Larry Klaes (
Fri, 02 Jul 1999 10:32:43 -0400

>Date: Thu, 01 Jul 1999 12:04:04 -0400
>From: HST News Release <>
>X-Mailer: Mozilla 4.07 [en] (WinNT; I)
>EMBARGOED UNTIL: 12 NOON (EDT) July 1, 1999
>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
>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: For additional information, please contact Robert T.
>Astronomy Dept., Univ. of Virginia, P.O. Box 3818, Charlottesville, VA
>(phone) 804-924-4904, (fax) 804-924-3104, (e-mail)
>Image files and photo caption are available on the
>Internet at:
> or via links in
> and
>Higher resolution digital versions (300 dpi JPEG and TIFF) of the
>release photo are available at:
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