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15.7: Milky Way

Difficulty Level: At Grade Created by: CK-12
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“The Milky Way is nothing else but a mass of innumerable stars planted together in clusters.” — Galileo Galilei

It's sad that there is so much light pollution in most cities that many people have never seen the Milky Way. On a clear night away from lights the view is of a bright white river of stars. You don't need a telescope or even binoculars to see it. The view of the Milky Way is so bright because you're looking at the stars in your own galaxy.

The Milky Way Galaxy

The Milky Way Galaxy, which is our galaxy. The Milky Way is made of millions of stars along with a lot of gas and dust. It looks different from other galaxies because we are looking at the main disk from within the galaxy. Astronomers estimate that the Milky Way contains 200 to 400 billion stars.

Shape and Size

Although it is difficult to know what the shape of the Milky Way Galaxy is because we are inside of it, astronomers have identified it as a typical spiral galaxy containing about 100 billion to 400 billion stars (Figure below).

An artist’s rendition of what astronomers think the Milky Way Galaxy would look like seen from above. The Sun is located approximately where the arrow points.

Like other spiral galaxies, our galaxy has a disk, a central bulge, and spiral arms. The disk is about 100,000 light-years across and 3,000 light-years thick. Most of the Galaxy’s gas, dust, young stars, and open clusters are in the disk.

What evidence do astronomers find that lets them know that the Milky Way is a spiral galaxy?

1. The shape of the galaxy as we see it (Figure below).

An infrared image of the Milky Way shows the long thin line of stars and the central bulge typical of spiral galaxies.

2. The velocities of stars and gas in the galaxy show a rotational motion.

3. The gases, color, and dust are typical of spiral galaxies.

The central bulge is about 12,000 to 16,000 light-years wide and 6,000 to 10,000 light-years thick. The central bulge contains mostly older stars and globular clusters. Some recent evidence suggests the bulge might not be spherical, but is instead shaped like a bar. The bar might be as long as 27,000 light-years long. The disk and bulge are surrounded by a faint, spherical halo, which also contains old stars and globular clusters. Astronomers have discovered that there is a gigantic black hole at the center of the galaxy.

The Milky Way Galaxy is a big place. If our solar system were the size of your fist, the Galaxy’s disk would still be wider than the entire United States!

A video closeup of the Milky Way Galaxy is seen here: http://www.space.com/common/media/video/player.php?videoRef=black_holes#playerTopjjj.

Where We Are

Our solar system, including the Sun, Earth, and all the other planets, is within one of the spiral arms in the disk of the Milky Way Galaxy. Most of the stars we see in the sky are relatively nearby stars that are also in this spiral arm. We are about 26,000 light-years from the center of the galaxy, a little more than halfway out from the center of the galaxy to the edge.

Just as Earth orbits the Sun, the Sun and solar system orbit the center of the Galaxy. One orbit of the solar system takes about 225 to 250 million years. The solar system has orbited 20 to 25 times since it formed 4.6 billion years ago. Astronomers have recently discovered that at the center of the Milky Way, and most other galaxies, is a supermassive black hole, although a black hole cannot be seen.

This video describes the solar system in which we live. It is located in an outer edge of the Milky Way galaxy, which spans 100,000 light years (2a): http://www.youtube.com/watch?v=0Rt7FevNiRc (5:10).

The Universe contains many billions of stars and there are many billions of galaxies. Our home, the Milky Way galaxy, is only one (2a, 2b): http://www.youtube.com/watch?v=eRJvB3hM7K0&feature=related (5:59).


  • We view the Milky Way Galaxy from within so it looks like a river of stars.
  • From outside the galaxy, the Milky Way would appear as a spiral.
  • A supermassive black hole resides at the center of the galaxy, just like within most other galaxies.


Use these resources to answer the questions that follow.


1. What type of galaxy is our galaxy?

2. Where are we located in the galaxy?

3. How old is our galaxy?


4. How wide is our galaxy?

5. How long does it take our solar system to revolve around the center of our galaxy?

6. How far away is the center of our galaxy?

7. Explain the Local Group.


1. Why do astronomers think that the Milky Way is a spiral galaxy?

2. Where is Earth within the Milky Way Galaxy?

3. What are some of the features found within the Milky Way Galaxy?

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Milky Way Galaxy The spiral galaxy in which Earth and our solar system reside.

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Difficulty Level:
At Grade

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Date Created:
Feb 24, 2012
Last Modified:
Aug 15, 2016
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