Black holes

What is a black hole and why is it called that?

A black hole is a star that has collapsed onto itself. Very large stars, ones that are much larger than the Sun, end their lives in a titanic explosion called a supernova, which blows off most of the star into space. What's left of the star collapses onto itself due to the strong gravity. If the star shrinks enough, the surface gravity gets so strong that not even light can escape. Since no light can get out, the object is black. We believe that no objects in the Universe can travel faster than light, so anything that falls into a star like this can never get out. That's why it's a hole. Get it? A black hole!

No one has ever actually seen a black hole, but there is a lot of evidence they exist. Anything near a black hole will orbit it very quickly, and we think we see material orbiting objects at the right speeds. Sometimes we see a star in orbit around another star, but we can't see the second star! There are ways to figure out the mass of the star that cannot be seen (mostly by knowing how fast the other one is going around it) and if it's massive enough we know it's a black hole.

It looks very likely that there is a huge black hole in the center of our own Galaxy. The mass of that black hole may be more than a million times the mass of the Sun!

There are many many sites on the web with black hole information. There is even a Mad Scientist question I answered about them. Go to that page and read the answer I wrote. There are more links to black hole information on the web there.