Because no light can get out, people can't see black holes. Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently from other ones.

　　Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Mass is the amount of matter, or “staff”， in an object.

　　Another kind of black hole is called “stellar”(星球黑洞). Its mass can be up to 20 times more than the mass of the sun. There may be many stellar mass black holes in Earth's galaxy. Earth's galaxy is called the Milky Way.

　　The largest black holes are called “supermassive”(超大质量黑洞). These black holes have masses that are more than one million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy has a mass equal to about four million suns and would fit inside a very large ball that could hold a few million Earths.

　　Scientists think the smallest black holes formed when the universe began. Stellar black holes are made when the center of a very big star falls in upon itself, or falls apart. When this happens, it exploded part of the star into space. Scientists think supermassive black holes were made at the same time as the galaxy they are in.

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