I never expected that Hubble would see things as clearly as it does. The results are so exciting that they are a dream come true. Bahcall and astrophysicist Jeremiah Ostriker of Princeton University first proposed in 1975 that M15 might harbor a black hole. While distinguished by its extreme density of stars, M15 is in other respects similar to the rest of the dozens of globular clusters that freckle space in and around our Milky Way. Each cluster is like a miniature galaxy, with 100,000 to one million stars in a compact spherical blob.
Globular clusters contain almost no gas or dust and show few signs of recent star formation.
Astronomers believe they are primordial remnants, left over from the birth of the Milky Way. As such, they are ideal laboratories for studying how stars evolve. Cluster stars also provide a limit on the age of the universe, independent of the expansion of the universe itself. Stars at the core of M15 may be crowded closer together than anywhere in the Milky Way except in the galaxy's hidden heart. Attempted studies of this exotic locale with ground-based telescopes proved frustrating.