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Galaxies

by Ron Kurtus (8 March 2006)

A galaxy is a collection of a huge number of stars that are rotating around a common center or axis. There is also dust and other material dispersed among the stars. There are many billions of galaxies in the Universe. Many of the "stars" you see in the night-time sky are really galaxies consisting of a great many actual stars. Our Sun is one star in the Milky Way galaxy.

Questions you may have include:

• What is the most obvious part of a galaxy?
• What other material is in a galaxy?
• How big are galaxies?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

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Stars in galaxies

A typical galaxy consists of between 10 million (10⁷) and 1 trillion (10¹²) stars slowly rotating around a central axis. It is possible that many of those stars are solar systems with planets rotating around them. Beside all the stars, there is also interstellar gas and dust rotating around the center.

Top view of galaxy with spiral arms of stars and dust

Dark matter

Some astronomical measurements indicate there is "something else" among the stars and interstellar material. This is called dark matter, because it cannot be seen, but it apparently has mass and does affect other matter and gravitational fields.

Although so-called dark matter appears to account around 90% of the mass of most galaxies, the nature of this material is not well understood.

At the center of some galaxies, there may be super massive black holes. Again, they cannot be seen but are indicated on how they affect nearby stars.

Size and distance

Most galaxies are several thousand to several hundred thousand light years in diameter. A light year is about 5,900,000,000,000 miles (5.9*10¹² mi) or 9,500,000,000,000 kilometers (9.5*10¹² km).

Edge view of a galaxy 10,000 light years wide

Galaxies are usually separated from each another by millions of light years. There are about 125 billion (1.25*10¹¹) different galaxies in the Universe.

Called the "sombrero" galaxy because of its unusual shape

Summary

Many of the "stars" you see in the night sky are actually far-away galaxies. They consist on billions of stars rotating around a center. Gases, dust and dark matter are also part of a galaxy. Some galaxies include one or more black holes near their centers. Galaxies are in thousands of light years in diameter and are separated by millions of light years.

Answers to Readers' Questions

You are important in this huge Universe

Resources

The following resources provide information on this subject:

Websites

Astronomy Resources

Books

Top-rated books on Astronomy

Top-rated books on Galaxies

Miscellaneous

Mini-quiz to check your understanding

1. What are many of the stars in a galaxy?

2. Why do they call it "dark matter"?

3. What is a light year?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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