Dust Lots of dust in spiral galaxies Dust

Dust • 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 • • The disk The bulge The nucleus The halo – Globular clusters

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

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 • We can see through the dust in near infrared • • The disk The bulge The nucleus The halo – Globular clusters

The Bulge 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 disk The bulge The nucleus The halo – Globular clusters

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

The Nucleus 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 disk The bulge The nucleus The halo – Globular clusters

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 • 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 disk The bulge The nucleus The halo – Globular clusters

The Halo • 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 • • The disk The bulge The nucleus The halo – Globular clusters

Total Mass in the Galaxy • 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: MP 2 = a 3 Object Disk Stars Disk Gas Bulge Halo Stars Nucleus Mass (MSun ) 60 billion ~10 billiion 20 billion 1 billion 4 million 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 • 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 • • The disk The bulge The nucleus The halo – Globular clusters Massive Compact Halo Objects

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

MACHO’s: What the Data Tells Us • 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 • • The disk The bulge The nucleus The halo – Globular clusters • Most likely dark matter is some weird new particle