The Milky Way Galaxy Astronomy 315 Professor Lee Carkner Lecture 16

Via Lactia   The band forms a complete ring  We seem to be in the center of a disk of stars    Clouds of gas and dust block our view so it is hard to see beyond our local region

Our View of the Milky Way

The Milky Way from Outside

Discovering The Galaxy  In the early part of the century Harlow Shapley found the distance to globular clusters using Cepheid variables  Globular clusters  Unlike stars in the disk, we can see distant globulars  Cepheid variables  If we can find luminosity and flux we can get distance (F = L/4  d 2 )

The Center  Shapley found the distance to the globulars and plotted their positions  He found:    We are not at the center of the galaxy and the disk extends out much further than we can easily see

Changing Views of the Galaxy

How Do We Learn About The Milky Way?  Optical observations   Radio observations   Infrared observations   Observing other galaxies  Since we are in the middle of the Milky Way we can’t get an overview of it

Structure of the Milky Way  Disk   Nucleus   Halo  Spherical distribution of old stars and globular clusters around disk and bulge

Edge-on and Face-on

The Disk  The disk is very thin   Younger stars and star forming regions near the center, older stars above and below   Disk exhibits differential rotation (inner parts rotating faster than outer)

Differential Galactic Rotation

Spiral Structure  We know that other galaxies have spiral structure, but it is harder to see the Milky Way’s  We find spiral arms by tracing:    They are not uniformly distributed but are found in a loose spiral structure  How do spiral arms form?

Local Spiral Arms

Density Waves  Spiral arms are like traffic jams   This can trigger star formation in the arms   The clouds eventually move out the other side   The spiral arm material changes, only the pattern stays the same

Density Wave

At the Core   The nucleus is the hardest part of the galaxy to observe due to all the gas and dust   One, Sagittarius A* (Sgr A*), may be the center of the galactic core  What is it?

Properties of the Core  Stars near the core are very close together and moving very fast   Sgr A* is emitting enormous amounts of energy   A black hole

Massive Black Holes  Sgr A* does not move and may have jets and and accretion disk   Mass of ~1 million solar masses?    Our own is very hard to observe

The Halo  The halo are stars orbiting in a large sphere around the galaxy  The halo is composed of old stars   Halo stars -- Population II -- metal poor  Disk stars -- Population I -- metal rich   Halo stars formed formed early from relatively unprocessed material

Globular Clusters   Size:  Shape:  Contents:  Globulars are in elliptical orbits around the galactic center  Unlike open clusters in the disk, globular clusters are very tightly gravitationally bound

History of the Milky Way  How did the galaxy form?    Basic theory has Milky way forming from large spinning cloud of gas   Halo formed first, and so is older and more metal poor  Disk is denser and so keeps forming stars and building up metals

Next Time  Quiz #2  Covers lectures  same format as quiz #1  multiple choice and short answer/problems  Read and do homework for Friday  Observing Thursday (April 20), 8:30-9:30 pm  If you have not yet observed