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The galaxy M 100, seen through the Hubble telescope

The galaxy M 100, seen through the Hubble telescope

What is a galaxy?

A galaxy is a group of millions of stars that are relatively near each other. Most stars seem to be in one or another galaxy. Earth‘s own star, the Sun, is in the Milky Way galaxy.

What is the sun made of?
What are stars made of?
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How many stars are in a galaxy?

Small galaxies have about ten million stars, while big galaxies sometimes have a trillion stars in them. There are probably more than a hundred billion galaxies in the Universe.

How do big numbers work?

Gravity and galaxies

Gravity holds the stars together into a galaxy, just as gravity holds planets in orbit around a star. In most galaxies, the stars also orbit around the center of the galaxy the way planets orbit around a star.

What is gravity?
More about planets
What are black holes?

But we don’t know what is in the middle of galaxies. Some astronomers think there might be giant black holes in the middle of galaxies. The only parts of galaxies we can see are the stars (and sometimes planets), but there is probably other stuff in galaxies as well, that doesn’t send out energy or light waves, so we can’t see it and don’t know much about it.

What are light waves?

Galaxies and empty space

What is space?

Bibliography and further reading about galaxies and stars:

More about Stars
What is Space?
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