Where do black holes come from?

Question posed by Jennie.

First off, this post has the potential to be huge, so I'm going to cut out whatever I think I can get away with. If you'd like to know more about what black holes are, more about the birth, life and death of stars, or any of the other things that I may well have skimmed over in the course of this post, please, as usual, comment with your requests!

What is a black hole?
A black hole is, simply put, a region of space which is defined by a gravitational field so strong that nothing, not even light, can escape its grasp. There are two parts to a black hole: a singularity and an event horizon.

No black holes have been directly observed*, but they have been predicted by mathematics and appear to fill certain holes** and fit certain shoes in space science and astronomy. It is, for example, theorised (and widely accepted) that the supermassive object that exists at the centre of our galaxy (the Milky Way) is a black hole, and that there are black holes at the centre of most (if not every) large galaxy.

Where do they come from?
Black holes can be made by different processes. For the purposes of this blogstronomy post, I'll concentrate on the 'main'*** process, and provide a few words on the other possibilities to whet your appetite afterwards.

Gravitational collapse
Stars are big. The mass of our closest star, the Sun, is around 300,000 times the mass of the Earth, and it's not a particularly large star. Star masses are often stated, for reasons of convenience, as a multiple of the mass of the Sun. For example, a star described as having a mass of 'one solar mass' has exactly the same mass as the Sun. A star with 'two solar masses' is twice as massive as the Sun, and so on.

Stars are all formed in pretty much the same way, but have different fates based on their mass. When a star is 'born,' its large mass causes it to begin collapsing in on itself due to gravity****. This collapse provides the necessary pressures to start the fire burning, which stops the star collapsing.
At the end of its life, when all of its fuel has been used up, the star begins to die. I won't go through the death-throes of a star (that's another post's worth by itself), but in short, the fires go out, and the mass that's left starts to collapse in on itself again. This time, however, there is no fuel left, and so the fires do not re-start. What happens from here onwards is determined by the mass of the star:

• Stars of around one solar mass (up to about 4 solar masses maximum) like our Sun will collapse until their atoms can be pushed no closer together. They become a very dense, cooling body known as a white dwarf, which is a bit bigger than the Earth. This will slowly cool until it becomes a black dwarf.
• Stars of between 4 and 8 solar masses will collapse more violently and explode, blowing off the outer layers of the star. This explosion is called a supernova. The left over central region of the star continues to collapse under gravity, but goes beyond the white dwarf stage, and the atoms themselves are crushed, forcing protons and electrons together to form neutrons. This is a neutron star. Neutron stars are around 1.4 times the mass of the Sun, but crammed into a ball only 20km in diameter: if you weighed a teaspoon of neutron star material on Earth, it'd weigh around a billion tons.
• Stars any bigger than that would collapse in a similar manner, but the collapse wouldn't stop at the neutron star stage: the star continues to collapse, crushing the neutrons even further to form a singularity.

Note: Black holes can form from neutron stars that have collected extra mass through accretion.


Other methods of black hole formation
Primordial black holes
Black holes can be formed even at lower masses than are required for the process outlined above if there is an external force that can provide the pressure required. One possibility for this is at the beginning of the universe, shortly after the big bang. The matter in the universe was at this time so densely packed that, assuming the distribution of the matter wasn't smooth, it is possible that some black holes were created. Depending on the conditions at this early stage of the universe, these black holes could have ranged anywhere from very tiny masses up to hundreds of thousands of solar masses.

High energy collisions
Another way to provide the pressures required to form a black hole is to smash particles together at very high velocities. In principle, low mass black holes could be created in particle accelerator experiments (such as those undertaken at the Large Hadron Collider).


Some extra interesting snippets
If you're interested in the subject, these could be starting points for your own research, or feel free to ask questions!

• Black holes can grow by absorbing or accreting additional matter and radiation.
• Black holes can evaporate via a process that results in Hawking radiation.
• The 'photo' at the top of this post is an example of gravitational lensing.
  

Have a question about this topic? Comment below! Got an astronomy related question of your own? Ask it here.

* Here, the pedant may like to point out that due to their nature they are not able to be directly observed... But you get the point.
** No pun intended. Honest.
*** That is, the most popularly recognised
**** It has been commented upon that our solar system's largest planet, Jupiter, may have needed only a few times more mass in order to undergo this process. It has been labelled a 'failed star' by some people for this reason.