• Question: how big is a black hole

    Asked by 496sptm42 to Ollie, Guy on 2 Mar 2018.
    • Photo: Guy Rixon

      Guy Rixon answered on 2 Mar 2018:

      I had to look this one up!

      The “size” of the black hole is to do with its event horizon – the place beyond which nothing can escape, not even light. Inside the horizon, physics comes unstuck rather fast and sizes don’t mean much.

      The size of the event horizon depends on the mass of the hole. There’s a web page http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/blkhol.html that can work it out for you: just type in the mass. For a hole of 3 times the mass of the sun, which is the smallest size made by collapsing stars, the radius is about 9km. For the big black hole at the centre of our galaxy, I get a value of about 10,000,000km. If you could make a hole with the mass of a person it would be tiny.

    • Photo: Alexander Burke

      Alexander Burke answered on 2 Mar 2018:

      I agree with Guy. The size of a black hole depends on how heavy the black hole is. Black holes can come in all sizes… from microscopic (super tiny) black holes, which Stephen Hawking came up with, to absolute massive monster black holes which could be 1 billion times heavier than our sun. Unfortunately, the microscopic black holes will be nearly impossible to detect since they “evaporate” almost instantaneously due to a thing called Hawking radiation. Think about it this way – A very small puddle takes less time to evaporate than a lake!

      The largest black hole ever discovered is roughly 66 billion times heavier than our sun. In fact, if my calculations are correct, in a Boeing 737 (and assuming we are not pulled into the centre of the black hole) it would take us well over 20,000 years to fly from the event horizon into the centre of the black hole… this black hole is an absolute monster!

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