Messier 15 is a gathering of  100,000 very old stars that orbits the center of the Milky Way. It
is one of the densest globular clusters known, with most of its mass concentrated at its core.
 It could also hide a rare type of black hole or a collection of dark neutron stars.

Messier 15 is located 35,000 light-years away in the constellation of Pegasus, The Winged Horse). It is one of the oldest globular clusters known, with an age of around 12 billion years.

In a new Hubble image, very hot blue stars and cooler golden stars are seen swarming together, becoming more concentrated towards the cluster's bright center. 

Astronomers believe that Messier 15 harbors an intermediate-mass black hole at its center, as does the massive globular cluster Mayall II.

Messier 15's very hot blue stars and cooler golden stars swarming together, becoming more concentrated towards the cluster's bright center. This new image is made up of observations from Hubble's Wide Field Camera 3 and Advanced Camera for Surveys in the ultraviolet, infrared, and optical parts of the spectrum. Credit: NASA, ESA

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 [3], and could tell us about how black holes grow and evolve within clusters like Messier 15, and within galaxies.

As well as this black hole, Messier 15 is 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.