TOO CLOSE FOR COMFORT
This Hubble Space Telescope view of the core of
one of the
nearest globular star clusters, called NGC 6397,
a treasure chest of glittering jewels. The cluster
8,200 light-years away in the constellation Ara.
Here, the stars are jam-packed together. The stellar
is about a million times greater than in our Sun's
neighborhood. The stars are only a few light-weeks
while the nearest star to our Sun is over four light-years
The stars in NGC 6397 are in constant motion, like
of angry bees. The ancient stars are so crowded
that a few of them inevitably collide with each
in a while. Near misses are even more common. Even
collisions only occur every few million years or
thousands of collisions in the 14-billion-year lifetime
These Hubble images were taken for a research program
at studying what is left behind when such collisions
misses occur. When direct collisions occur, the
two stars may
merge to form a new star called a "blue straggler";
bright, young stars stand out among the old stars
that make up
the vast majority of stars in a globular cluster.
bright blue stars are visible near the center of
in the Hubble Heritage image.
If two stars come close enough together without
colliding, they may "capture" each other
gravitationally bound. One type of binary that might
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
the white dwarf in an "accretion disk,"
and eventually falls
onto it. The result of this accretion process is
variables are, as the name suggests, variable in
heat generated by the accreting material also generates
amounts of ultraviolet and blue light.
To search for cataclysmic variables, the program
a series of 55 images of the cluster taken over
a period of about
20 hours. Most of the images were taken in ultraviolet
filters; a few images were also taken at green and
wavelengths. By comparing the brightness of all
the stars in all
the images, the Hubble astronomers were able to
cataclysmic variable stars in the cluster. Comparison
brightness in the different filters confirmed that
emitting copious amounts of ultraviolet light. A
few of these
stars can be seen in the Hubble Heritage image as
or violet stars.
One of the more intriguing results of this study
unexpected. Three faint blue stars can be seen near
of the cluster -- in the Hubble Heritage image they
These three stars don't vary in brightness at all,
and were clearly
not cataclysmic variables. These stars may be very-low-mass
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
to its normal evolution, exposing its hot, blue
core. The end result
will be a white dwarf of a smaller mass than would
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
studied. These stars appear throughout the cluster,
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
images, the brightest of which can be seen here
as faint blue stars.
This Hubble image is a mosaic of two sets of images
years apart by the Wide Field Planetary Camera 2.
Archival data from
science teams led by Jonathan Grindlay (Harvard
Ivan King (University of California, Berkeley),
taken in 1997 and
1999, were combined with Hubble Heritage data taken
Adrienne Cool (San Francisco State University),
who was also on
both archival science teams, worked with the Hubble
to acquire the new observations.
Credit: NASA and The Hubble Heritage Team
Acknowledgment: A. Cool (SFSU)