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EMBARGOED UNTIL: 12 NOON (EST) July 1, 1999

PHOTO NO.: STScI-PRC99-26

HUBBLE IMAGES A SWARM OF ANCIENT STARS

This stellar swarm is M80 (NGC 6093), one of the densest of the
147 known globular star clusters in the Milky Way galaxy.
Located about 28,000 light-years from Earth, M80 contains
hundreds of thousands of stars, all held together by their mutual
gravitational attraction. Globular clusters are particularly
useful for studying stellar evolution, since all of the stars in
the cluster have the same age (about 15 billion years), but cover
a range of stellar masses. Every star visible in this image is
either more highly evolved than, or in a few rare cases more
massive than, our own Sun. Especially obvious are the bright red
giants, which are stars similar to the Sun in mass that are
nearing the ends of their lives.

By analyzing the Wide Field and Planetary Camera 2 (WFPC2)
images, including images taken through an ultraviolet filter,
astronomers have found a large population of "blue stragglers" in
the core of the cluster. These stars appear to be unusually young
and more massive than the other stars in a globular cluster.
However, stellar collisions can occur in dense stellar regions
like the core of M80 and, in some cases, the collisions can
result in the merger of two stars. This produces an unusually
massive single star, which mimics a normal, young star. M80 was
previously unknown to contain blue stragglers, but is now known to
contain more than twice as many as any other globular cluster
surveyed with NASA's Hubble Space Telescope (HST). Based on the
number of blue stragglers, the stellar collision rate in the core
of M80 appears to be exceptionally high.

M80 is also unusual because it was the site of a nova explosion
in the year 1860. Nova outbursts occur when a close companion
star transfers fresh hydrogen fuel to a burned-out white dwarf.
Eventually the hydrogen ignites a thermonuclear explosion on the
surface of the white dwarf, giving rise to the nova outburst.
The ultraviolet Hubble observations have revealed the hot, faint
remnant of this exploding star, which was named T Scorpii in the
19th century. Curiously, however, the WFPC2 observations have
revealed only two other nova-like close binary stars in M80, far
fewer than expected theoretically based on the stellar collision
rate.

So the blue stragglers in M80 seem to indicate that there are
lots of collisions, yet the nova-like stars suggest only a few.
Sometimes life for astronomers isn't so simple, but it is from
exploring discrepancies like this that our understanding
eventually deepens.

This high-resolution image was created from 2 separate pointings
of HST. One WFPC2 data set was obtained by Francesco R. Ferraro
(ESO, Bologna Obs.), Barbara Paltrinieri (U. La Sapienza), Robert
T. Rood (U. Virginia), and Ben Dorman (Raytheon/STX), who study
blue stragglers. The other data set was acquired by Michael Shara
(STScI, AMNH), David Zurek (STScI), and Laurent Drissen (U. Laval)
to search for dwarf novae.

Image Credit: The Hubble Heritage Team (AURA/STScI/NASA)


NOTE TO EDITORS: Image files and photo caption are available on the
Internet at:
http://heritage.stsci.edu/
http://oposite.stsci.edu/pubinfo/pr/1999/26 or via links in
http://oposite.stsci.edu/pubinfo/latest.html and
http://oposite.stsci.edu/pubinfo/pictures.html

Higher resolution digital versions of (300 dpi JPEG and TIFF) of the
release photo are available at:
http://oposite.stsci.edu/pubinfo/pr/1999/26/pr-photos.html

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