This multi-colored firework display is a cluster of stars known as Messier 15, located some 35,000 light-years away in the constellation Pegasus the Winged Horse. It is one of the oldest globular clusters known, with an age of around 12 billion years.
Hot blue stars and cooler golden stars are seen swarming together in this image, becoming more concentrated toward the cluster’s bright center. Messier 15 is one of the densest globular clusters known, with most of its mass concentrated at its core.
This sparkling bauble, however, has hidden secrets. Astronomers studying the cluster with Hubble in 2002 found something dark and mysterious lurking at its heart. It could either be a collection of dark neutron stars or an intermediate-mass black hole. Of the two possibilities, it is more likely that Messier 15 harbors a black hole at its center, as does the massive globular cluster Mayall II.
Intermediate-mass black holes are thought to form either from the merging of several smaller stellar-mass black holes or as a result of a collision between massive stars in dense clusters. A third possibility is that they were formed during the Big Bang. In terms of mass, they lie between the more commonly found stellar-mass and supermassive types of black hole and could tell us about how black holes grow and evolve within clusters like Messier 15 and within galaxies.
Messier 15 is also known to house a planetary nebula, Pease 1 — and it was the first globular known to contain one of these objects. This nebula is visible as the bright blue object just to the left of the cluster’s center.
