This Hubble Space Telescope view of the core of one of the
nearest globular star clusters, called NGC 6397, resembles
a treasure chest of glittering jewels. The cluster is located
8,200 light-years away in the constellation Ara.
Here, the stars are jam-packed together. The stellar density
is about a million times greater than in our Sun’s stellar
neighborhood. The stars are only a few light-weeks apart,
while the nearest star to our Sun is over four light-years away.
The stars in NGC 6397 are in constant motion, like a swarm
of angry bees. The ancient stars are so crowded together
that a few of them inevitably collide with each other once
in a while. Near misses are even more common. Even so,
collisions only occur every few million years or so. That’s
thousands of collisions in the 14-billion-year lifetime of
the cluster.
These Hubble images were taken for a research program aimed
at studying what is left behind when such collisions and near
misses occur. When direct collisions occur, the two stars may
merge to form a new star called a “blue straggler”; these hot,
bright, young stars stand out among the old stars that make up
the vast majority of stars in a globular cluster. Several such
bright blue stars are visible near the center of the cluster
in the Hubble Heritage image.
If two stars come close enough together without actually
colliding, they may “capture” each other and become
gravitationally bound. One type of binary that might form
this way is a “cataclysmic variable”– a pairing of a normal,
hydrogen-burning star and a burned-out star called a white dwarf.
In a binary system, the white dwarf will pull material
off the surface of the normal star. This material encircles
the white dwarf in an “accretion disk,” and eventually falls
onto it. The result of this accretion process is that cataclysmic
variables are, as the name suggests, variable in brightness. The
heat generated by the accreting material also generates unusual
amounts of ultraviolet and blue light.
To search for cataclysmic variables, the program consisted of
a series of 55 images of the cluster taken over a period of about
20 hours. Most of the images were taken in ultraviolet and blue
filters; a few images were also taken at green and infrared
wavelengths. By comparing the brightness of all the stars in all
the images, the Hubble astronomers were able to identify several
cataclysmic variable stars in the cluster. Comparison of their
brightness in the different filters confirmed that they were
emitting copious amounts of ultraviolet light. A few of these
stars can be seen in the Hubble Heritage image as faint blue
or violet stars.
One of the more intriguing results of this study was completely
unexpected. Three faint blue stars can be seen near the center
of the cluster — in the Hubble Heritage image they appear turquoise.
These three stars don’t vary in brightness at all, and were clearly
not cataclysmic variables. These stars may be very-low-mass white
dwarfs, formed in the cores of giant stars whose evolution is somehow
interrupted before a full-fledged white dwarf has time to form.
Such an interruption might occur as the result of a stellar collision
or an interaction with a binary companion. When a giant star interacts
with another star, it can lose its outer layers prematurely, compared
to its normal evolution, exposing its hot, blue core. The end result
will be a white dwarf of a smaller mass than would have otherwise
ensued. In any case, these unusual stars are yet more evidence that
the center of a dense globular cluster is a perilous place to reside.
A large number of normal white dwarfs were also identified and
studied. These stars appear throughout the cluster, since they
form through normal stellar evolution processes and don’t involve
any stellar interactions, which occur predominantly near the cluster
center. Nearly 100 such burned-out stars were identified in these
images, the brightest of which can be seen here as faint blue stars.
This Hubble image is a mosaic of two sets of images taken several
years apart by the Wide Field Planetary Camera 2. Archival data from
science teams led by Jonathan Grindlay (Harvard University) and
Ivan King (University of California, Berkeley), taken in 1997 and
1999, were combined with Hubble Heritage data taken in 2001.
Adrienne Cool (San Francisco State University), who was also on
both archival science teams, worked with the Hubble Heritage team
to acquire the new observations.
Image Credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: A. Cool (SFSU)
NOTE TO EDITORS: For additional information, please contact
Adrienne Cool, San Francisco State University, 1600 Holloway Avenue,
San Francisco, CA 94132, (phone) 415-338-6450, (fax) 415-338-2178,
(e-mail) cool@stars.sfsu.edu or
Keith Noll, Hubble Heritage Team, Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218, (phone)
410-338-1828, (fax) 410-338-4579, (e-mail) noll@stsci.edu
Electronic images and additional information are available at
The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, MD. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).
