Normally we think of black holes as stellar black holes, black holes formed from the collapse of a sufficiently sized star. And rightly so! They make up most of the black holes in the universe. We’re also familiar with supermassive black holes which are generally located in the centre of most galaxies, getting bigger and bigger every day by eating the matter that spirals into them in their accretion disk.
And there’s also a class of black holes called intermediate mass black holes, black holes with a mass of 100 to one million solar masses, being more massive than your typical stellar black hole but less massive than supermassive black holes. Another type of black hole, although theoretical,is a primordial black hole, a black hole formed at the very early stages of the universe,immediately after the Big Bang. While we have all these different types or classifications of black holes, they all have one particular thing in common: they can all collide and merge. When this happens, it is one of the most extraordinary events in the universe. There are two possible paths where two black holes can collide and merge. Firstly, in a binary star system.
These stars have formed together at the same time in a stellar nebula, and are much more massive than our sun, having masses several hundred times that of the mass of the sun. Because of their exceptional mass, the two of them only have a lifetime of a few million years. After orbiting each other for a few million years, one of the stars explodes in a supernova and eventually becomes a black hole. Now we have still a massive star but orbiting a black hole. Soon enough the other star detonates and becomes a black hole. Now what’s interesting is that all stars have a finite lifetime because eventually they run out of fuel however black holes do not. Well in truth, black holes most likely will die but the process takes exceptionally long.
It’s thought that black holes evaporate and shrink due to Hawking radiation. Anyways, this binary black hole system, like everything else in the universe, will emit gravitational waves in every direction at the speed of light. The emission of gravitational waves decays the orbit of the black holes and decreases the angular momentum of each black hole, meaning that over time the black holes will spiral closer and closer into each other. The beginning of this inevitable inward spiral takes the longest, as the gravitational wave emission is very low since the black holes aren’t very close.
As the orbit shrinks the speed increases and the emission of gravitational waves increases. Eventually the orbit becomes so small that the two black holes collide and coalesce into one black hole with a mass of about the sum of the two previous black holes. The mass of the end product black hole will be less than the sum of the two because some of the mass will have been converted into energy in the form of gravitational waves. With sizeable black holes, this tends to be a few solar masses. The third detection of gravitational waves,for instance, involved the merger of two black holes, one with a mass of 31 solar masses and the other with 19. The final black hole had a mass of 49 solar masses.
When that final black hole is created the emission of gravitational waves peaks. The other possibility where a pair of black holes can collide is if they happen to run into each other. This event includes two black holes forming completely independently of each other then coming close enough to each other that they become locked in orbit, and then following the same path as we described earlier in the inward spiral and the eventual merge. I’d say that this event is a lot less common than the other possibility we discussed since the chances of two black holes coming across each other aren’t high.
My guess would make sense since only one of the gravitational wave detections has black holes that has evidence that they did not originate together. This event also has a lot more variations in how it can occur. As far as we can tell most galaxies have a supermassive black hole in the centre. So two galaxies merging involves the supermassive black holes in the centre of each galaxy colliding and merging as well.
It’s thought that the Milky Way galaxy and Andromeda galaxy will collide in about 4 billion years and take a few billion years to actually coalesce. By then the Sun won’t have begun its helium burning process, so it won’t be a red giant yet however Earth will become un inhabitable by then as the Sun will increase in enough luminosity to wipe all life out on Earth. Although the two supermassive black holes will merge, it’s very unlikely that there will be any star collisions because of the large distances between each star system.
The supermassive black hole in the centre of our galaxy, Sagittarius A*, and the supermassive black hole in the centre of Andromeda will begin to radiate very strong gravitational waves once they’re only a light year apart. It’s estimated that in about 150 billion years from now every other galaxy in the Local Group will have merged into this galaxy. Comment down below your thoughts on black holes colliding and merging. PLEASE VISIT CONCEPTOFSCIENCE.COM TO READ MORE FASCINATING THINGS.
Originally published at https://www.conceptofscience.com on November 11, 2020.