, temperature, mass, and size-a fact that helps
astronomers understand stellar structure and evolution.
Stellar temperatures
can be determined from stars' colors or stellar spectra.
Stars are classified
into spectral types (O, B, A, F, G, K, and M) based on their spectra.
The
spectral type of a star is directly related to its surface
temperature.
Types of
Stars
The Hertzsprung-Russell
(H-R) diagram is a graph on which luminosities of stars are plotted against
their spectral types (or, equivalently, their absolute magnitudes are plotted
against surface temperatures).
The H-R diagram reveals the existence of four
major groupings of stars: main-sequence stars, giants, supergiants, and white
dwarfs.
The
mass-luminosity relation expresses a direct correlation between a main-sequence
star's mass and the total energy it emits.
Distances to stars can
be determined using their spectral type and luminosity class.
Binary Stars and
Stellar Masses
Binary stars are
surprisingly common.
Those that can be resolved into two distinct star images
by an Earth-based telescope are called visual binaries.
The masses of the two
stars in a binary system can be computed from measurements of the orbital period
and orbital dimensions of the system.
Some binaries can be
detected and analyzed, even though the system may be so distant (or the two
stars so close together) that the two star images cannot be resolved with
Earth-based telescopes.
A spectroscopic binary
is a system detected from the periodic shift of its spectral lines.
This
shift is caused by the Doppler effect as the orbits of the stars carry them
alternately toward and away from the Earth.
An eclipsing binary is
a system whose orbits are viewed nearly edge-on from the Earth, so that one star
periodically eclipses the other.
Detailed information about the stars in an
eclipsing binary can be obtained by studying its light curve.
Mass transfer occurs
between binary stars that are close together.
Protostars and
Pre-Main-Sequence Stars
Enormous, cold clouds
of gas and dust, called giant molecular clouds, are scattered about the disk of
the Galaxy.
Star formation begins
when gravitational attraction causes clumps of gas and dust called protostars to
coalesce within a giant molecular cloud.
As a protostar contracts, its matter
begins to glow.
When the contraction slows down, the protostar becomes a
pre-main-sequence star.
When the pre-main-sequence star's core temperature
becomes high enough to begin hydrogen fusion and stop contracting, it becomes a
main-sequence star.
The most massive
pre-main-sequence stars take the shortest time to become main-sequence stars (O
and B stars).
They emit strong ultraviolet radiation that ionizes hydrogen in
the surrounding molecular cloud, creating reddish emission nebulae called H II
regions.
In
the final stages of pre-main-sequence contraction, when hydrogen fusion is about
to begin in the core, pre-main-sequence stars undergo vigorous chromospheric
activity that ejects large amounts of matter into space.
Such gas-ejecting
stars are called T Tauri stars.
Ultraviolet radiation
and stellar winds from the OB association at the core of an H II region create
shock waves that compress the gas cloud, triggering the formation of more
protostars.
Supernova explosions also compress gas clouds and trigger star
formation.
A
collection of a few hundred or a few thousand newborn stars is called an open
cluster.
Stars escape from open clusters, most of which eventually
dissipate.
Main-Sequence and
Giant Stars
The Sun has been a
main-sequence star for 4.6 billion years and should remain so for about another
5 billion years.
Less massive stars than the Sun evolve more slowly and have
longer main-sequence lifetimes.
More massive stars than the Sun evolve more
rapidly and have shorter main-sequence lifetimes.
Main-sequence stars
with less than 0.4M0 convert all of their mass into helium and then
stop fusing.
Their lifetimes are hundreds of billions of years and so none of
these stars have yet left the main sequence.
Core hydrogen fusion
ceases when hydrogen is exhausted in the core of a main-sequence star with M >
0.4M0, leaving a core of nearly pure helium surrounded by a shell
where hydrogen fusion continues.
Shell hydrogen fusion adds more helium to
the star's core, which contracts and becomes hotter.
