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Things to Know Before You Go

Galaxies are enormous swarms of stars, dust, gas, and dark matter held together by gravity. The sun is one of about 100 billion stars in our own galaxy, called the Milky Way. If you think that?s incredible, imagine this: The Milky Way is just one of billions in the observable universe!

Most galaxies are found in clusters of about 150 galaxies, bound together by each other?s gravity. Our Milky Way is part of a small cluster of some 30 galaxies called the Local Group. Clusters of galaxies are often collected in superclusters. Our Local Group is part of the Virgo Supercluster, which contains several thousand galaxies.

Galaxies range in size, containing anywhere from 100,000 to 3 trillion stars! They also come in different shapes. There are three major types of galaxies:

Spiral galaxies are shaped like disks and look like pinwheels from above. Young stars are found in the arms, and older stars are found in the central bulge, or nucleus.

Elliptical galaxies are the oldest and largest galaxies. They are smooth and oval and contain many old stars. There are many more elliptical galaxies in the universe than spiral galaxies.

Irregular galaxies don?t have a distinct shape and are not symmetrical like spiral or elliptical galaxies. They may be young galaxies that have not yet formed a symmetrical shape, or their irregular shape may be caused by two galaxies colliding.

Answers From the Expert
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Frank Summers, American Museum of Natural History
? How do astronomers count 100 billion stars without counting ones they already counted?

Click here for Frank's answer.

? How do spiral galaxies form?

Click here for Frank's answer.

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