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.
These galaxies can be up to two or three times larger than the Milky Way Galaxy. They are smooth shaped and have no spiral arms. Irregular Galaxies come in many shapes and are usually smaller than the Milky Way Galaxy. There are several different types of Irregular Galaxies like Type I, Type II, Peculiar. Some common properties that galaxies are measured by are distance and size. Distance, which is the most basic property, is not an exact science. Galaxies do not have sharp edges and vary in size, so it’s very hard to get an exact measurement.
Galaxies may be as few as thousands of light years in diameter and as much as 600,000 light years. Size is measured by luminosity. Two issues must be addressed to calculate luminosity. First, one must measure the total range of radiation given off by the galaxy. Second, one must compensate for the fact that the light given off is not evenly distributed across its diameter. With these two issues in consideration astronomers can make a model of light distribution and use it to calculate luminosity.
Color and rotation are two other properties used to classify galaxies. But the most important and hardest to measure is mass. There is a relationship between mass and luminosity. The more mass the brighter the star and the reverse is also true. Also, about 90% of the mass of a galaxy is dark matter. Galaxies also can do other interesting exhibits. They can collide, interact and merge with each other. They can also form clusters and super clusters. These big blobs of gas and dust are magnificent and almost have a life of their own.