The outer atmosphere
expands considerably, and the star becomes a giant.
When the central
temperature of a giant reaches about 100 million K, the thermonuclear process of
helium fusion begins.
This process converts helium to carbon, then to
oxygen.
In a massive giant, helium fusion begins gradually.
In a less
massive giant, it begins suddenly in a process called the helium flash.
The age of a stellar
cluster can be estimated by plotting its stars on an H-R diagram.
The upper
portion of the main sequence disappear first, because more massive main-sequence stars
become giants before low mass stars do.
Relatively young stars are
metal-rich; ancient stars are metal-poor.
Giants undergo
extensive mass loss, sometimes producing shells of ejected material that
surround the entire star.
Variable
Stars
When a star's
evolutionary track carries it through a region called the instability strip in
the H-R diagram, the star becomes unstable and begins to pulsate.
RR Lyrae variables are
low-mass, pulsating variables with short periods.
Cepheid variables are
high-mass, pulsating variables exhibiting a regular relationship between the
period of pulsation and luminosity.
Mass can be transferred
from one star to another in close binary systems.
When this occurs the
evolution of the two stars changes.
Stars with different
masses fuse different elements.
Stars lose mass via
stellar winds throughout their giant and supergiant phases.
Low-Mass Stars
and Planetary Nebulae
A low-mass (below 8
M0) main-sequence star becomes a giant when shell hydrogen fusion
begins.
It becomes a horizontal branch star when core helium fusion
begins.
It enters the asymptotic giant branch and becomes a supergiant when
shell helium fusion starts.
Stellar winds during
the thermal pulse phase can also eject mass from the star's outer
layers.
The
burned-out core of a low-mass star becomes a dense carbon-oxygen body, called a
white dwarf, with about the same diameter as the Earth.
The maximum mass of a
white dwarf (the Chandrasekhar limit) is 1.4 M0.
Explosive hydrogen
fusion may occur in the surface layer of a white dwarf in a close binary system,
producing the sudden increase in luminosity that we call a nova.
An accreting white
dwarf in a close binary system can also become a supernova when carbon fusion
ignites explosively throughout such a degenerate star.
Such a detonation is
called a Type Ia supernova.
High-Mass Stars
and Supernovae
After exhausting its
central supply of hydrogen and helium, the core of a high-mass (above 8
M0) star undergoes a sequence of other thermonuclear
reactions.
These include carbon fusion, neon fusion, oxygen fusion, and
silicon fusion.
This last fusion eventually produces an iron core.
A high-mass star dies
in a supernova explosion that ejects most of the star's matter into space at
very high speeds.
This Type II supernova is triggered by the gravitational
collapse of the doomed star's core.
Neutrinos were detected
from Supernova 1987A, which was visible to the naked eye.
Neutron Stars and
Pulsars
A high-mass star's dead
core becomes a neutron star or even a black hole.
A neutron star is a very
dense stellar corpse consisting of closely packed neutrons in a sphere roughly
20 km in diameter.
The maximum mass of a neutron star, called the
Oppenheimer-Volkov limit, is about 3 M0.
A pulsar is a rapidly
rotating neutron star with a powerful magnetic field that makes it a source of
periodic radio and other electromagnetic pulses.
Energy pours out of the
polar regions of the neutron star in intense beams that sweep across the
sky.
Some
X-ray sources exhibit regular pulses.
These objects are thought to be neutron
stars in close binary systems with ordinary stars.
Explosive helium fusion
may occur in the surface layer of a companion neutron star, producing the sudden
increase in X-ray radiation called an X-ray burster.
If a stellar corpse is
more massive than about 3 M0, gravitational compression overcomes
neutron degeneracy or the equivalent quark pressure and forces it to collapse
further and become a black hole.
A black hole is an
object so dense that the escape velocity from it exceeds the speed of
light.
The Evidence for
Black Holes
According to general
relativity, mass causes space to curve and time to slow down.
