Date: Oct 17 20:11 UTC
Subject: Secrets of Star Birth
HUBBLE YIELDS SECRETS OF STAR BIRTH IN THE EARLY UNIVERSE
NASA's Hubble Space Telescope has provided new insights into how stars
might have formed many billions of years ago in the early universe.
Hubble observations of a pair of star clusters suggest a new link in
the stellar evolution processes.
The pair of clusters are 166,000 light-years away from Earth in the
southern hemisphere constellation, Doradus, within the Large Magellanic
Cloud (LMC). The clusters are unusually close together for being
distinct and separate objects, according to Hubble astronomers.
A preliminary assessment of the HST observation indicates that these
two compact clusters contain many more massive stars than expected.
"If this were also the case billions of years ago, it could have
altered drastically the early history of the universe," says Dr. Nino
Panagia of the Space Telescope Science Institute (STScI) in Baltimore,
MD, and the European Space Agency (ESA).
Panagia, R. Gilmozzi (also of STScI/ESA) and their co-investigators
utilized HST's unique capabilities -- ultraviolet sensitivity, ability
to see faint stars, and high resolution -- to identify three separate
populations in this concentration of nearly 10,000 stars. (Previous
observations with ground-based telescopes have been able to resolve
less than 1,000 stars in this region.)
About 60 percent of these stars belong to the dense cluster called NGC
1850, estimated to be 50 million years old. However, Hubble found that
a loose distribution of extremely hot, massive stars in a separate
cluster within the same region --representing about 20 percent of the
stars in the Hubble image--are only about 4 million years old. (The
third group of stars observed by Hubble are simple field stars in the
LMC.)
The significant difference between the ages of the two clusters
suggests that they are two separate star groups that lie along the same
line of sight. The younger, more open cluster probably lies 200
light-years beyond the older cluster, says Panagia, because if it were
in the foreground, then dust from the younger cluster would obscure
stars in the older cluster.
Having two well-defined star populations separated by such a small gap
of space is very unusual. This juxtaposition suggests that the
clusters might be linked in an evolutionary sense. The possible
scenario proposed by the Hubble researchers is that an expanding
"bubble" of hot gas from more than 1,000 supernova explosions in the
older cluster triggered the birth of the younger cluster.
The bubble expanded across space for 45 million years before plowing
into a wall of cool gas and dust. The resulting shock front then
caused the gas to contract and precipitate a new generation of star
formation. The massive, hot stars produced by this contraction are
destined to explode in a few million years, and thus create yet another
expanding bubble of gas.
Previously, such detailed studies of stellar population were restricted
to nearby star-forming regions within the plane of our Milky Way
Galaxy. However, Hubble's high-quality images enable these studies to
be extended a hundred times farther into the universe, out to the
distance of a neighboring galaxy.
The LMC is a natural laboratory for studying the birth and evolution of
stars because it lies outside the clutter of the Milky Way and its
stars have few heavy elements, so their composition is believed to be
more like the primordial stars that formed in the early universe.
The findings will be published in the Nov. 1, 1994 issue of the
Astrophysical Journal Letters.
Co-investigators are R. Gilmozzi (STScI, Baltimore, MD and ESA), E.K.
Kinney (STScI); S.P. Ewald (California Institute of Technology,
Pasadena, CA), N. Panagia (STScI and ESA, and University of Catania,
Italy); and M. Romaniello (University of Pisa, Italy).
The Hubble Space Telescope is a project of international cooperation
between NASA and the ESA.
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- T o d d E. V a n H o o s e a r -
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