Great question! There are actually two types of black holes that scientists consider: "stellar black holes" and "supermassive black holes".
A stellar black hole is created when a big star runs out of fuel. When this happens the star explodes, and whatever material is left over is super compacted. This super compact core has such strong gravity that it collapses into a black hole. For this process to occur, the original star has to be more than twenty times the mass of our sun, otherwise there won't be enough mass left over from the explosion to form a black hole.
Supermassive black holes are not as well understood. We know that they can have over a million times as much mass as our sun, and that they are frequently located at the center of galaxies. These black holes are so big that they could not have formed from any stars that exist in the universe today. Scientists are still trying to understand where these supermassive black holes come from, but one theory is that they are the result of smaller black holes combining together.
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