Galaxies started to form when the Universe was about a billion years old and was filled with gas.
Astronomers think that the galaxy formed out of a large ball cold gas in space.
The cloud began to fall in on itself and started to form stars around the edges. These early stars are called population 2 stars.
The cloud continued to fall in, with more and more stars being made. These later stars are called population 1 stars.
The type of galaxy that is formed depends on how fast the cloud spins. If it spins quickly it forms a spiral galaxy, and if it spins slowly it makes an elliptical galaxy.
Irregular galaxies arenít made in the same way as the others. They are created when other types of galaxies hit each other.
Click on the links below to find out more about galaxies.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001