Galaxy groups and clusters are the largest gravitationally-bound objects in the Universe. Groups and clusters may contain from ten to thousands of galaxies. The clusters themselves are often associated with larger groups called superclusters. Groups of galaxies are the smallest aggregates of galaxies. They typically contain fewer than 50 galaxies in a diameter of 1 to 2 megaparsecs (3 to 6 million light-years). Their mass is approximately ten trillion solar masses. The group which contains our own galaxy, the Milky Way, is called the Local Group, and contains more than 40 galaxies. Clusters are larger than groups, although there is no sharp dividing line between the two.