Astrophys. J (in the press); preprint at http://arxiv.org/abs/1505.03407 (2015)

Credit: NASA, 2002

A single star without a host galaxy is virtually invisible to us until it explodes in a supernova. Then it becomes a probe for the vast empty spaces between galaxy clusters. But ascertaining that there is no host poses a technical challenge. Indeed, Melissa Graham and co-workers observed that a suspected lone star (detected by the Canada–France–Hawaii Telescope) is actually part of a faint dwarf galaxy or globular cluster, thanks to the resolution of the Hubble Space Telescope (pictured). It may be the first instance of a type 1a supernova in such a small galaxy. Usually, supernovae are found in galaxies comprising hundreds of billions of stars.

In the same study, two other stars (plus one additional probable case) were confirmed as being true intracluster stars that have been stripped from their host galaxies. However, the loners are unable to escape the gravitational potential of their former hosts. Detections of such hostless stars are often serendipitous discoveries, but they are useful for estimating the amount of unseen baryonic mass in galaxy clusters and for studying the structure of the Universe.