THE MILKY WAY GALAXY I

The Milky Way Galaxy is a spiral galaxy about 100,000 light years across. The sun is about 2/3rds the way out from the center and orbits in the plane of the galaxy. It takes about 250 million years for the galaxy to rotate; the sun has made about 20 orbits since it was born. There are about 100 billion stars in the Milky Way.

The gas in the Milky way is confined to a very thin disk. New stars form in the sprial arms. Spirals arms consist of a dark cloud region, a star forming region (reddish emission nebulae), and a bright region dominated by new O and B stars that have exited the star forming region of the sprial arm.

Spiral galaxies have a thin layer of gas and dust in the center of their disks. These gas clouds are where new stars form.

The three main components of a spiral galaxy are:

• disk
  - contains ISM where star formation is on going; spiral arms are in disk
• bulge
  - central spherical region with old stars, very little gas
• halo
  - extended spherical region surrounding the visible galaxy; about 10x bigger than visible galaxy; contains old stars, globular clusters, and very thin gas called the circumgalactic medium (CGM)

The halo and bulge are part of the spherical component. Globular clusters are old clusters of stars found in the halo. The globular clusters and the bulge are redder in color because the stars are old and have lived a long time. Thus they are mostly K and M stars, which are low mass, cool, and redder. The spiral ares are bluer in color because their light is dominated by the most luminous stars, which are O and B stars. These stars are massive, short lived, hot, and blur in color.

In the 1700s, William Hershel tried to find the center of the "universe" (at the time) by counting stars. He made a big assumption. All stars are identical, i.e. have the same luminosity. He then assumed that the brightness of a star depended only upon its distance. He then made a 3-D map of the stars. His map placed us at then center of the star system. He was wrong for two reasons. 1. Stars are not all the same. 2. He did not know about gas and dust clouds that block the light from the stars, or what we call "interstellar extinction".

A powerful method for measuring star distances is their luminosity variability. Some stars, called Cepheid variables, ot Variable Stars. One measures the time period of the variability in the brightness. The longer the period the more luminous the star. This is called the Period-Luminosity Relation. One measure's the period of variability and this provides the star's luminosity, form which the distance to the star can be determined.

In the 1920s, Harlow Shapely used Cepheid variables in globular clusters to try and find our location in the galaxy. He assumed that globular clusters orbit around the galaxy center. He was correct. He measured the distance and sky locations of all the globular clusters and found the direction and distance to the galaxy center.