Artist's Conception

1. Take a Giant Step Outside the Milky Way Artist's Conception Example (not to scale)

2. Another galaxy: NGC 4414. The Milky Way roughly resembles it.

3. The Three Main Structural Components of the Milky Way 1. Disk - 30,000 pc diameter (or 30 kpc) - contains young and old stars, gas, dust. Has spiral structure - vertical thickness roughly 100 pc - 2 kpc (depending on component. Most gas and dust in thinner layer, most stars in thicker layer) 2. Halo - at least 30 kpc across - contains globular clusters, old stars, little gas and dust, much "dark matter" - roughly spherical

4. 3. Bulge - About 4 kpc across - old stars, some gas, dust - central black hole of 3 x 106 solar masses - Spherical or football shaped (as seen from side)

5. Measuring the Milky Way We have already encountered variable stars – novae, supernovae, and related phenomena – which are called cataclysmic variables. There are other stars whose luminosity varies in a regular way, but much more subtly. These are called intrinsic variables. Two types of intrinsic variables have been found: RR Lyrae stars and Cepheids.

6. Measuring the Milky Way The upper plot is an RR Lyrae star. All such stars have essentially the same luminosity curve, with periods from 0.5 to 1 day. The lower plot is a Cepheid variable; Cepheid periods range from about 1 to 100 days.

7. Measuring the Milky Way The variability of these stars comes from a dynamic balance between gravity and pressure – they have large oscillations around stability.

8. Measuring the Milky Way The usefulness of these stars comes from their period–luminosity relationship.

9. Measuring the Milky Way • This allows us to measure the distances to these stars. • RR Lyraestars all have about the same luminosity; knowing their apparent magnitude allows us to calculate the distance. • Cepheidshave a luminosity that is strongly correlated with the period of their oscillations; once the period is measured, the luminosity is known and we can proceed as above.

10. Shapley (1917) found that Sun was not at center of Milky Way Shapley used distances to variable “RR Lyrae” stars (a kind of Horizontal Branch star) in Globular Clusters to determine that Sun was 16 kpc from center of Milky Way. Modern value 8 kpc.

11. Measuring the Milky Way We have now expanded our cosmic distance ladder one more step.

12. Precise Distance to Galactic Center Distance = 7.94 +/- 0.42 kpc SgrA* Eisenhauer et al. 2003 Orbital motion 6.37 mas/yr

13. Where is our solar system located? A: near the center of the Milky Way Galaxy in the bulge. B: 4 kpc from the center of the Milky Way in the halo. C: 8 kpc from the center of the Milky Way in the disk. D: 20 kpc from the center of the Milky Way in the disk. Clicker Question:

14. What lurks at the center of our galaxy? A: A 3 million solar mass black hole. B: A giant star cluster. C: A 30 solar mass black hole. D: Darth Vader Clicker Question:

15. Galactic Structure This infrared view of our Galaxy shows much more detail of the galactic center than the visible-light view does, as infrared is not as much absorbed by gas and dust.

16. Stellar Orbits Halo:stars and globular clusters swarm around center of Milky Way. Very elliptical orbits with random orientations. They also cross the disk. Bulge:similar to halo. Disk:Stars and gas rotate in circular orbits. Stellar orbits in the disk are in a plane and in the same direction; orbits in the halo and bulge are much more random.

17. The Formation of the Milky Way Any theory of galaxy formation should be able to account for all the properties below.

18. The Formation of the Milky Way The formation of the galaxy is believed to be similar to the formation of the solar system, but on a much larger scale.

19. Rotation of the Disk Sun moves at 225 km/sec around center. An orbit takes 240 million years. Stars closer to center take less time to orbit. Stars further from center take longer. => rotation not rigid like a phonograph record or a merry-go-round. Rather, "differential rotation". Over most of disk, rotation velocity is roughly constant. The "rotation curve" of the Milky Way

21. Spiral Structure of Disk Spiral arms best traced by: Young stars and clusters Emission Nebulae HI Molecular Clouds (old stars to a lesser extent) Disk not empty between arms, just less material there.

