A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying.
Because no light can get out, it is apparently invisible to naked eye. Space telescopes with special tools can find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.
How Big Are Black Holes?
Black holes can be big or small. 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 "stuff," in an object. Another kind of black hole is called "stellar". 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.
The largest black holes are called "supermassive." These black holes have masses that are more than 1 million suns together. Every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns.
How Do Black Holes Form?
Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova. A supernova is an exploding star that blasts part of the star into space. The supermassive black holes were made at the same time as the galaxy they are in.
A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Satellites and telescopes in space are used to see the high-energy light.
Black holes do not go around in space eating stars, moons and planets. Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that.
Even if a black hole the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.
The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.