1. Dust • The disk • The bulge • The nucleus • The halo • Globular clusters • Lots of dust in spiral galaxies • Dust absorbs and scatters short wavelength light • The color of objects is reddened if we look through thin dust clouds • Dust clouds look blue • You can peek through dust in nearinfrared light Dust cloud

2. Dust • The disk • The bulge • The nucleus • The halo • Globular clusters • Dust is warmed by absorbing light from stars • It glows in the infrared

3. A Reflection Nebula – the Pleiades • The disk • The bulge • The nucleus • The halo • Globular clusters

4. A Reflection Nebula – the Merope Nebula • The disk • The bulge • The nucleus • The halo • Globular clusters

5. Reflection Nebulae IC 349 Witch Head Nebula

6. Spiral Arms • The disk • The bulge • The nucleus • The halo • Globular clusters

7. Spiral Arms – Signs of Rotation • The disk • The bulge • The nucleus • The halo • Globular clusters • Disk - rotation vs. gravity • Spiral arms - what causes them? • Simple winding? No! • Density waves? Yes! Complicated!

8. Simple Winding – The Wrong Theory • Spiral arms wind up in one cycle • 20 cycles since beginning of galaxy • Something else is going on Q. 90: Problem With the Winding Theory

9. An Analogy from Driving • Knot in traffic causes slowdown • Slowdown causes other cars to slow down • Knot in traffic moves • Different cars at different times

10. Density Waves – The explanation • Gas is in a disk • Inevitably, some parts are more dense than others • Gravity/rotation causes dense parts to become denser • Different gas goes in/out • Dense parts form new/bright stars

11. The Bulge • Our view of it is (mostly) blocked by dust • We can see through the dust in near infrared • The disk • The bulge • The nucleus • The halo • Globular clusters

12. The Bulge • The disk • The bulge • The nucleus • The halo • Globular clusters The Sun • Flattened sphere approximately 20 kly across • Composition • Mostly older stars • Little gas anddust • Recent evidenceindicates it is barshaped, with leftside closer to usthan right side.

13. The Bulge – Color and Appearance • Older stars give it a redderappearance

14. The Nucleus • The disk • The bulge • The nucleus • The halo • Globular clusters The Sun • At the center of our galaxy lies a complex region • Fast star formation • Recent supernovae remnants • Hot gas • Fast motion • Density of stars is very high here • Intense radio sources can penetrate the gas and dust

15. The Nucleus in Radio • Close in, we see streamers of gas apparently flowing in • At the heart is an intense radio source called Sagittarius A*

16. What is That Thing in the Nucleus? • Copious quantities of X-rays close in • Stars orbit this source very quickly • Kepler’s Laws tell us mass X-ray image Q. 91: Who Says It’s a Black Hole? • It appears to be a 4 million solar mass black hole

17. The Monster in the Middle • The disk • The bulge • The nucleus • The halo • Globular clusters • Radio waves can’t come from black hole itself • Gas from nearby attracted by gravity • Accelerates to near light speed • Friction creates heat/X-rays/etc. • More efficientthan anyotherpowersource • Black hole • 4.0 million MSun

18. The Halo • The disk • The bulge • The nucleus • The halo • Globular clusters • Roughly spherical shape • At least 2  size of disk • Stars: • Old stars (about 10 Gyr) • Orbits well out of plane • Globular Clusters • Up to a million stars each • Oldest stars (up to about 13 Gyr) • Little gas or dust

19. Total Mass in the Galaxy ObjectMass (MSun ) Disk Stars 60 billion Disk Gas ~10 billiion Bulge 20 billion Halo Stars 1 billion Nucleus 4 million • Total of a few hundred billion stars • Total mass about 100 billion MSun • Stars mostly concentrated near center • Treat gravity as if it all comes from a point source in the center • Use Kepler’s Third law: MP2 = a3 Q. 92: Rotation Curves for Galaxies

20. Where is the Mass? • Measure rotational velocities using Doppler shift of 21 cm line • Plot vs. distance from center of galaxy

21. Dark Matter Massive Compact Halo Objects • The disk • The bulge • The nucleus • The halo • Globular clusters • 85% of mass is not concentrated in the center • It does not glow, it is dark • We do not know what it is • We know a lot of things it isn’t: • Living Stars • Gas • Dust • Could it be MACHOs? What is the dark matter? A) Neutron Stars B) Black Holes C) White Dwarfs D) Planets without stars E) Something else

22. MACHOs How to catch a MACHO • All of these objects are dark • But they do have gravity! • Einstein says they can bend light • They can magnifya distant star MACHOs Neutron Stars Black Holes White Dwarfs Planets without stars We see: MACHO:

23. MACHO’s: What the Data Tells Us • The disk • The bulge • The nucleus • The halo • Globular clusters • MACHO’s do exist • Substantial fraction of stars • But not the majority • There are not enough of them to account for the dark matter • They are probably mostly white dwarfs • Most likely dark matter is some weird new particle