This mosaic of the globular cluster M15 (fifteenth object in the Messier catalog of star clusters and nebulae) contains over 30, 000 stars. The Hubble Space Telescope probed the core of M15, the most tightly packed cluster of stars in our galaxy, to look for evidence of either a massive black hole or another remarkable phenomenon: a 'core collapse' driven by the intense gravitational pull of so many stars in such a small volume of space.

Credit: P. Guhathakurta (UCO/Lick Observatory, UC Santa Cruz), B. Yanny (Fermi National Accelerator Lab), D. Schneider (Pennsylvania State Univ.), J. Bahcall (Inst. for Advanced Study), and NASA/ESA