Black holes are expected to form via two distinct channels. According to the first pathway, they are stellar corpses, so they form when massive stars die. Stars whose birth masses are above roughly 8 to 10 times mass of our sun, when they exhaust all their fuel — their hydrogen — they explode and die leaving behind a very compact dense object, a black hole. The resulting black hole that is left behind is referred to as a stellar mass black hole and its mass is of the order of a few times the mass of the sun.
Not all stars leave behind black holes, stars with lower birth masses leave behind a neutron star or a white dwarf. Another way that black holes form is from the direct collapse of gas, a process that is expected to result in more massive black holes with a mass ranging from 1000 times the mass of the sun up to even 100,000 times the mass of the sun. This channel circumvents the formation of the traditional star, and is believed to operate in the early universe and produce more massive black hole seeds.