Colliding Black Holes

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Normally we think of black holes as stellarblack holes, black holes formed from the collapse of a sufficiently sized star. And rightly so! They make up most of the black holes in theuniverse. We’re also familiar with supermassive blackholes which are generally located in the centre of most galaxies, getting bigger and biggerevery day by eating the matter that spirals into them in their accretion disk. And there’s also a class of black holescalled intermediate mass black holes, black holes with a mass of 100 to one million solarmasses, being more massive than your typical stellar black hole but less massive than supermassiveblack 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 orclassifications of black holes, they all have one particular thing in common: they can allcollide and merge. When this happens, it is one of the most extraordinaryevents in the universe. There are two possible paths where two blackholes can collide and merge. Firstly, in a binary star system. These stars have formed together at the sametime in a stellar nebula, and are much more massive than our sun, having masses severalhundred times that of the mass of the sun. Because of their exceptional mass, the twoof them only have a lifetime of a few million years. After orbiting each other for a few millionyears, one of the stars explodes in a supernova and eventually becomes a black hole. Now we have still a massive star but orbitinga black hole. Soon enough the other star detonates and becomesa black hole. Now what’s interesting is that all starshave a finite lifetime because eventually they run out of fuel however black holes donot. Well in truth, black holes most likely willdie but the process takes exceptionally long.

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It’s thought that black holes evaporateand shrink due to Hawking radiation. Anyways, this binary black hole system, likeeverything else in the universe, will emit gravitational waves in every direction atthe speed of light. The emission of gravitational waves decaysthe orbit of the black holes and decreases the angular momentum of each black hole, meaningthat over time the black holes will spiral closer and closer into each other. The beginning of this inevitable inward spiraltakes the longest, as the gravitational wave emission is very low since the black holesaren’t very close. As the orbit shrinks the speed increases andthe emission of gravitational waves increases. Eventually the orbit becomes so small thatthe two black holes collide and coalesce into one black hole with a mass of about the sumof the two previous black holes. The mass of the end product black hole willbe less than the sum of the two because some of the mass will have been converted intoenergy in the form of gravitational waves. With sizeable black holes, this tends to bea 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 massesand the other with 19. The final black hole had a mass of 49 solarmasses. When that final black hole is created theemission of gravitational waves peaks. The other possibility where a pair of blackholes can collide is if they happen to run into each other. This event includes two black holes formingcompletely independently of each other then coming close enough to each other that theybecome locked in orbit, and then following the same path as we described earlier in theinward spiral and the eventual merge. I’d say that this event is a lot less commonthan the other possibility we discussed since the chances of two black holes coming acrosseach other aren’t high. My guess would make sense since only one ofthe gravitational wave detections has black holes that has evidence that they did notoriginate together. This event also has a lot more variationsin how it can occur. As far as we can tell most galaxies have asupermassive black hole in the centre.

So two galaxies merging involves the supermassiveblack holes in the centre of each galaxy colliding and merging as well. It’s thought that the Milky Way galaxy andAndromeda galaxy will collide in about 4 billion years and take a few billion years to actuallycoalesce. By then the Sun won’t have begun its heliumburning process, so it won’t be a red giant yet however Earth will become uninhabitableby then as the Sun will increase in enough luminosity to wipe all life out on Earth. Although the two supermassive black holeswill merge, it’s very unlikely that there will be any star collisions because of thelarge distances between each star system. The supermassive black hole in the centreof our galaxy, Sagittarius A*, and the supermassive black hole in the centre of Andromeda willbegin to radiate very strong gravitational waves once they’re only a light year apart. It’s estimated that in about 150 billionyears from now every other galaxy in the Local Group will have merged into this galaxy. Comment down below your thoughts on blackholes colliding and merging. Make sure you subscribe for more content onastronomy and futurism. If you enjoyed this video check out my mostrecent video, a previous video or my playlist on black holes. Thank you so much for watching, have a niceday.

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Colliding Black Holes. (2022, February 17). Edubirdie. Retrieved April 17, 2024, from
“Colliding Black Holes.” Edubirdie, 17 Feb. 2022,
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