1. Announcements Tests are graded! (Homework, too) Take the test today, tomorrow, or Friday before class (all about stars) Sunspot viewing at end of class today!

2. The Milky Way 15 November 2006

3. Today : Observing the Milky Way; light pollution Finding our place in the Milky Way Measuring distances beyond the range of parallax Interstellar “dust” and how to see through it

4. Observing the Milky Way

5. Light Pollution Goodwood, Ontario, power blackout 14 Aug 2003

6. Light Pollution

7. Light Pollution: Solutions Full cut-off fixtures (no light comes out above a horizontal plane) Motion sensors on security lights Low-pressure sodium lights Good old-fashioned moderation! All these steps will also save energy and money

8. How far do the stars go? Herschel (late 1700’s) counted stars in all directions and found more along the Milky Way than elsewhere (surprise!) But no big differences between different parts of the Milky Way William Herschel

9. Structure of our star system Using variable stars as standard candles, we can measure distances to globular clusters Result: They’re centered on a point about 25,000 light-years away, behind the constellation Sagittarius--the center of the Milky Way “galaxy”!

10. Structure of our star system Huge disk, 100,000 light-years across We’re about 25,000 light-years out from center. Globular clusters (and some stars) are not confined to disk, but are in random orbits about the center, going around in all directions. Estimated to contain about 100 billion stars (10 11 ) Spiral arms are hard to discern from within. Why can’t we see the galactic center?

11. Interstellar gas and “dust” (more like smoke)

12. Gas and dust in Orion

13. Looking toward the galactic center

14. Milky Way in Sagittarius

15. Toward the galactic center

16. Milky Way from Cygnus to Sagittarius

17. Southern Milky Way and Magellanic Clouds

18. Southern Milky Way, full sky

19. Full sky composite photo

20. Infrared light penetrates the dust!

21. Milky Way at many wavelengths

22. Cold hydrogen emits 21-cm radiation