Donald Savage
Headquarters, Washington, DC           November 15, 1994
(Phone:  202/358-1547)                 EMBARGOED 
                                       UNTIL NOON

Jim Elliott
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/286-6256)

Ray Villard
Space Telescope Science Institute, Baltimore
(Phone:  410/338-4562)

RELEASE:  94-188

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, and leader of one of the teams.

       The group, led by Bahcall and Andrew Gould of Ohio State
University, Columbus, OH, (formerly of the Institute for Advanced
Study), showed that faint red dwarf stars, which were thought to be
abundant, actually are sparse in the 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 space telescope 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.

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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 the 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 therefore
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 the 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 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 envelops the Milky Way's spiral
disk of stars, of which Earth's Sun is one inhabitant. 

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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 (Space Telescope Science Institute
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 cast-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. 

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                                 - 4 -

       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 observations, 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.

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                          -5-

       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).

                           - end -

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