These effects
are significant only near large masses or compact objects.
Observations indicate
that some binary star systems harbor black holes.
In such systems, gases
captured by the black hole from the companion star heat up and emit detectable X
rays.
Supermassive black
holes originated in the cores of some galaxies.
Very low mass black holes may
have formed at the beginning of the universe.
Inside a Black
Hole
The event horizon of a
black hole is a spherical boundary where the escape velocity equals the speed of
light.
No matter or electromagnetic radiation can escape from inside the
event horizon.
The distance from the center of the black hole to the event
horizon is called the Schwarzschild radius.
The matter inside a
black hole collapses to a singularity.
The singularity for nonrotating matter
is a point at the center of the black hole.
For rotating matter, the
singularity is a ring inside the event horizon.
Matter inside a black
hole has only three physical properties: mass, angular momentum, and electrical
charge.
Nonrotating black holes
are called Schwarzschild black holes.
Rotating black holes are called Kerr
black holes.
The event horizon of a Kerr black hole is surrounded by an
ergoregion in which all matter must constantly move to avoid being pulled into
the black hole.
Matter approaching a
black hole's singularity is torn by extreme tidal forces generated by the black
hole, light from the matter is red shifted, and time appears to slow.
Black holes can
evaporate by the Hawking process, in which virtual particles near the black hole
become real.
This transition decreases the mass of a black hole until
eventually it disappears.
Galaxies
A century
ago, astronomers were divided on whether all stars and nebulae are part of the
Milky Way Galaxy.
The Shapley-Curtis
debate was the first major discussion among astronomers of whether the Milky Way
contains all the stars in the universe.
Cepheid variable stars
are important in determining the distance to other galaxies.
Edwin Hubble first
determined that there are other galaxies far outside the Milky
Way.
Types of
Galaxies
The Hubble
classification system groups galaxies into four major types: spiral, barred
spiral, elliptical, and irregular.
The arms of spiral and
barred spiral galaxies are sites of active star formation.
According to the theory
of self-propagating star formation, spiral arms of flocculent galaxies are
caused by the births and deaths of stars over extended regions of a
galaxy.
Differential rotation of a galaxy stretches the star-forming regions
into elongated arches of stars and nebulae that we see as spiral arms.
According to the spiral
density wave theory, spiral arms of grand-design
galaxies are caused by density waves.
The gravitational field of a spiral
density wave compresses the interstellar clouds that pass through it, thereby
triggering the formation of stars, including OB associations, which highlight
the arms.
Elliptical galaxies
contain much less interstellar gas and dust than do spiral galaxies; little star
formation is occurring in elliptical galaxies.
Clusters and
Superclusters
Galaxies group into
clusters rather than being randomly scattered through the universe.
A rich cluster contains
hundreds or even thousands of galaxies; a poor cluster may contain only a few
dozen.
A regular cluster has a nearly spherical shape with a central
concentration of galaxies; in an irregular cluster, the distribution of galaxies
is asymmetrical.
Our Galaxy is a member
of a poor, irregular cluster called the Local Group.
Rich, regular clusters
contain mostly elliptical and lenticular galaxies; irregular clusters contain
more spiral and irregular galaxies.
Giant elliptical galaxies are often found
near the centers of rich clusters.
No cluster of galaxies
has an observable mass large enough to account for the observed motions of its
galaxies; a large amount of unobserved mass must be present between the
galaxies.
Hot
intergalactic gases emit X rays in rich clusters.
When two galaxies
collide, their stars initially pass each other but their interstellar gas and
dust collide violently, either stripping the gas and dust from the galaxies or
triggering prolific star formation.
The gravitational effects of a galactic
collision can cast stars out of their galaxies into intergalactic
space.
Galactic mergers occur;
a large galaxy in a rich cluster may grow steadily through galactic cannibalism,
perhaps producing a giant elliptical galaxy.
