If a supernova’s original star is massive enough to form a black hole, why is there any explosion? Why doesn’t the entire mass of the original star simply fall onto the newly formed black hole and instantly vanish?

For some massive stars, it may be possible that collapse leads directly to the formation of a black hole with no explosion. Astronomers have proposed that such stars might just wink out of existence, and there is a recent report that such an event might have been detected. It seems a star that appeared in a Hubble Space Telescope image years ago has vanished.

For moderately sized stars, from about 10 to upward of perhaps 100 times the mass of the Sun, the situation is more complicated. The problem is that such stars form cores of iron that absorb energy and trigger the collapse. In this case, the collapsing material first forms a neutron star. In most circumstances, the energy released in that process turns the implosion into an explosion.

The nature of that reversal is hotly debated and may involve the precise mass of the star, the degree of its rotation, the strength of magnetic fields, and other aspects. In this context, a black hole might form if some matter initially blown away from the neutron star in the explosion does not achieve escape velocity.

If sufficient material falls back to accumulate on the newly formed neutron star, it may be crushed to form a black hole despite the fact that the outer parts of the star exploded as a supernova. In this case, the result would be a supernova leaving behind a black hole rather than a neutron star.