A Spectacle of Lights
wondered what a million stars looks like? This image of the
globular cluster Omega Centauri (ω Cen), taken with the
Hubble Space Telescope has almost 2 million stars. Counting
one star per second, it would take 470 hours, almost 20 days
nonstop, to count each star in this image. Luckily, astronomers
can rely on the aid of computers to help identify individual
stars in this gravitationally-bound cluster.
image doesn't even contain all of the cluster but merely its
core! Omega Centauri, also known as NGC 5139 is the largest
and most massive globular cluster of more than 200 known clusters
in our Milky Way galaxy. It is predicted that the entire cluster
contains more than 10 million members, and has a mass of more
than 5 million times that of our Sun. It is easily visible as
a 4th magnitude object from southern latitudes, and even appears
low in the sky to northern locations such as Kitt Peak, Arizona
if one knows when and where to look.
central core of Omega Centauri taken with the Hubble Telescope
roughly 17,000 light-years from earth in the direction of the
southern constellation Centaurus, Omega Centauri was listed
in Ptolemy’s catalogue as a star and given the stellar
designation “Omega” by Johann Bayer in 1603. It
was first found to be made of individual stars by Edmond Halley
other globular clusters, Omega Centauri contains several generations
of stars. The bright reddish stars are red giants. They are
nearing the ends of their lives, having run out of hydrogen
fuel in their cores. They have expanded to be hundreds of times
larger than they were before they ran out of fuel. They are
furiously burning residual hydrogen in a thin layer around a
small dense core of helium ash.
moderately bright blue stars are in an even later stage of evolution
than the red giants. They have ignited the helium in their cores
and are turning it into carbon and oxygen. These stars are all
similar in brightness because ignition occurs when a very specific
amount of helium has built up in a red giant’s core (about
half the mass of the Sun). They appear blue because they have
shrunk back down to normal star dimensions but are more luminous
than normal stars.
and oxygen will be the last elements created in the stars in
Omega Centauri. Once they are forged, the blue stars will expand
once again, blow off their outer layers, and leave behind tiny,
dense, white-hot glowing embers about the size of the Earth.
Over 2000 of these so-called “white dwarfs” are
present in this image alone. Because they are so small compared
to normal stars, white dwarfs are exceedingly faint, and can
be seen only in a very high-resolution rendition of this object.
Ground-based, wide-field image of Omega
Centauri by Robert
is also a distinct possibility that Omega Centauri is not a
globular cluster at all but instead the remnant of a dwarf galaxy
that is been consumed by the Milky Way. There is much support
to this idea, including Omega Centauri being quite unusual among
Milky Way globular clusters, what with containing stars of multiple
metallicities and multiple ages.
image was taken in June 2002 with Hubble's Advanced Camera for
Surveys (ACS). The globular cluster was imaged in red and blue
filters in nine overlapping fields. The resultant 3×3
mosaic covers 10 arcminutes in the sky and nearly 50 light-years
in physical space.
been really wonderful about this data set is what a wide variety
of investigations it has engendered, by lots of different scientists.
These data have proven to be extremely versatile images. The
fact that astronomers imaged the cluster with a 3×3 mosaic
will allow a complete study of the core; a single ACS field
does not contain the entire core, and it is hard to do cluster
structure work without having the whole core (such work involves
finding the center, examining the density and behavior of stars
at the center, etc.).
Centauri may be able to answer some important questions on how
galaxies or clusters of different sizes may chemically enrich
themselves. This ACS field will serve as an anchor for Omega
Centauri studies for many years to come.