Galaxies are huge and contain billions of stars. They contain all of the stars of the Herzspring-Russell diagram, open cluster, emission, nubula, reflection nubula, nova remnants and supernova remnants. We have three different and distinct types of galaxies, which are spiral galaxies, eluptical galaxies, and last but not least irregular galaxies.
The Milky Way galaxy, which is a representative of a spiral galaxy, is basically lens-shaped. Its circumference is about 120,000 light years across. On a dark summer night, we are able to see a few of the Milky Way's arm. The Milky Way, as all spiral galaxies, rotates on its axis. However, it does not rotate uniformly. The budge rotates much faster than the arms and it takes about 250 million years to make a single rotation. In the Milky Way galaxy, as in all spiral galaxies, there are two star populations. Population I Stars are similar to our sun. Those stars are relatively young and orbit in circular orbits in the galactic disk. The small mass of these stars consists of "heavy" elements. These elements are heavier than helium and include carbon, oxygen, silicon and iron. The Population II Stars are nearly pure hydrogen and helium with a small percentage of heavy elements. These stars are in the center of our galaxy. Their orbits are random around the bulge. These stars as a group usually have less mass and are redder in color.
Eliptica Galaxies are mostly made of old stars and contain almost no dust. These stars can range from dwarf size to very massive. .
