What Is A Galaxy?

A galaxy could consists of millions of stars held together by gravity. Spiral galaxies, like the Milky Way are of a flattened shape. There will be a bulge of stars at the centre while younger stars cascade around the perimeter in arms which resemble a Catherine wheel.

Barred Spirals are spiral galaxies which have a bar of stars across the centre. Spiral arms poke out from the ends of this. The arms of spirals contain an assortment of dust and gas from which the new stars form in a continuous process.

Elliptical galaxies have old stars and not much gas. Among this group are the largest galaxies yet found made up of trillions of stars. Some of them at least could have been formed by spiral galaxies merging.
There are also irregular galaxies. Most galaxies seem to happen in clusters. They may contain in number from just a few thousand stars upwards. The Milky Way to which our own sun belongs is about 100,000 light years in diameter, and has 100 billion stars.

A system of stars held together by gravity and separated from other systems by immense space.