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1996-01-12
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FOR RELEASE: August 28, 1995
CONTACT: Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4514)
Harvey Richer
University of British Columbia, Vancouver, Canada
(Phone: 604-822-4134)
Howard Bond
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4718)
PRESS RELEASE NO.: STScI-PR95-32
HUBBLE SPACE TELESCOPE FINDS STELLAR GRAVEYARD
Peering deep into the globular star cluster M4 with NASA's Hubble Space
Telescope, Canadian and American astronomers have discovered a large
number of "stellar corpses," called white dwarf stars, which may be
used eventually to refine age estimates of the universe.
The observation, made by a team led by Harvey Richer of the University
of British Columbia, Vancouver, Canada, was so sensitive that even the
brightest of the detected white dwarfs was no more luminous than a
100-watt light bulb seen at the Moon's distance (239,000 miles).
The Hubble results will allow astronomers to refine theoretical
predictions of the rate at which white dwarfs cool -- an important
prerequisite for making reliable estimates for the age of the universe
and our Milky Way galaxy, based on white dwarf temperatures. Present
estimates for the universe's age range from eight to twenty billion
years, and refining this value is a key goal for modern astronomy and
the Hubble telescope.
A white dwarf is the burned-out core of a collapsed star that, like a
dying ember, slowly cools and fades away. However, the universe is not
yet old enough for any white dwarfs to have cooled off completely to
become invisible black dwarfs. White dwarf temperatures can therefore
be used as "cosmic clocks" for estimating the age of the universe
independently from other techniques.
A globular cluster like M4 contains hundreds of thousands of stars
visible with ground-based telescopes. "We expected that the typical
globular cluster should also contain about 40,000 white dwarfs.
However, white dwarfs are extremely faint, and to date no ground or
space-based telescope has been able to reveal more than a handful of
them in any star cluster," said Richer. By exposing with Hubble's Wide
Field and Planetary Camera 2 for five hours, Richer's team was able to
detect more than 75 white dwarfs in one small area of M4. Analysis of
the Hubble images, done with computer software developed by Peter
Stetson at the National Research Council of Canada, Victoria, British
Columbia. The faintest white dwarfs are 40 times fainter than the
brightest ones in the cluster.
"Even longer exposures with Hubble could conceivably reveal the ages of
the faintest and oldest white dwarfs in M4. This would be a crucial
way to distinguish between recent divergent values for the age of the
universe, since its age cannot be less than the age of the oldest white
dwarfs in M4," said team member Howard Bond of the Space Telescope
Science Institute in Baltimore, MD.
A white dwarf contains most of the original mass of a star, but has
contracted to an extremely dense and faint object about the size of the
Earth. A golf ball-sized piece of a white dwarf would weigh more than
a ton. Because of its small size, high density, and initially hot
temperature, it takes billions of years for a white dwarf to radiate
all of its residual heat into space.
Located 7,000 light-years away in the direction of the constellation
Scorpius and visible in a pair of binoculars, M4 (the fourth object in
the Messier catalog of star clusters and nebulae) is the nearest
globular cluster to the Earth. Globular clusters like M4 were born
early in the history of the Milky Way, and today are veritable stellar
retirement communities. M4 is so ancient (estimated to be 14 billion
years old) that all of its stars that began with 80% or more of the
Sun's mass have already evolved to become red giants, followed by a
collapse to a white dwarf. (Our Sun will not become a white dwarf for
another five billion years.)
Details of the M4 study will be published in the Astrophysical Journal
Letters in September. Other participants in the research are Gregory
Fahlman, Rodrigo Ibata, and Georgi Mandushev (University of British
Columbia), Roger Bell (University of Maryland), Michael Bolte
(University of California, Santa Cruz), William Harris (McMaster
University), James Hesser (National Research Council of Canada),
Carlton Pryor (Rutgers University), and Don VandenBerg (University of
Victoria).
* * * *
The Space Telescope Science Institute is operated by the Association of
Universities for Research in Astronomy, Inc. (AURA) for NASA, under
contract with the Goddard Space Flight Center, Greenbelt, MD. The
Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).
Image files in GIF and JPEG format may be accessed on Internet via
anonymous ftp from ftp.stsci.edu in /pubinfo:
GIF JPEG
PRC95-32 White Dwarfs in M4 gif/M4WD.gif jpeg/M4WD.jpg
The same images are available via World Wide Web from URL
http://www.stsci.edu/pubinfo/Latest.html, or via
links in http://www.stsci.edu/pubinfo/public.html.
Space Telescope Science Institute press release text and other
information are available automatically by sending an Internet
electronic mail message to listserv@stsci.edu. In the body of the
message (not the subject line) users should type the words "subscribe
pio <Name>" (don't use quotes, brackets, or user/account names; i.e.
someone named Jane Doe would type subscribe pio Jane Doe). The system
will reply with a confirmation via e-mail of each subscription. E-mail
will be received with new press releases.