Clayton Robinson asked:
Watching the water drain away down the plug hole got me thinking....At the centre of every spiral galaxy is there a massive black hole or a star of huge mass, in turn, if all the galaxies rotate around a certain point in space...could there also be a black hole or giant star there?
We put this question to space scientist Judith Croston...
Judith - Okay, yes. So the questioner is right. There is a black hole in the centre of our galaxy and black holes are these very strange places in space where, essentially, no light and no matter, nothing can escape. An awful lot of matter has been squashed down - a bit like the neutron stars I was talking about before but even worse. Everything gets squashed down really, really far so there is a lot of gravity to do with a black hole.
But whatís interesting really is that black holes are only weird if you get close the them. Once youíre a little bit further away from the black hole, they just behave like any other star that had the same amount of mass. So actually, although in our galaxy we have stars spiralling around, we have these sort of beautiful spiral arms. But itís not the black hole at the centre thatís actually controlling that, itís actually the gravity from everything else in the galaxy. So the things that the stars are orbiting around is not really the black hole, itís all the stuff in the galaxy. And thereís actually much more stuff in the stars, and even more stuff in the dark matter. So itís actually ninety-five per cent of the mass in the galaxy is this strange, invisible dark matter and thatís actually what controls everything moving around.
So going to the bigger picture - talking about could there be a really monster black hole and all the galaxies were spiralling around a really monstrous black hole. Well firstly, the biggest black holes we know about are about a billion suns and thatís still a lot less matter than in one single galaxy. So when you think about how much gravitational pull that will have, even the biggest black holes we know about couldnít make an entire galaxy move around.
So again, when you think about whatís happening and the way that galaxies are moving in the universe, which is something we can actually measure quite a lot - we can do lots of different observations - in fact again, itís the dark matter thatís out there thatís really controlling everything - itís not the black holes. Itís pretty impossible to imagine a black hole that would be big enough to be able to control the movement of whole galaxies.