Galaxies
 A. The Milky Way is the spiral galaxy in which we live. It has two
component parts, the disc and the sphere. The disc is about 100,000 light years
in diameter, with the sun about two-thirds of the way out on a spiral arm. The
disc stars are thought to be mostly metal-rich Population I stars, moving in
circular orbits that lie in the plane of the disc. The disc exhibits
differential rotation. Those stars near the center orbit the spherical region
faster than stars near the perimeter. The spherical component consists of a
nuclear bulge at the center and a collection of thinly scattered stars and
globular clusters that encircle the disc. These stars are metal-poor Population
II stars. The size of our galaxy was initially determined by Harlow Shapley, and
the position of our sun within the galaxy by studying the globular clusters.
B. Spiral density wave theory suggests that the spiral arms of
galaxies are regions of compression that move through the disc. These are the
areas where stars are formed, as gas clouds smash into the compression waves.
C. Galaxies are thought to have formed out of spherical clouds of
rotating gas. The younger the stars, the more metal-rich they are and the more
circular and flat their orbits.
D. The nucleus of the galaxy is invisible at optical wavelengths. Radio and
x-ray radiation reveal crowded central features expanding outward.
E. Galaxies are divided into three classes- elliptical, spiral and
irregular.
- Elliptical galaxies used up all of their gas and dust in a
sudden burst of star formation when they were young.
- Spiral galaxies formed more slowly,
and conserved their gas and dust and thereafter flattened into discs.
- Irregular galaxies
may have formed from turbulent gas clouds.
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