1. What does our galaxy look like?

2. The Milky Way galaxy appears in our sky as a faint band of light

3. We see our galaxy edge-on
Primary features: disk, bulge, halo, globular clusters

4. Halo: No ionization nebulae, no blue stars
 no star formation
Disk: Ionization nebulae, blue stars  star formation

5. If we could view the Milky Way from above the disk, we would see its spiral arms

6. How do stars orbit in our galaxy?

7. Stars in the disk all orbit in the same direction with a little up-and-down motion

8. Orbits of stars in the bulge and halo have random orientations

9. Sun’s orbital motion (radius and velocity) tells us mass within Sun’s orbit:
1.0 x 1011 MSun

10. Much of star formation in disk happens in spiral arms
Ionization Nebulae
Blue Stars
Gas Clouds
Whirlpool Galaxy

11. Spiral arms are waves of star formation
Gas clouds get squeezed as they move into spiral arms
Squeezing of clouds triggers star formation
Young stars flow out of spiral arms

13. Saturn’s Rings: Natural Color

14. 15km/s
< 1cm/s
15km/s

15. Unseen Influences
Dark Matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence
Dark Energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

16. Mass within Sun’s orbit:
1.0 x 1011 MSun
Total mass:
~1012 MSun

17. What is the evidence for dark matter in galaxies?

18. Spiral galaxies all tend to have flat rotation curves indicating large amounts of dark matter

19. Contents of Universe “Normal” Matter: ~ 4.4% Dark Matter: ~ 25%
Normal Matter inside stars: ~ 0.6%
Normal Matter outside stars: ~ 3.8%
Dark Matter: ~ 25%
Dark Energy ~ 71%

20. We can measure rotation curves of other spiral galaxies using the Doppler shift of the 21-cm line of atomic H

21. Gravitational lensing, the bending of light rays by gravity, can also tell us a cluster’s mass

22. All three methods of measuring cluster mass indicate similar amounts of dark matter

23. Clusters contain large amounts of X-ray emitting hot gas
Temperature of hot gas (particle motions) tells us cluster mass:
85% dark matter
13% hot gas
2% stars

24. Our Options
Dark matter really exists, and we are observing the effects of its gravitational attraction
Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter

25. How dark is it?
… not as bright as a star.

26. Two Basic Options Ordinary Dark Matter (MACHOS)
Massive Compact Halo Objects:
dead or failed stars in halos of galaxies
Extraordinary Dark Matter (WIMPS)
Weakly Interacting Massive Particles:
mysterious neutrino-like particles

27. Halo stars formed first as gravity caused cloud to contract

28. Remaining gas settled into spinning disk

29. Detailed studies: Halo stars formed in clumps that later merged

30. Our Galaxy Earth = 100 nm = virus Sun = 10 μm = cell
Earth orbit = ¼ cm = pin head
Solar system = cm = saucer
Nearest star = m = lawn
Jewel Box
Cluster
Naked eye
stars
Eagle
Nebula
Crab
Nebula
Solar system
Galaxy Center

31. Typical galaxy.
[A]
100 billion stars, like city – people born & die
Measure rate of star birth – ~1 per year.
 only 14 billion stars ?? : LESS than known number
We INFER that the rate of star birth was GREATER in the past.
[B]
Also : stars dying  pollution/enrichment by elements.
gradual buildup of elements ----
expect oldest stars to have few elements
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34. NGC 3310
Spiral Galaxy

35. NGC 5194
Spiral Galaxy

36. NGC 1365
Barred Spiral Galaxy

38. Interacting
galaxies