Stellar streams consist of groups of stars moving in orbit together. They are usually remnants of small galaxies that were absorbed by larger galaxies or former star clusters. The Phoenix stream discovered four years ago is the latter. It was, as researchers show in an article in Nature, once a globular cluster, and a very special one at that.
Globular clusters are special objects in themselves. Imagine the night sky full of gleaming stars shining much brighter than the brightest planets in our Solar System. The average distance between two stars of a globular cluster is only 0.1 light-years, while the closest star to the Sun is 4.5 light-years away. Every cubic parsec holds between 1000 to 10,000 stars (the stellar density in the vicinity of the Sun is around 0.14 stars per cubic parsec).