Photo: Andromeda Galaxy (Creative Commons: Mironovfoto).
Types of Galaxies
There are hundreds of billions of galaxies in the universe. One estimate is that there are 500 billion galaxies in the universe. A galaxy is a very large object. The smallest galaxy may have about 10 million stars. The largest galaxy may contain over a trillion stars.
Our solar system is located in the Milky Way galaxy. There about 400 billion stars in the Milky Way galaxy. It is considered a middle sized galaxy.
Galaxies are classified by their shape. The Milky Way is a spiral galaxy. There are three basic types of galaxies:
- the spiral galaxies,
- the elliptical galaxies, and
- the irregular galaxies.
Origin of a Galaxy
Right after the Big Bang, the universe was uniform. At first, the universe was only gas. There were no galaxies at that time. Then, the universe slowly began to cool down. Gases began to condense into areas filled with dark matter. Gases such as hydrogen and helium began to form in these areas.
Slowly, these gases began to combine into stars. Large groups of stars made the first galaxy. The first galaxies began to appear when the universe was about 500 million years old.
Improving Your Reading and Thinking Skills
Think for a few moments. Try to recall the major ideas in this article. When you are done, click the box below for a list of main ideas.
How did you do? With practice, you should be able to identify many of the major ideas. You may even discover some important ideas that aren’t listed.