Globular cluster NGC 1851. Globular clusters are spherical groups of very old stars that are tightly bound by gravity and orbit a dense central core. They can contain over a million stars within a volume only a few hundred light years across. They formed billions of years ago, and are distributed in halos around many galaxies, including the Milky Way. NGC 1851 has a mass of 551,000 solar masses. It is around 39,500 light years from Earth, in the constellation of Columba. This image was obtained on 17 November 2015.