22. Problem: How do spiral arms survive? Given differential rotation, arms should be stretched and smeared out after a few revolutions (Sun has made 20 already): The Winding Dilemma

23. The spiral should end up like this: Real structure of Milky Way (and other spiral galaxies) is more loosely wrapped.

24. Proposed solution: Arms are not material moving together, but mark peak of a compressional wave circling the disk: A Spiral Density Wave Traffic-jam analogy:

25. Now replace cars by stars and gas clouds. The traffic jams are actually due to the stars' collective gravity. The higher gravity of the jams keeps stars in them for longer. Calculations and computer simulations show this situation can be maintained for a long time. Traffic jam on a loop caused by merging

26. Video Links: Density Waves & Spiral Arms Density Waves & Traffic Jams Spiral Galaxy Simulation Molecular gas clouds pushed together in arms too => high density of clouds => high concentration of dust => dust lanes. Also, squeezing of clouds initiates collapse within them => star formation. Bright young massive stars live and die in spiral arms. Emission nebulae mostly in spiral arms. So arms always contain same types of objects, but individual objects come and go.

27. Galactic Spiral Arms As clouds of gas and dust move through the spiral arms, the increased density triggers star formation. This may contribute to propagation of the arms. The origin of the spiral arms is not yet understood.

28. The Mass of the Milky Way Galaxy The orbital speed of an object depends only on the amount of mass between it and the galactic center.

29. observed curve Milky Way Rotation Curve Curve if Milky Way ended where visible matter pretty much runs out. 90% of Matter in Milky Way is Dark Matter Gives off no detectable radiation. Evidence is from rotation curve: 10 Solar System Rotation Curve: when almost all mass at center, velocity decreases with radius ("Keplerian") Rotation Velocity (AU/yr) 5 1 30 1 10 20 R (AU)

30. What is “dark matter”? • What could this “dark matter” be? It is dark at all wavelengths, not just the visible. Accounts for 90% of mass • Stellar-mass black holes? • Probably contributes but no way enough could have been created • Brown dwarfs, faint white dwarfs, and red dwarfs? • Currently the best star-like option • Weird subatomic particles, mostly in the halo? • Could be, although no evidence so far

31. What is “dark matter”? The bending of spacetime can allow a large mass to act as a gravitational lens: Observation of such events suggests that low-mass white dwarfs could account for about half of the mass needed. The rest is still a mystery.

32. The Galactic Center This is a view toward the galactic center, in visible light. The two arrows in the inset indicate the location of the center; it is entirely obscured by dust.

33. The Galactic Center These images, in infrared, radio, and X ray, offer a different view of the galactic center.

34. The Galactic Center • The galactic center appears to have • a stellar density a million times higher than near Earth • a ring of molecular gas 400 pc across • strong magnetic fields • a rotating ring or disk of matter a few parsecs across • a strong X-ray source at the center

35. The Galactic Center Apparently, there is an enormous black hole at the center of the galaxy, which is the source of these phenomena. An accretion disk surrounding the black hole emits enormous amounts of radiation.

36. The Galactic Center These objects are very close to the galactic center. The orbit on the right is the best fit; it assumes a central black hole of 3.7 million solar masses.

37. How long does it take our solar system to orbit once around the Milky Way? A: 1 year B: 2 million years C: 250 million years D: 250 billion years (longer than the age of the universe) Clicker Question:

38. What makes up most of the mass (90%) of the Milky Way Galaxy? A: hydrogen gas B: stars C: dead stars (white dwarfs, neutron stars, and black holes) D: we don’t know Clicker Question:

39. Hubble’s Galaxy Classification Hubble’s “tuning fork” is a convenient way to remember the galaxy classifications, although it has no deeper meaning.

40. Hubble’s Galaxy Classification Type Sa has the largest central bulge, Type Sb is smaller, and Type Sc is the smallest. Type Sa tends to have the most tightly bound spiral arms, with Types Sb and Sc progressively less tight, although the correlation is not perfect. The components of spiral galaxies are the same as in our own Galaxy: disk, core, halo, bulge, spiral arms.

41. Hubble’s Galaxy Classification Spiral galaxies are classified according to the size of their central bulge.

42. Hubble’s Galaxy Classification Similar to the spiral galaxies are the barred spirals.

43. Elliptical galaxies have no spiral arms and no disk. They come in many sizes, from giant ellipticals of trillions of stars, down to dwarf ellipticals of fewer than a million stars. Ellipticals also contain very little, if any, cool gas and dust, and show no evidence of ongoing star formation. Many do, however, have large clouds of hot gas, extending far beyond the visible boundaries of the galaxy.

44. Ellipticalsare classified according to their shape, from E0 (almost spherical) to E7 (the most elongated).

45. S0 (lenticular) and SB0 galaxies have a disk and bulge, but no spiral arms and no interstellar gas.

46. The irregular galaxies have a wide variety of shapes. These galaxies appear to be undergoing interactions with other galaxies. Irregular galaxies are the most common type of galaxy

47. Hubble’s Galaxy Classification A summary of galaxy properties by type