Globular Cluster Messier 15 (M15)

M15 is one of nearly 170 known globular clusters that form a vast halo surrounding our Milky Way Galaxy. M15 harbors hundreds of thousands of stars. But the Hubble Space Telescope spotted evidence of an unexpected guest: an intermediate-sized black hole deep within the cluster's dense core. Black holes are invisible, but Hubble's exacting resolution could measure the velocities of stars whirling around the crowded core. When studying the motions of stars orbiting the core of M15, astronomers discovered that the stars move too fast to be easily explained without the presence of a black hole. The new findings promise a better understanding of how galaxies and globular clusters first formed billions of years ago. Globular star clusters contain the oldest stars in the universe. If these clusters have black holes now, then they most likely had black holes when they formed billions of years ago. The suspected black hole in M15 is thought to be about 4,000 times more massive than our Sun.