Our solar system is located in a spiral galaxy composed of billions of stars, the Milky Way. With the naked eye, we can see some 3000 stars in a dark night. However, if Earth would reside within an ultra-diffuse galaxy, we would only spot a few dozen stars on the sky. Galaxies of this type were either not able to produce more stars in the first place, or they got stripped of their stars by tidal forces.

Intriguingly, though, larger telescopes and improved imaging techniques have recently led to the discovery of many ultra-diffuse galaxies in the harshest environments possible: galaxy clusters.

"We have been asking ourselves how these fragile objects are able to survive among such dense, massive accumulations of hundreds of large and small galaxies", explains Carolin Wittmann, PhD student at the Astronomisches Rechen-Institut (ARI) of the Zentrum für Astronomie der Universität Heidelberg (ZAH). Using very deep optical images obtained in 2012 with the Prime Focus Camera (PFIP) of the William Herschel Telescope (WHT), Ms Wittmann identified about 90 such galaxies in the core of the Perseus Cluster, 240 million light-years away.