Hubble peers into the heart of the densest known star cluster
This mosaic of the globular cluster M15 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.
The larger picture shows the central portion of M15, photographed with
Hubble’s Wide Field Planetary Camera 2. The image is about 28 light-
years across. The cluster center is in the upper right, on the
highest- resolution part of the image. The inset is an enlargement of
the innermost 1.6 light-years of the cluster. Images in ultraviolet,
blue, and visual light were combined for this picture, so that the
colors roughly correspond to the surface temperatures of stars in M15.
Hot stars appear blue, while cooler stars appear reddish-orange.
The density of stars rises all the way into the cluster center, marked
by a green cross. Careful analysis of the distribution of these stars
suggest that at some point in the distant past, the stars converged on
M15’s core, like bees swarming to their hive. This runaway collapse,
long theorized by researchers but never seen in such detail, may have
lasted a few million years–a flash in the 12-billion-year life of the
cluster. A precise reading of the speeds at which stars move near
M15’s core would reveal whether the stars are packed so tightly because
of the influence of a single massive object, or simply by their own
mutual attraction. Stars would orbit more quickly in the gravitational
grip of a black hole, which would be several thousand times more
massive than our sun. Such measurements are time consuming but
possible with Hubble.
The images were taken in April 1994 and will be published in the
January 1996 issue of Astronomical Journal. Members of the research
team are Puragra Guhathakurta (UCO/Lick Observatory, UC Santa Cruz),
Brian Yanny (Fermi National Accelerator Laboratory), Donald Schneider
(Pennsylvania State University), and John Bahcall (Institute for
Advanced Study).
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.
The HST image is also available as:
Hi-res jpeg:
95-06.jpg [602k];
Lo-res jpeg:
M15GC.jpg [84k].
- More images of M15
- Amateur images of M15
Last Modification: 18 Jun 1999, 23:45 MET