1. 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.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. 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.

10. What have we learned? What are the life stages of a high-mass star? – They are similar to the life stages of a low- mass 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. Your Preparation Read Quick Quiz after each chapter Read Use the internet Read Read!

17. 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/

18. 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 26 th of June.