Question 1: What
is a gloublar cluster?
Omega Centauri is a dense swarm of stars
called a "globular cluster". Located
some 17,000 light-years from Earth, Omega
Centauri is a massive 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
(as seen in the image at left). 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. The ground-based
image at left shows roughly a 50 arcminute
field of view. The entire globular has a
diameter of approximately 36 arcminutes.
2: How many stars are in this Hubble image?
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.
3: Do the stars in globular clusters ever
The stars in the core of Omega Centauri are
so densely packed that occasionallythey do
collide with another one. Although stellar
collisions are infrequent, even in the densest
part of the cluster's core, Omega Centauri
is so old that many thousands of collisions
have occurred over time. 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.
Astronomers 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 astronomers to pinpoint
these two faint stars among the myriad of
other faint stars in the cluster.