The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/94_49.txt

Text File | 1996-01-12 | 10KB | 188 lines

CONTACT:  Ray Villard, STScI       EMBARGOED UNTIL: 2:00 P.M. EDT
          [phone]           Wednesday, October 26, 1994

          Dr. Wendy L.  Freedman   PRESS RELEASE NO.: STScI-PR94-49
          Carnegie Observatories
          [phone]


            HUBBLE SPACE TELESCOPE MEASURES PRECISE DISTANCE
                      TO THE MOST REMOTE GALAXY YET

An international team of astronomers using NASA's Hubble Space
Telescope announced  today the most accurate measurement yet of the
distance of the remote galaxy M100, located in the Virgo cluster of
galaxies.

This measurement will help provide a precise calculation of the
expansion rate of the universe, called the Hubble Constant, which is
crucial to determining the age and size of the universe.

"Although this is only the first step in a major systematic program to
measure accurately the scale, size, and age of the universe," noted
Dr.  Wendy L. Freedman, of the Observatories of the Carnegie
Institution of Washington, "a firm distance to the Virgo cluster is a
critical milestone for the extragalactic distance scale, and it has
major implications for the Hubble Constant."

HST's detection of Cepheid variable stars in the spiral galaxy M100, a
member of the Virgo cluster, establishes the distance to the cluster as
56 million light-years (with an uncertainty of +/- 6 million
light-years).  M100 is now the most distant galaxy in which Cepheid
variables have been measured accurately.

The precise measurement of this distance allows astronomers to
calculate that the universe is expanding at the rate of 80 km/sec per
megaparsec (+/- 17 km/sec).  For example, a galaxy  one million
light-years away will appear to be moving away from us at approximately
60,000 miles per hour.  If it is twice that distance, it will be seen
to be moving at twice the speed, and so on.  This rate of expansion is
the Hubble Constant.

These results are being published in the October 27 issue of the
journal Nature.  The team of astronomers is jointly led by Freedman,
Dr. Robert Kennicutt (Steward Observatory, University of Arizona), and
Dr. Jeremy Mould (Mount Stromlo and Siding Spring Observatories,
Australian National University).

Dr. Mould noted,  "Those who pioneered the development of  the Hubble
Space Telescope in  the 1960s and 1970s recognized its unique potential
for finding the value of the Hubble Constant.  Their foresight has been
rewarded by the marvelous data that we have obtained for M100."

Using Hubble's Wide-Field and Planetary Camera (WFPC2), the team of
astronomers repeatedly imaged a field where much star formation
recently had taken place, and was, therefore, expected to be rich in
Cepheids  -- a class of pulsating stars used for determining
distances.  Twelve one-hour exposures, strategically placed in a
two-month observing  window, resulted in the discovery of 20 Cepheids.
About 40,000 stars were measured in the search for these rare, but
bright, variables.  Once the periods and intrinsic brightness of these
stars were established from the careful measurement of their pulsation
rates, the researchers calculated a distance of 56 million light-years
to the galaxy.  (The team allowed for the dimming effects of distance
as well as that due to dust and gas between Earth and M100.)

Many complementary projects are currently being carried out from the
ground with the goal of providing values for the Hubble Constant.
However, they are subject to many uncertainties which HST was designed
and built to circumvent.  For example, a team of astronomers using the
Canada-France-Hawaii telescope at Mauna Kea recently have arrived at a
distance to another galaxy in Virgo that is similar to that found for
M100 using HST -- but their result  is tentative because it is based on
only three Cepheids in crowded star fields.

"Only Space Telescope can make these types of observations routinely,"
Freedman explained.   "Typically, Cepheids are too faint and the
resolution too poor, as seen from ground-based telescopes, to detect
Cepheids clearly in a crowded region of a distant galaxy."

Although M100 is now the most distant galaxy in which Cepheid variables
have been discovered, the Hubble team emphasized that the HST project
must link into even more distant galaxies before a definitive number
can be agreed on for the age and size of the universe.  This is because
the galaxies around the Virgo Cluster are perturbed by the large mass
concentration of galaxies near the cluster.  This influences their rate
of expansion.


