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A
UK-led team of astronomers has discovered a completely new type of star cluster
around a neighbouring galaxy.
The
new-found clusters contain hundreds of thousands of stars, a similar number to
the so-called “globular” star clusters which have long been familiar to
astronomers.
What distinguishes them from the globular clusters is
that they are much larger – several hundred light years across – and hundreds of
times less dense. The distances between the stars are, therefore, much greater
within the newly discovered “extended
clusters”.
The
discovery was made during the course of an unprecedentedly broad and detailed
survey of the Milky Way’s nearby sister, the Andromeda Galaxy (often referred to
by the catalogue number, M31). The survey has so far covered more than 50 square
degrees of sky, compared with only a few degrees covered by all previous CCD
surveys.
Part of this study involved a search for globular
clusters around M31, during which the new “extended clusters” were found. The
new clusters are distributed in a spherical ‘halo’ region extending about
200,000 light years from the giant M31 spiral galaxy.
“How these
objects formed, and why there are no similar clusters in the Milky Way is still
a mystery,” said Avon Huxor, a PhD student at the
“What is clear is that these clusters, like the
globulars, are ancient. They are billions of years old - possibly amongst the
first objects to form in the Universe.”
“It
may be they were originally created not in M31, but as part of other small,
so-called dwarf galaxies, which have subsequently between pulled apart and
merged with the giant M31 galaxy,” commented team member Mike Irwin (University
of Cambridge).
“That would be particularly exciting since they might
then be more properly considered as the very smallest galaxies rather than star
clusters, and help explain the apparent scarcity of such objects compared to
theoretical predictions,” added Nial Tanvir, another University of Hertfordshire
astronomer, who led this part of the work.
The
data for the survey were acquired with the 2.5 m Isaac Newton Telescope in La
Palma, Canary Islands, and the 3.6 m Canada-France-Hawaii Telescope in Hawaii.
The observations were made using sensitive electronic CCD cameras; previous
surveys of these regions had used photographic technology, which had failed to
detect the faint clusters.
The team also
included astronomers from
CONTACTS:
On Tuesday 5
April, Dr. Tanvir and Dr. Huxor can be contacted via the
Normal contact
details:
Dr.
Nial Tanvir
Tel
Mobile: +44 (0)7980-136499
E-mail:
nrt@star.herts.ac.uk
Dr.
Tel
E-mail: ahuxor@star.herts.ac.uk
Dr. Mike
Irwin
Tel: +44 (0)1223-337524
E-mail:
mike@ast.cam.ac.uk
NOTES FOR EDITORS
Globular clusters are spherical star systems composed of
hundreds of thousands or millions of closely packed stars. Our Milky Way is
thought to contain around 200 of these clusters in a huge halo that surrounds
the galactic centre. Many of the stars in these globular clusters are very
ancient, having existed for most of the history of the Universe. A number of
globular clusters have also been found around the Andromeda galaxy (M31), the
nearest large spiral galaxy. Andromeda lies about 2.5 million light years away,
and is just visible with the naked eye.
The 2005 RAS
National Astronomy Meeting is hosted by the
IMAGES CAN BE FOUND ON THE WEB
AT:
http://star-www.herts.ac.uk/~nrt/m31
Captions:
1. The new clusters are distributed in a spherical
region extending out to about 200,000 light years around the giant M31 spiral
galaxy. M31 itself is about 2.5 million light years from the Milky Way,
and contains about 200 billion stars.
2. A close-up picture of one
of the new clusters, together with a more typical globular cluster for
comparison.