Black Hole

A black hole is a place in space where gravity pulls so much that even light cannot get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Since no light can get out, people can't see black holes. They are invisible. Space telescopes with special tools can help find black holes. The special tools can see how stars  very close to black holes act differently than other stars.

How big are Black Holes?

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 "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. Earth's galaxy is called the Milky Way.

The largest black holes are called "super-massive." These black holes have masses more than 1 million suns together. Scientists have found proof that every large galaxy contains a super-massive black hole at its center. The super-massive black hole at the center of the Milky Way galaxy is called Sagittarius A. It has a mass equal to about 4 million suns and would inside a very large ball that could hold a few million Earths.

How do Black Holes form?

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 collapses. When this happens, it causes  a supernova. A supernova is an exploding star that blasts part of the star into space. Scientists think super-massive black holes were made at the same time when the galaxy was made.

Gravitational Collapse

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. For stars this usually occurs either because a star has too little "fuel" left to maintain it's temperature through stellar nucleosynthesis or because a star that would have been stable receive extra matter in a way that does not raise it's core temperature. In either case the star's temperature is no longer high enough to prevent it from collapsing under its own weight. The collapse may be stopped by the degeneracy pressure of the star's constituent allowing the condensation of matter into a exotic denser state.