REFINING THE HUBBLE CONSTANT

These first HST results are a critical step in converging on the true
value of the Hubble Constant, first developed by the American
astronomer Edwin Hubble in 1929.  Hubble found that the farther a
galaxy is, the faster it is receding away from us.  This "uniform
expansion" effect is strong evidence the universe began in an event
called the "Big Bang" and that it has been expanding ever since.

To calculate accurately the Hubble Constant, astronomers must have two
key numbers: the recession velocities of galaxies and their distances
as estimated by one or more cosmic "mileposts," such as Cepheids.  The
age of the universe can be estimated from the value of the Hubble
Constant, but it is only as reliable as the accuracy of the distance
measurements.

The Hubble constant is only one of several key numbers needed to
estimate the universe's age.  For example, the age also depends on the
average density of matter in the universe, though to a lesser extent.

A simple interpretation of the large value of the Hubble Constant, as
calculated from HST observations, implies an age of about 12 billion
years for a low-density universe, and 8 billion years for a
high-density universe.  However, either value highlights a
long-standing dilemma.  These age estimates for the universe are
shorter than the estimated ages of some of the oldest stars found in
the Milky Way and in globular star clusters orbiting our Milky Way.
Furthermore, small age values pose problems for current theories about
the formation and development of the observed large-scale structure of
the universe.


COSMIC MILEPOSTS

Cepheid variable stars rhythmically change in brightness over intervals
of days (the prototype is the fourth brightest star in the circumpolar
constellation Cepheus).  For more than half a   century, from the early
work of the renowned astronomers Edwin Hubble, Henrietta Leavitt, Allan
Sandage, and Walter Baade, it has been known that there is a direct
link between a Cepheid's pulsation rate and its intrinsic brightness.
Once a star's true brightness is known, its distance is a relatively
straightforward calculation because the apparent intensity of light
drops off at a geometrically predictable rate with distance.  Although
Cepheids are rare, once found, they provide a very reliable "standard
candle" for estimating intergalactic distances, according to
astronomers.

Besides being an ideal hunting ground for the Cepheids, M100 also
contains other distance indicators that can in turn be calibrated with
the Cepheid result.  This majestic, face-on, spiral galaxy has been
host to several supernovae, which are also excellent distance
indicators.  Individual supernovae (called Type II, massive exploding
stars) can be seen to great distances, and, so, can be used to extend
the cosmic distance scale well beyond Virgo.

As a crosscheck on the HST results, the distance to M100 has been
estimated using the Tully-Fisher relation (a means of estimating
distances to spiral galaxies using the maximum rate of rotation to
predict the intrinsic brightness) and this independent measurement also
agrees with both the Cepheid and supernova "yardsticks."

HST Key Projects are scientific programs that have been widely
recognized as being of the highest priority for the Hubble Space
Telescope and have been designated to receive a substantial amount of
observing time on the telescope.  The Extragalactic Distance Scale Key
Project involves discovering Cepheids in a variety of important
calibrating galaxies to determine their individual distances.  These
distances then will be used to establish an accurate value of the
Hubble Constant.

  ----------------------------------------------------------------------

The Key Project Team on the  Extragalactic Distance Scale consists of
Sandra Faber, Garth Illingworth & Dan Kelson (Univ. of California,
Santa Cruz), Laura Ferrarese & Holland Ford (Space Telescope Science
Institute), Wendy Freedman, John Graham, Robert Hill & Randy Phelps
(Carnegie Institution of Washington), James Gunn (Princeton
University), John Hoessel & Mingsheng Han (University of Wisconsin),
John Huchra (Harvard-Smithsonian Center for Astrophysics), Shaun Hughes
(Royal Greenwich Observatory), Robert Kennicutt, Paul Harding, Anne
Turner & Fabio Bresolin (Univ. of Arizona), Barry Madore & Nancy
Silbermann (JPL, Caltech), Jeremy Mould (Mt. Stromlo, Australian
National University), Abhijit Saha (Space Telescope Science Institute),
and Peter Stetson  (Dominion Astrophysical Observatory).



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


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).

The Wide Field and Planetary Camera 2 was developed by the Jet
Propulsion Laboratory (JPL) and is managed by the Goddard Space Flight
Center for NASA's Office of Space Science.