A description of the cluster courtesy of the ESA:
Both very hot blue stars and cooler golden stars can be seen swarming together in the image, becoming more concentrated towards the cluster’s bright centre. Messier 15 is one of the densest globular clusters known, with most of its mass concentrated at its core. As well as stars, Messier 15 was the first cluster known to host a planetary nebula, and it has been found to have a rare type of black hole at its center.
The final images is made up of photos taken by Hubble’s Wide Field Camera 3 and Advanced Camera for Surveys. To see a full res version of the image, click here. And if you’d like more information about the M15 cluster, head over to Wikipedia or the ESA’s Hubble website by following the corresponding links.