The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/94_41.txt

Text File | 1996-01-12 | 12KB | 237 lines

CONTACT:  Ray Villard, STScI       EMBARGOED UNTIL: 12:00 NOON, EST
          [phone]           Tuesday, November 15, 1994

                                   PRESS RELEASE NO.: STScI-PR94-41
          John Bahcall, Institute for Advanced Study
          [phone]

          Francesco Paresce, STScI
          [phone]


          HUBBLE RULES OUT A LEADING EXPLANATION FOR DARK MATTER


Two teams of astronomers, working independently with NASA's Hubble
Space Telescope (HST), have ruled out the possibility that red dwarf stars
constitute the invisible matter, called dark matter, believed to account
for more than 90 percent of the mass of the universe.

Until now, the dim, small stars were considered ideal candidates for dark
matter.  Whatever dark matter is, its gravitational pull ultimately will
determine whether the universe will expand forever or will someday
collapse.

"Our results increase the mystery of the missing mass.  They rule out a
popular but conservative interpretation of dark matter," said Dr. John
Bahcall, professor of natural science at the Institute of Advanced Study,
Princeton, NJ, a leader of one of the teams.

The group led by Bahcall and Andrew Gould of Ohio State University,
Columbus, Ohio, (formerly of the Institute for Advanced Study) showed
that faint red dwarf stars, which were thought to be abundant, actually
are sparse in the Milky Way, Earth's home galaxy, and in the universe by
inference.

The team, led by Dr. Francesco Paresce of the Space Telescope Science
Institute in Baltimore, MD, and the European Space Agency, determined
that the faint red stars rarely form and that there is a cutoff point
below which nature does not make this type of dim, low-mass star.

The pair of HST observations involved accurately counting stars and
gauging their brightness. The observations overturn several decades of
conjecture, theory and observation about the typical mass and abundance of
the smallest stars in the universe.


PREVIOUS GROUND-BASED RESULTS INCONCLUSIVE

In our own stellar neighborhood, there are almost as many red dwarfs as
there are all other types of stars put together.   The general trend
throughout our galaxy is that small stars are more plentiful than larger
stars, just as there are more pebbles on the beach than rocks.  This led
many astronomers to believe that they were only seeing the tip of the
iceberg and that many more extremely faint red dwarf stars were at the
limits of detection with ground-based instruments.

According to stellar evolution theory, stars as small as eight percent of
the mass of our Sun are still capable of shining by nuclear fusion
processes.

Over the past two decades, theoreticians have suggested that the lowest
mass stars also should be the most prevalent and, so, might provide a
solution for dark matter.  This seemed to be supported by previous
observations with ground-based telescopes that hinted at an unexpected
abundance of what appeared to be red stars at the faintest detection levels
achievable from the ground.

However, these prior observations were uncertain because the light from
these faint objects is blurred slightly by Earth's turbulent atmosphere.
This makes the red stars appear indistinguishable from the far more
distant, diffuse-looking galaxies.


PINNING DOWN THE LONG-SOUGHT HALO POPULATION

Hubble's capabilities made it possible for a team of astronomers led by
Bahcall and Gould to observe red stars that are 100 times dimmer than
those detectable from the ground -- a level where stars can be distinguished
easily from galaxies.  Hubble Space Telescope's extremely high resolution
also can separate faint stars from the much more numerous galaxies by
resolving the stars as distinct points of light, as opposed to the "fuzzy"
extended signature of a remote galaxy.

Bahcall and Gould, with their colleagues Chris Flynn and Sophia Kirhakos
(also of the Institute for Advanced Study, Princeton) used images of
random areas in the sky taken with the HST Wide Field Planetary Camera
2 (in WF mode) while the telescope was performing scheduled
observations with other instruments.  By simply counting the number of
faint stars in the areas observed by HST, the scientists demonstrated that
the Milky Way has relatively few faint red stars.

The HST observations show that dim red stars make up no more than six
percent of the mass in the halo of the Galaxy, and no more than 15 percent
of the mass of the Milky Way's disk. The Galactic halo is a vast spherical
region that envelopes the Milky Way's spiral disk of stars, of which Earth's
Sun is one inhabitant.


FAINT RED STARS MISSING FROM A GLOBULAR CLUSTER

By coincidence, Paresce pursued the search for faint red dwarfs after his
curiosity was piqued by an HST image taken near the core of the globular
cluster NGC 6397.  He was surprised to see that the inner region was so
devoid of stars, he could see right through the cluster to far more distant
background galaxies.  Computer simulations based on models of stellar
population predicted the field should be saturated with dim stars -- but it
wasn't.

