Q:

How long does it take a star to form a black hole after its death?

A:

Black Hole formation during the collapse of massive stars can proceed in two different ways: either the massive star collapses directly into a black hole without a supernova explosion or an explosion occurs, but its energy is too low to completely blow away the stellar envelope, and a significant part of the star falls back to form a black hole. In either case, calculations indicate that a black hole is formed in a time that ranges from about a tenth of a second to half a second!

An alternative path involves the collapse of a neutron star into a black hole if it accretes so much material from a nearby companion star, or merges with the companion star that it is pushed over the neutron star limit and collapses to become a black hole. This process could take a million years or more depending on how quickly it accretes the material, but once the neutron star is over the limit, which is about 3 solar masses, the collapse to a black hole occurs in less than a second.