Superclusters in
Motion
A simple linear
relationship exists between the distance from the Earth to galaxies in other
superclusters and the red shifts of those galaxies (a measure of the speed at
which they are receding from us).
This relationship is the Hubble law,
recessional velocity = H0 � distance, where H0 is the Hubble
constant.
Astronomers use
standard candles-Cepheid variables, the brightest supergiants, globular
clusters, H II regions, supernovae in a galaxy-and the Tully-Fisher relation to
estimate intergalactic distances.
Because of difficulties in measuring the
distances to remote galaxies, the value of the Hubble constant, H0, is not known
with complete certainty.
The Big
Bang
Most astronomers
believe that the universe began as an exceedingly dense cosmic singularity that
expanded explosively in an event called the Big Bang.
The Hubble law
describes the ongoing expansion of the universe and the rate at which
superclusters of galaxies move apart.
The observable universe
extends about 14 billion light-years in every direction from the Earth to what
is called the cosmic light horizon.
We cannot see any objects that may exist
beyond the cosmic light horizon because light from these objects has not had
enough time to reach us.
According to the theory
of inflation, matter originally near our location in the universe spread
throughout a volume of the universe so large that we cannot yet observe much of
it.
The observable universe today is thus a growing volume of space
containing matter and radiation that was in close contact with our matter and
radiation during the first instant after the Big Bang.
This explains the
isotropic appearance of the universe.
A Brief Physical
History of Matter.
Four basic
forces-gravity, electromagnetism, the strong nuclear force, and the weak nuclear
force-explain the interactions observed in the universe.
According to current
theory, all four forces were identical just after the Big Bang.
At the end of
the Planck time (about 10-43s after the Big Bang), gravity "froze
out" to become a separate force.
A short time later, the strong nuclear force
became a distinct force.
A final "freeze-out" separated the electromagnetic
force from the weak nuclear force.
Before the Planck time,
the universe was so dense that known laws of physics do not properly describe
the behavior of spacetime, matter, and energy.
In its first 500,000
years, the universe was radiation-dominated, during which time photons prevented
matter from forming clumps.
Then it was matter-dominated, during which time
superclusters and smaller clumpings of matter formed.
Today it is
dark-energy-dominated.
Dark energy apparently supplies a repulsive
gravitational force that causes superclusters to accelerate away from each
other.
During
the first 500,000 years of the universe, astronomers believe that matter and
energy formed an opaque plasma called the primordial fireball.
Cosmic
microwave background radiation is the greatly redshifted remnant of the universe
as it existed about 500,000 years after the Big Bang.
By 500,000 years after
the Big Bang, spacetime expansion caused the temperature of the universe to fall
below 3000 K, enabling protons and electrons to combine to form hydrogen
atoms.
This event is called the era of recombination.
The universe became
transparent during the era of recombination, meaning that the microwave
background radiation contains the oldest photons in the universe.
Clusters of galaxies
and individual galaxies formed from dark energy? Which, if either, of the
current candidates for dark energy, the cosmological constant or quintessence,
is correct? One of the things that makes this time in human existence so
fascinating is that it is likely we will have answers to most of these questions
within your lifetime.
Extraterrestrial
Life
The chemical building
blocks of life exist throughout the Milky Way Galaxy.
Organic molecules have
been discovered in interstellar clouds, in some meteorites, and in
comets.
The
Drake equation is used to estimate the number of technologically advanced
civilizations in the Galaxy whose radio transmissions we might
discover.
Astronomers are using
radio telescopes to search for signals from other self-aware life in the
Galaxy.
This effort is called the search for extraterrestrial intelligence,
or SETI.
SETI is primarily done at frequencies where radio waves pass most
easily through the interstellar medium.
So far, these searches have not
detected any life off Earth.
Everyday radio and
television transmissions from Earth, along with intentional broadcasts into
space, may be detected by other life-forms.
Final
Review