Peering into the
Core of a Globular Cluster
Astronomers have used NASA's Hubble
Space Telescope to peer into the center of a dense
swarm of stars called Omega Centauri. Located some
17,000 light-years from Earth, Omega Centauri is
a massive globular star cluster, containing several
million stars swirling in locked orbits around a
common center of gravity. The stars are packed so
densely in the cluster's core that it is difficult
for ground-based telescopes to make out individual
stars. Hubble's high resolution is able to pick
up where ground-based telescopes leave off, capturing
distinct points of light from stars at the very
center of the cluster.
Omega Centauri is so large in our
sky that only a small part of it fits within the
field of view of the Wide Field and Planetary Camera
2 (WFPC2) on the Hubble Space Telescope. Yet even
this tiny patch contains some 50,000 stars, all
packed into a region only about 13 light-years wide.
For comparison, a similarly sized region centered
on the Sun would contain about a half dozen stars.
The vast majority of stars in this
Hubble image are faint, yellow-white dwarf stars
similar to our Sun. The handful of bright yellow-orange
stars are red giants that have begun to exhaust
their nuclear fuel and have expanded to diameters
about a hundred times that of the Sun. A number
of faint blue stars are also visible in the image.
These are in a brief phase of evolution between
the dwarf stage and the red-giant stage, during
which the surface temperature is high. The stars
in Omega Centauri are all very old, about 12 billion
years. Stars with a mass as high as that of our
Sun have already completed their evolution and have
faded away as white dwarfs, too faint to be seen
even in the Hubble image.
The stars in the core of Omega Centauri
are so densely packed that occasionally one of them
will actually collide with another one. Even in
the dense center of Omega Centauri, stellar collisions
will be infrequent. But the cluster is so old that
many thousands of collisions have occurred.
What happens when stars collide? These
Hubble images were taken to help answer that question.
When stars collide head-on, they probably just merge
together and make one bigger star. But if the collision
is a near miss, they may go into orbit around each
other, forming a close binary star system.
Searching for a needle in a haystack,
scientists have found two binary star systems in
these Hubble images that may have had such an origin.
Both of them are close pairs in which one component
is a white dwarf that pulls gas off of its companion.
When the gas falls onto the surface of the white
dwarf, it is heated to the point that it emits ultraviolet
light. These unusual emissions enabled scientists
to pinpoint these two faint stars among the myriad
of other faint stars in the cluster.
Omega Centauri is the most luminous
and massive globular star cluster in the Milky Way.
It is one of the few globular clusters that can
be seen with the unaided eye. Named by Johann Bayer
in 1603 as the 24th brightest object in the constellation
Centaurus, it resembles a small cloud in the southern
sky and might easily be mistaken for a comet.
This Hubble WFPC2 image was taken
on June 11, 1997 in ultraviolet, red, and H-alpha
filters. The science team, led by Dr. Adrienne Cool
of San Francisco State University, includes Jennifer
Carson, a former SFSU student who is now at UCLA,
Charles Bailyn at Yale and Jonathan Grindlay at
Harvard. These data are currently being used by
Jeff Carlin and Daryl Haggard, two SFSU students,
to look for optical counterparts of X-ray sources
recently discovered with the Chandra Observatory.
This image was produced by the Hubble Heritage Team
Credits: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: A. Cool (SFSU)