NASA - National Aeronautics and Space Administration

Stars are huge balls of glowing gas in the sky. The sun is a star. It is the only star close enough to the Earth to look like a ball to us. The other stars are so far away that they look like tiny dots of light. But even though they look small to us, stars are very big. Some of them are 1,000 times bigger than the sun.

Image to left: A globular cluster is a tightly grouped swarm of stars held together by gravity. Credit: NASA

Stars shine because they give off energy. Deep inside, a gas called hydrogen is changed into gas called helium. As it changes, it gives off energy. The star then gets very hot and gives off light.

Stars have different temperatures and sizes. As a result, some stars are brighter than others, and they have different colors. Some stars look yellow, like the sun. Others glow blue, white, or red.

Stars seem to twinkle because we see starlight through the moving layers of air that surround the Earth. Stars shine all day and night, but we can see them only when the sky is dark and clear.

Scientists think there are about 10 billion trillion stars. Stars are not spread out evenly in space. They are in groups called galaxies. The sun is in a galaxy called the Milky Way.

People have studied the stars for thousands of years. The scientific study of stars is called astronomy.

How to cite this article: To cite this article, World Book recommends the following format: "Star." The World Book Student Discovery Encyclopedia. Chicago: World Book, Inc., 2005.

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