Galaxies began to form about a billion years after the birth of the Universe. At this time the Universe was filled with gas - mainly hydrogen, with some helium.
Astronomers believe that the galaxy formed out of a large, fairly spherical cloud of cold gas in space. At some point in time, the cloud began to collapse in on itself, or condense, in the same way that the clouds which formed individual stars also condensed. Initially, some stars may have formed as the gas cloud began to fragment around the edges, with each fragment condensing further to form a star or group of stars. At such early times, these stars consisted only of the hydrogen and helium gas that made up the cloud.
The cloud continued to collapse, with more and more stars forming as it did so. As the formation of new stars continued, some of the early massive stars evolved to the end of their lifetimes and began to shed their atmospheres or explode in huge supernova events. In this process, the remnants of these old stars enriched the gas in the collapsing cloud with the new, heavier elements that they had created, which were then incorporated into the new stars as they were created in the disc regions. These younger, enriched stars are called population 1 stars, while the older, mainly hydrogen and helium stars, are called population 2.
This process of star formation, the creation of heavier elements inside stars and the final release of these elements into the gas between the stars (called the interstellar medium) still continues today, constantly enriching the medium so that the gas within our galaxy now consists not only of Hydrogen and Helium, but also the other elements which are needed to form the Earth and the rest of the Universe we see around us.
As the young clouds collapsed, they also rotated. The speed of rotation determined the type of galaxy they would form. Fast rotating clouds flattened out into a disc. Stars formed early on were distributed in a spherical halo around the outside of the galaxy, while those formed later were concentrated in the disc. These are spiral galaxies.
Clouds rotating more slowly weren’t able to form a disc, and so the gas continued to fall to the centre in a spherical shape. Star formation continued throughout the entire galaxy and formed the elliptical galaxies.
Irregular galaxies usually form due to interactions or collisions between other types of galaxy.
Click on the links below to find out more about galaxies.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001