How do black holes form?

The  most  common  way  for  a  black  hole  to  form  is probably in a supernova, an exploding star. When a star with about 25 times the mass of the Sun ends its life, it explodes. The  outer  part  of  the  star  screams  outward at  high  speed,  but  the  inner  part  of  the  star,  its  core, collapses down. If there is enough mass, the gravity of the collapsing core will compress it so much that it can become a black hole. When it’s all over, the black hole will have a few times the mass of the Sun. This is called a  “stellar-mass  black  hole”,  what  many  astronomers think of as a “regular” black hole. But there are also monsters, called supermassive black holes.  These  lurk  in  the  centers  of  galaxies,  and  are huge:  they can be millions or even billions of  times  the mass  of  the  Sun!  They  probably  formed  at  the  same time  as  their  pa rent  galaxies,  but  exactly  how  is  not known for sure. Perhaps each one started as a single huge star which exploded  to  create  a black  hole,  and  then accumulated  more material  (including other  black  holes).  Astronomers  think there is a  supermassive black hole in the center of  nearly  every  large galaxy, including our own Milky Way. Stellar-mass black holes also form when two orbiting neutron  stars – ultra-dense  stellar  cores  left over  from one kind of supernova –   merge  to produce a short gamma ray burst, a tremendous blast of energy detectable across the entire observable Universe. Gamma-ray bursts are in a sense the birth cries of black holes.