What have we learned?
• What does our galaxy look like?
– Our galaxy consists of a disk of stars and gas,
with a bulge of stars at the center of the disk,
surrounded by a large spherical halo.
• How do stars orbit in our galaxy?
– Stars in the disk orbit in circles going in the
same direction with a little up-and-down
motion.
– Orbits of halo and bulge stars have random
orientations.
What have we learned?
• How is gas recycled in our galaxy?
– Gas from dying stars mixes new elements into
the interstellar medium, which slowly cools,
making the molecular clouds where stars
form.
– Those stars will eventually return much of
their matter to interstellar space.
• Where do stars tend to form in our
galaxy?
– Active star-forming regions contain molecular
clouds, hot stars, and ionization nebulae.
– Much of the star formation in our galaxy
happens in the spiral arms.
What have we learned?
• What clues to our galaxy’s history do halo
stars hold?
– Halo stars are all old, with a smaller
proportion of heavy elements than disk stars,
indicating that the halo formed first.
• How did our galaxy form?
– Halo stars formed early in the galaxy’s
history; disk stars formed later, after much of
the galaxy’s gas settled into a spinning disk.
What have we learned?
• How are the lives of galaxies connected
with the history of the universe?
– Galaxies generally formed when the universe
was young and have aged along with the
universe.
• What are the three major types of
galaxies?
– The major types are spiral galaxies, elliptical
galaxies, and irregular galaxies.
– Spirals have both disk and spheroidal
components; ellipticals have no disk.
What have we learned?
• How do we observe the life histories of
galaxies?
– Deep observations of the universe show us
the history of galaxies because we are seeing
galaxies as they were at different ages.
• How did galaxies form?
– Our best models for galaxy formation assume
that gravity made galaxies out of regions in
the early universe that were slightly denser
than their surroundings.
What have we learned?
• Why do galaxies differ?
– Some of the differences between galaxies
may arise from the conditions in their
protogalactic clouds.
– Collisions can play a major role because they
can transform two spiral galaxies into an
elliptical galaxy.
What have we learned?
• How does a star’s mass determine its life
story?
– Mass determines how high a star’s core
temperature can rise and therefore
determines how quickly a star uses its fuel
and what kinds of elements it can make.
What have we learned?
• How does a star’s mass affect nuclear
fusion?
– A star’s mass determines its core pressure
and temperature and therefore determines
its fusion rate.
– Higher mass stars have hotter cores, faster
fusion rates, greater luminosities, and shorter
lifetimes.
What have we learned?
• What are the life stages of a low-mass
star?
– Hydrogen fusion in core (main sequence)
– Hydrogen fusion in shell around contracting
core (red giant)
– Helium fusion in core (horizontal branch)
– Double shell burning (red giant)
• How does a low-mass star die?
– Ejection of hydrogen and helium in a
planetary nebula leaves behind an inert white
dwarf.
What have we learned?
• What are the life stages of a high-mass
star?
– They are similar to the life stages of a lowmass star.
• How do high-mass stars make the
elements necessary for life?
– Higher masses produce higher core
temperatures that enable fusion of heavier
elements.
• How does a high-mass star die?
– Its iron core collapses, leading to a
supernova.
What have we learned?
• What is a white dwarf?
– A white dwarf is the inert core of a dead star.
– Electron degeneracy pressure balances the
inward pull of gravity.
• What can happen to a white dwarf in a
close binary system?
– Matter from its close binary companion can
fall onto the white dwarf through an
accretion disk.
– Accretion of matter can lead to novae and
white dwarf supernovae.
What have we learned?
• What is a neutron star?
– It is a ball of neutrons left over from a
massive star supernova and supported by
neutron degeneracy pressure.
• How were neutron stars discovered?
– Beams of radiation from a rotating neutron
star sweep through space like lighthouse
beams, making them appear to pulse.
– Observations of these pulses were the first
evidence for neutron stars.
What have we learned?
• What can happen to a neutron star in a
close binary system?
– The accretion disk around a neutron star can
become hot enough to produce X rays,
making the system an X-ray binary.
– Sudden fusion events periodically occur on a
the surface of an accreting neutron star,
producing X-ray bursts.
What have we learned?
• What is a black hole?
– A black hole is a massive object whose radius
is so small that the escape velocity exceeds
the speed of light.
• What would it be like to visit a black hole?
– You can orbit a black hole like any other
object of the same mass—black holes don’t
suck!
– Near the event horizon, time slows down and
tidal forces are very strong.
What have we learned?
• What lies in the center of our galaxy?
– Orbits of stars near the center of our galaxy
indicate that it contains a black hole with 4
million times the mass of the Sun.
Your Preparation
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Read
Quick Quiz after each chapter
Read
Use the internet
Read
Read!
Text and Other Resources
• The Cosmic Perspective, Fundamentals
• Voyager: Skygazer
• Other resources
– JPL (www.jpl.nasa.gov/index.cfm)
– NASA (www.nasa.gov/news/index.html)
– www.msnbc.msn.com/id/3033063/
Another SkyGazer Project
• We can view both Mars and Saturn in the sky this
evening.
• Which planet will first stop being visible, and
approximately when will that occur?
• Criteria?
– Negative altitude angles?
– Sunrise/Sunset?
• Time steps?
• Answers due (with brief description of procedure)
on Tuesday, the 26th of June.