Dust Dust cloud The disk Lots of dust in spiral galaxies 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 Dust cloud

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

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

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

Reflection Nebulae IC 349 Witch Head Nebula

Spiral Arms The disk The bulge The nucleus The halo Globular clusters

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!

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

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

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

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

The Bulge The Sun The disk The bulge The nucleus The halo Globular clusters The Sun Flattened sphere approximately 20 kly across Composition Mostly older stars Little gas and dust Recent evidence indicates it is bar shaped, with left side closer to us than right side.

The Bulge – Color and Appearance Older stars give it a redder appearance

The Nucleus The Sun 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

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*

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 It appears to be a 4 million solar mass black hole Q. 91: Who Says It’s a Black Hole?

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 efficient than any other power source Black hole 4.0 million MSun

The Halo The disk Roughly spherical shape The bulge The nucleus 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

Total Mass in the Galaxy Object Mass (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

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

Dark Matter The disk 85% of mass is not concentrated in the center 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 Massive Compact Halo Objects

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

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