How Do Black Holes Form?

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X-Ray, Optical, and Infrared composite image of the remnant from the supernova observed by Kepler in 1604. Image Credit: NASA/ESA/JHU/R.Sankrit & W.Blair

Black holes are astronomical objects that have collapsed to the point that the gravitational force at their surfaces prevents anything from escaping.

Objects can fall into, but never escape from, black holes – so where do black holes come from?

Stellar black holes form in the explosive death throes of massive stars. Supermassive black holes, found in the cores of galaxies, form from the collisions and mergers of many stars and stellar black holes.

There may also be primordial black holes that formed during the Big Bang.

Supernovas and Black Hole Formation

Ordinary stellar black holes form from the deaths of massive stars via type II supernova explosions. (Caveat: Astronomers talk about star birth, star death, and stellar lives, but stars are not alive in any biological sense.)

During the majority of their lives, which astronomers call the main sequence, stars produce their energy by hydrogen-burning nuclear fusion reactions in their cores. In these fusion reactions, four hydrogen atoms fuse to make one helium atom, while releasing energy.

All stars eventually exhaust their hydrogen fuel and then begin to die out. Stars with the same mass as the Sun have enough hydrogen fuel to last about 10 billion years. Very massive stars burn their fuel much more rapidly, and may run out in less than a million years.

When stars run out of hydrogen fuel, they expand into red giant or red supergiant stars. The largest red supergiants are so big that they would extend as far as the area between the orbits of Mars and Jupiter if placed at the Sun’s location. After expanding into red giants, our Sun, and other stars with relatively low masses, will quietly collapse into white dwarf stars and, over billions of years, gradually cool like the last burning embers of a fire.

The most massive stars, however, end their lives much more violently. Expanding into red supergiants, these stars continue to fuse successively heavier elements in their cores until they have an iron core. The iron core cannot undergo energy-generating nuclear reactions, so it collapses.

When the gravitational forces can compress the atoms no further, the atoms rebound, like a basketball bouncing off the floor. However, in this case, each falling ball, which is an atomic particle, has a series of similar atomic particles above it. Each atomic particle gets a big kick upwards from the particle below it. Hence the collapsing core rebounds with a huge amount of energy. The rebound triggers a type II supernova explosion; the violent death of the massive star. During the supernova explosion, the massive star, in about a year, emits about as much total energy as the Sun will emit in its entire 10- billion-year hydrogen-burning main sequence lifetime.

The type II supernova blasts much of the star back into space to be recycled into the next generation of stars, but a small core remains. If this core is less than between 2 and 3 times the Sun’s mass, it will collapse into a neutron star. If, however, it is more than 2 to 3 (the exact number is uncertain) times the Sun’s mass, the stellar core will collapse into a black hole.

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