• Question: How wide is the black hole?

    Asked by DJSTobyM to Hannah, Lucy, Phil, Stephen on 14 Mar 2017.
    • Photo: Lucy Kissick

      Lucy Kissick answered on 14 Mar 2017:

      It’s difficult to tell how big a black hole gets in terms of distance, but astronomers can estimate how many solar masses (the size of the sun) one contains. The largest might be as many as 21 billion times the size of the Sun.

      I’m really, really, really glad it is far away.

    • Photo: Phil Sutton

      Phil Sutton answered on 14 Mar 2017:

      A black doesn’t really have any spatial dimensions. The defining feature of a black hole is that it is a star that was so big when it came to the end of its life the huge gravity causes all of the matter that was once part of the star to be compressed down into a singularity. A singularity has no volume associated with it but it does have a mass. Therefore it has an infinite density. Now we can’t see black holes because once inside the event horizon even light can not escape the gravity. A black hole does have an event horizon which will have a width or size proportional to its mass. So more massive black holes will have a wider event horizon. We should also say at this point we actually haven’t imaged a black hole or an event horizon but we have measured the mass of black holes. How do we do this? Well we can we work out the mass of astronomical objects by how things nearby orbit around them. We have looked towards the supermassive black hole at the centre of the milkyway and watched as stars orbit very fast around an invisible object and calculated its mass to be approximately 4 million times the mass of the Sun.

    • Photo: Stephen Pulker

      Stephen Pulker answered on 15 Mar 2017:

      Phil gives a good answer to this. The dimensions of a black hole (the region from which you can never escape) is roughly a sphere with size proportional to the amount of mass inside it.

    • Photo: Hannah Sargeant

      Hannah Sargeant answered on 15 Mar 2017:

      As everyone else has said, the black holes can be very big indeed! The biggest ones are thought to be at the centre of galaxies. That means there’s a supermassive black hole at the centre of the milkyway!