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.
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 four main components of a spiral galaxy are:
- disk
- bulge
- halo
- corona
The halo and bulge are part of the spherical component. Globular clusters are old clusters of stars found in the halo.
The orbits of stars in the disk are fairly circular and confined to the disk. The orbits of stars in the bulge and halo are highly elliptical and have random orientations
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 stars.
A powerful method for measuring star distances is their luminosity variability. Some stars, called Cepheid variables, are variable. One measures the time period of the variability in the brightness. This provides the luminosity. Now you can solve for the distance.
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.
We think galaxies form via the collapse of giant rotating gas structures.