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Dust

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 near infrared light.

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Dust

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  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

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