Black holes are some of the most interesting and mysterious objects in the cosmos. These bodies are a place in space with such intense gravity that even light can not escape from them. As nothing can go faster than light, this means that nothing can go out from black holes. How do black holes form?
In this post we will explore how different types of black holes form: some of them form when a star collapses by its own gravity, but there are still some questions to answer about these mysterious bodies.
Stellar-mass black holes
In general, black holes form when a supermassive star explodes in the form of a supernova. Throughout its life, a star produces energy through a process known as nuclear fusion, which consists in fusing atoms, to create incredible quantities of energy. However, in the case of the biggest stars, a moment arrives in which this process consumes energy instead of creating it. At this moment, the star collapses on its own weight, in one of the most energetic and bright phenomenons of the universe: the supernovae.
In this explosion, the outer layers of the star are violently ejected into space. On the other hand, if the nucleus, the only thing left over from the star, has a mass above three solar masses, the extreme gravitational field of the object would make all the material concentrate in an infinitely small point with infinite density: a black hole would have been created.
Supermassive black holes
The process we have described previously explains how stellar-mass black holes form, but there are other types of black holes that are too big to have formed from the collapse of a star. They are supermassive black holes. They are generally found in the centre of galaxies, and can be billions of times bigger than the Sun. For example, the Milky Way contains a black hole that is a million times more massive than the Sun in its centre.
Even if there are some hypothesis, the origin and the formation of these enormous bodies is still a mystery for scientists.
At first moment, we could think that supermassive black holes come from stellar black holes that, throughout all their life, have been absorbing matter or fusing with other black holes to arrive at their current size.
However, it is calculated that it would be necessary more time than the actual age of the universe for a stellar-mass black hole to become a supermassive black hole. Moreover, astronomers have discovered objects in space named quasars that glow with more intensity than thousands of galaxies, and that are thought to be supermassive black holes (much bigger than the one of our galaxy) that consume incredible quantities of matter. Some of these bodies formed one billion years after the Big Bang. This would be impossible with the previously mentioned hypothesis.
Another theory states that supermassive black holes could have been formed even before the formation of the first stars, due to the collapse of enormous clouds of gas.