Galactic centres

What exactly lies at the heart of any galaxy, let alone our own, is a question that has not been satisfactorily answered. There are plenty of theories, some of which look very promising, but debate on this topic continues.

Galaxies form because all matter - the Earth, the stars, even you - attracts all other matter with gravity. Admittedly, on an everyday scale the attraction is very small, but it's what sticks you to the ground and keeps us going around the sun. But, if gravity makes things come together, why do we not fall into the Sun? It is for the same reason that the ball in a roulette wheel spins around and around rather than falling into the middle.

roulette wheel While not spinning around the wheel, a marble placed on the slope runs down into the centre, obviously enough. But spin it and the marble stays up at the top. It slows down as the marble loses energy to the noise it makes and to friction, and eventually falls down the sloping walls into the well at the bottom.

The stars in a galaxy are rather like lots and lots of marbles all going at different speeds, and hence at different heights up the wall and different distances from the centre. The stars don't have any friction like the marbles though, so they just keep going round and round.

Unlike the marbles however, the stars move at hundreds of kilometres every second. For a star to complete one revolution takes many millions of years (depending on how far out it is).

Bodies moving in circular motion possess angular momentum. The more of it a body has, the farther out from the centre the body will want to stay.