When do you have a black hole?

Answer 1

In one sentence, a black hole appears when a stellar mass, e.g. star, becomes so dense that the curvature of space around the mass becomes so great that light can not escape from a volume around the star called the event horizon.

There are more details for those who are interested.

A black hole is a result the curvature of space due to gravity. This is predicted by Einstein's theory of gravity. Black holes have been confirmed to the satisfaction of most scientists for the last three or four decades.

A black hole can be created when the mass of a star exceeds a certain critical mass and that mass is about 10 times the mas of the Sun. Normally, even a large star, hundreds of times more massive than the Sun, is not dense enough to create a black hole while it is burning in its normal fashion. It is only near the end of the life of a star that things happen which concentrate the mass near the center.

The process by which a star evolves to become a black hole is called gravitational collapse. This occurs when the star can not support its own weight. The material at the center of the star evolves normally as the star ages. It begins with a plasma of hydrogen atoms, evolves through the creation of helium and eventually heavier nuclei. During this process the center of the start gets very dense, but the pressure remains high enough to support the great mass above the center. For a large enough star, after burning for millions or billions of years, the fuel begins to run out and the pressures at the center crush the atoms.

Depending on the size of the star, life has three possible outcomes, a white dwarf star, a neutron star or a black hole. In the first case, light stars for a very massive white dwarf but do not suffer the gravitational collapse that obliterates all atoms. In the second case, a neutron star has collapsed, the nuclei of the original atoms all fused together but the mass remaining is not enough to curve space into a black hole. This neutron star formation may include a supernova explosion.

Finally, for the most massive stars, there is enough mass left at the end, even after a supernova explosion, to cause space to curve enough that light can not escape. That is a black hole.

There are more extensive details about the formation process and the properties of space-time around the black hole after it is formed that are outside of the present discussion.

Answer 2

A Black Hole is the result of a Star about twenty times more massive then our own Sun reaching the end of its life. They occur when ever one of these stars die, and since these stars have life-spans of only millions of years, they can occur fairly often, and can last for as long as there is more matter for them to absorb.

Also, it is theorized that a Mini Black Hole occurs when ever atoms are smashed in a Supercollider, however, these are not dangerous, and evaporate almost immediately.

Answer 3

Anywhere where a large star stops shining. If the remaining mass, after the supernova explosion, is large enough, the star will become a black hole.

Answer 4

The condition for a black hole to exist is that there is a sufficiently large mass, in a sufficiently small volume.

Answer 5

Very often because space never ends so there are billions of black holes out there all ready and they form most likely every second.

Answer 6

There is no definitive number as to how many black holes are formed within the universe in any given year.