Astronomy Picture of the Day

Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.

November 20, 1995

At the Core of M15
Credit: NASA, P. Guhathakurta (UCO/Lick Observatory, UC Santa Cruz)

Explanation: Densely packed stars in the core of the globular cluster M15 are shown in this Hubble Space Telescope (HST) image taken in April of 1994. The stars revealed are contained in an area 1.6 light years across and their colors roughly indicate their temperatures - hot stars appear blue, cooler stars look reddish-orange. M15 has long been recognized as one of the densest cluster of stars in our galaxy outside of the galactic center itself. Even the unprecedented resolving power of the HST cameras could not separate the individual stars in its innermost regions. However, this HST image reveals that the density of stars continues to rise toward the cluster's core, suggesting that a sudden, runaway collapse due to the gravitational attraction of many closely packed stars or a single central massive object, perhaps a black hole, could account for the core's extreme density.

Tomorrow's picture: M42: Orion Nebula Mosaic

Authors & editors: Robert Nemiroff (GMU) & Jerry Bonnell (USRA).
NASA Technical Rep.: Sherri Calvo. Specific rights apply.
A service of: LHEA at NASA/ GSFC