HST's sensitivity and resolution allowed Paresce, and co-investigators
Guido De Marchi (ST ScI, and the University of Firenze, Italy), and
Martino Romaniello (University of Pisa, Italy) to conduct the most
complete study to date of the population of the cluster (globular clusters
are ancient, pristine laboratories for studying stellar evolution).  To
Paresce's surprise, he found that stars 1/5 the mass of our Sun are very
abundant (there are about 100 stars this size for every single star the mass
of our Sun) but that stars below that range are rare. "The very small stars
simply don't exist, " he said.

A star is born as a result of the gravitational collapse of a cloud of
interstellar gas and dust. This contraction stops when the infalling gas is
hot and dense enough to trigger nuclear fusion, causing the star to glow
and radiate energy.

"There must be a mass limit below which the material is unstable and
cannot make stars," Paresce emphasizes." Apparently, nature breaks things
off below this threshold."

Paresce has considered the possibility that very low-mass stars formed long
ago but were thrown out of the cluster due to interactions with more
massive stars within the cluster, or during passage through the plane of our
Galaxy.  This process would presumably be common among the
approximately 150 globular clusters that orbit the Milky Way.  However,
the ast-off stars would be expected to be found in the Milky Way's halo,
and Bahcall's HST results don't support this explanation.


THE SEARCH FOR DARK MATTER

The HST findings are the latest contribution to a series of recent,
intriguing astronomical observations that are struggling to pin down the
elusive truth behind the universe's "missing mass."

Models describing the origin of helium and other light elements during the
birth of the universe, or "Big Bang," predict that less than 5% of the
universe is made up of "normal stuff," such as neutrons and protons.  This
means more than 90% of the universe must be some unknown material that
does not emit any radiation that can be detected by current instrumentation.
Candidates for dark matter include black holes, neutron stars and a variety
of exotic elementary particles.

Within the past year, astronomers have uncovered indirect evidence for a
dark matter candidate called a MACHO (MAssive Compact Halo Objects).
These previous observations detected several instances of an invisible
object that happens to lie along the line of sight to an extragalactic star.
When the intervening object is briefly aligned between Earth and a distant
star, it amplifies, or gravitationally lenses, the light from the distant
star.

The new HST finding shows that faint red stars are not abundant enough to
explain the gravitational lensing events attributed to MACHOs.  Bahcall
cautions, however, that his results do not rule out other halo objects that
could be smaller than the red stars such as brown dwarfs -- objects not
massive enough to burn hydrogen and shine in visible light.

Additional circumstantial evidence for dark matter in the halo of our galaxy
has been inferred from its gravitational influence on the motions of stars
within the Milky Way's disk.

Recently, this notion was further supported by ground-based observation,
made by Peggy Sachett of the Institute for Advanced Study, that show a
faint glow of light around a neighboring spiral galaxy that is the shape
expected for a halo composed of dark matter. This could either be light
from the dark matter itself or stars that trace the presence of the galaxy's
dark matter.

The reality of dark matter also has been inferred from the motions of
galaxies in clusters, the properties of high-temperature gas located in
clusters of galaxies and from the relative amounts of light elements and
isotopes produced in the Big Bang.

The ultimate fate of the universe will be determined by the amount of dark
matter present. Astronomers have calculated that the amount of matter - -
planets, stars and galaxies -- observed in the universe cannot exert enough
gravitational pull to stop the expansion which began with the Big Bang.
Therefore, if the universe contains less than a critical density of matter
it will continue expanding forever, but if enough of the mysterious dark
matter exists, the combined gravitational pull someday will cause the
universe to stop expanding and eventually collapse.

Bahcall stresses, "The dark matter problem remains one of the fundamental
puzzles in physics and astronomy.  Our results only sharpen the question of
what is the dark matter."

Bahcall's results appeared in the November 1, 1994 issue of the
Astrophysical Journal. Paresce's paper will appear in the February 10,
1995, issue of the Astrophysical Journal.


                    * * * * * * * * * * * * * * * * * *

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).

                                 * * * * *

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To electronically access this press release and associated captioned images
and background information, you can use the Internet or World Wide Web.


Image files are on ftp.stsci.edu in the directory /stsci/epa/gif:
      PR#                     GIF file            Caption text
     -----------          ------------      ---------------
     STScI-PRC94-41a          DarkMatA.gif         DarkMatA.txt
     STScI-PRC94-41b          DarkMatB.gif        DarkMatB.txt
     STScI-PRC94-41c          DarkMatC.gif         DarkMatB.txt

The PR text is in the file /stsci/epa/PR/94-41

The same data is available using WWW/Mosaic which is always
accessible via:
          http://www.stsci.edu/EPA/OPO.html
or to get right to the latest release:
          http://www.stsci.edu/EPA/Latest.html