Galaxy Cluster

The cluster Abell 1689
Credit: NASA Hubble

In the same way that gravity can influence stars to form clusters, observations have revealed that many groups and clusters of galaxies appear to be gravitationally bound.

Galaxy groups contain less than 50 galaxies and are 3 to 7 million light-years across. Their mass is around 10,000,000,000,000 (10 trillion) times that of the Sun. The Milky Way is part of the Local Group of galaxies, which contains around 40 galaxies.

Galaxy clusters contain between 50 and 1,000 galaxies. With a diameter of between 7 and 35 million light-years, clusters can contain 10 to 100 times more mass than galaxy groups. If you were to look at a cluster for a very long time, it would appear that the galaxies within that cluster were all swarming around a central point.