• Question: What happens when two black holes collide?

    Asked by edrienepadua to Christina, Colin, Jess, Samaneh, Steve on 14 Jun 2013.
    • Photo: Steven Gardner

      Steven Gardner answered on 14 Jun 2013:

      You get one heavier, bigger black hole.

      Around all black holes, there is a boundary where if you go past it, the gravity is so strong that nothing, not even light, can escape. This boundary is called the event horizon. How far away the event horizon is from the centre of the black hole depends on how heavy the black hole is. As more things disappear into it the gravity gets stronger and so the event horizon will get bigger. If the black hole swallows enough heavy stars or if enough of them combine, it gets so big that we call it a supermassive black hole. Supermassive black holes are found at the centre of galaxies (there’s one at the centre of ours), because that’s where stars are most densly packed.

      Supermassive black holes are absolutely huge, the one at the centre of our galaxy weights 4 million times as much as than the sun!

    • Photo: Christina Pagel

      Christina Pagel answered on 14 Jun 2013:

      Steven is right that you could definitely get one bigger black hole… Or, depending on how quickly they were spinning (almost all things in space spin – e.g. we spin (once a day), the sun spins about once every 4 weeks) – if they were spinning fast enough and hit each other at an angle, they could actually bounce off each other and just head off on their way in different directions…

      Five years ago, scientists at the Max Planck Institute in Germany saw exactly this – a black hole kicked out of its galaxy by bouncing into another black hole!

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