A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. There are two main types of black hole; stellar mass and supermassive black holes, and they differ in size, formation, and impact on their host galaxy. Stellar mass black holes, a few to dozens of times the mass of the sun, form from collapsing massive stars. Supermassive black holes, on the other hand, are millions to billions of times more massive and tend to live in the center of galaxies and grow through accretion and mergers.