A galaxy is often described as an enormous group of stars. And it’s true, galaxies contain from thousands to millions and millions of stars, along with cosmic dust and lots of other space stuff.
But calling a galaxy a group of stars sounds like they’re bundled together like a pile of dirty clothes. But galaxies can be very well organised, with a clear, recognisable shape – more like laundry that’s been neatly folded and put away.
Our galaxy, the Milky Way, is a spiral galaxy. Spiral galaxies are flat discs like a CD. But instead of a hole at the centre, there is a large blob. (This blob often has a supermassive black hole lurking in its centre!)
And of course, spiral galaxies have enormous arms that wind around the centre, like the twirling skirt on a ballroom dancer. Finally, the whole thing is wrapped in a so-called halo of old stars and mysterious, invisible stuff, like a bubble.
This picture, taken by the Very Large Telescope, shows a group of young stars called an Open Cluster. In spiral galaxies, open clusters are normally found inside the spiral arms. That’s because this is where you find lots of cosmic gas; the main ingredient for making stars.
Unlike some other groups of stars, the stars in open clusters slowly drift apart as they get older, over a few hundred million years. Actually, our own Sun was probably born in an open cluster with hundreds of siblings that have long since wandered off into space!
Our Galaxy contain around 1000 open clusters!