Because light can not escape a black hole they are invisible. The only way they can be detected is special instraments that observe stars abnormal tendencies.
Black holes can range in size. Scientists hypothesis that the smallest black hole can be as large as an atom. A black hole this small, however, would have the same mass as a mountain. Mass is a measurement of how much matter an object contains.
The next category is a "stellar" black hole. The mass of a stellar black hole can range up the mass of 20 of our own sun. These move around the universe and pull in anything that gets too close to them.
The largest black holes are called "supermassive" black holes. These have the mass exceeding 1 million of our suns together. Astronomers have confirmed that there is a supermassive black hole at the center of every galaxy. These black holes keep the galaxys from flying off in all directions. In the center of our galaxy, the black hole's mass is equal to approximately 5 million suns.
