1. AST101 Lecture 19 Discovery of the Galaxy

3. Northern Milky Way

4. Southern Milky Way

8. My God, it’s full of stars…

10. Star Counts

12. The Panchromatic Milky Way Different objects emit at different wavelengths For a blackbody, peak brightness is at λ ~ 1/T Other processes emit in different ways

13. Gamma-rays

14. X-rays - Hot Gas

15. Optical light - Stars

16. Near Infrared - old stars

17. Infrared light - Dust

18. 21 cm (radio) - Hydrogen

19. Radio - Electrons

21. Shape of the Galaxy

23. You are here

24. About 28,000 light years from the Center of the Galaxy. Our orbital velocity is about 220 km/s. The Galactic Year is about 220 million years long. The Sun is about 21 galactic years old.

25. The Mass of the Galaxy A star orbiting the center of the galaxy is the same as a planet orbiting the Sun. Use Newton’s laws M=v 2 r/G But, the mass depends on radius

26. Rotation-Velocity Curve

27. Mass of the Galaxy The mass of the Galaxy is 2x10 44 gm, or 10 11 solar masses. If the typical star is 1/4 solar masses, there are 4 x 10 11 stars in the Galaxy

28. The Center of the Galaxy

31. Radio: mini- spiral VLA, 6 cm

32. X-rays Chandra

33. Infrared

37. IR K band

38. K band Keck AO A. Ghez UCLA

39. Orbits at the Center

41. The central object Orbits  Mass ~ 2.5 x 10 6 M  Orbits  radius < 1 AU Density > 0.4 gm/cm 3 Unseen at any wavelength A black hole