Two weeks ago today, we explored the beautiful enigmas that are the black holes. We now have a basic understanding of what exactly they are, and have learnt of the scientific community’s recent efforts towards imaging the area around a black hole for the first time in history. Let’s now take a look at how the black hole came to be there and what it likes to do in its spare time, shall we?
There are three types of black hole that exist, classified based on their mass: stellar, supermassive, and miniature. Which one of these a black hole is depends on how it was formed. The most common type of black hole is the stellar, which is formed from the remains of a dying star. While the star is still alive, there is a constant tug of war between gravity pulling inwards and pressure pushing outwards which is generated from the energy of nuclear reactions in the core of the star. For most of its life, the two forces will balance each other exactly which means that the star is stable and stays at a relatively constant size, like our Sun (currently). However, when the star runs out of fuel, gravity becomes the dominant force and the star begins to collapse inwards under its own weight. For relatively smaller stars, this process will stop after a while as the repulsive forces among particles within the star eventually create enough pressure to prevent further collapse, known technically as degeneracy pressure. The star will then cool down and become what is known as a white dwarf.
However, when a larger star collapses, there are no known repulsive forces inside it that are powerful enough to prevent the implosion, so gravity continues to collapse the core, eventually forming a black hole. Any star with a mass of over about 20 times the mass of our Sun is capable of forming a black hole upon its death, known as Tolman–Oppenheimer–Volkoff limit.
The second type of black hole, the supermassive, is, as we’d expect it to be, super massive. Their size tends to be on the order of billions of solar masses. Here’s an animation which really demonstrates just how mind-bogglingly huge these things are.
The formation of these black holes still remains an open field of research. We know that they exist, however, through observing their effects on surrounding space. For example, the existence of quasars that radiate over a trillion times as much as energy as our Sun from a region about the size of our Solar System can only be explained through the mechanism of a black hole converting gravitational energy into light. They also account for the high orbital velocities of material surrounding the centre of galaxies. These black holes occupy the centre of most large galaxies, our own Milky Way being one of them with Sagittarius A* at its centre. As I discussed in my previous post, scientists are currently trying to image our galaxy’s black hole through using an array of telescopes spread across the whole globe, whose data will be accumulated together. Pictures are currently, as you read this, being taken, but due to long processing times will not be released until later this year.
The final type of black hole, the miniature or micro black hole, are actually completely theoretical, for which scientists have no evidence. Proposed by Stephen Hawking in 1971, these are thought to have existed in the short space of time just after the Big Bang, when density fluctuations were extremely variable, with some parts of space expanding or compressing more rapidly than others. These tiny black holes would pop in and out of existence constantly, kind of like a quantum primordial soup.They would have had event horizons as small as the width of an atom, and masses equivalent to Mount Everest. Some hypotheses predict that micro black holes could be formed at relatively low energies, which are available in the Large Hadron Collider. This caused lots of popular concern a few years ago as people thought miniature black holes would be created through the high-energy collisions that could demolish the planet.
See you next week. Stay curious,