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HOT, YOUNG STARS COOL DOWN A BIT The
most massive, hottest stars in the Universe are not as hot as we thought, according
to new research by NASA-sponsored astronomers. The result was obtained by comparing
a sophisticated new model of stellar atmospheres to recent observations of young,
massive stars in the Small Magellanic Cloud, a nearby galaxy. The observations
were obtained using a pair of NASA spacecraft: the Hubble Space Telescope and
the Far Ultraviolet Spectroscopic Explorer (FUSE). "We
are interested in hot, massive stars because we would not exist without them,"
said Dr. Sara Heap, an astronomer at NASA's Goddard Space Flight Center in Greenbelt,
Md. "Life-sustaining elements, like oxygen, are forged in the cores of these
stars and distributed back to space when they explode, where they become the raw
material for succeeding generations of stars and, ultimately, life. This new result
will help us understand how young, massive stars and their host galaxies evolve."
Heap is presenting her research June 3 during a poster session at the American
Astronomical Society's summer meeting in Albuquerque, N.M. If
stars had personalities, these massive stars, called "O" stars, would
be the flamboyant celebrities of the Universe. They are roughly between 20 and
100 times as massive as the Sun, with surface temperatures about 5 to 10 times
hotter than the Sun's 10,000 degrees Fahrenheit, and they shine up to a million
times more brightly. Like celebrities that live hard and die young, O stars never
really get a chance to grow old, quickly "burning up" their fuel via
nuclear reactions and exploding as supernova in a few million years, hardly a
moment compared to the Sun's estimated ten billion-year lifespan. The
new research indicates that O stars are actually between 5 to 20 percent cooler
than earlier estimates. "An accurate temperature is the key to unlocking
the nature of a star," said Dr. Thierry Lanz, a research scientist with the
University of Maryland stationed at Goddard who developed the new model with Dr.
Ivan Hubeny of the National Optical Astronomy Observatory in Tucson, Arizona.
If
O stars are actually cooler than previously estimated, their luminosities and
masses are subject to revision as well. Luminosity responds dramatically to temperature,
so if O stars are 5 to 20 percent cooler, their luminosities are cut between 20
and 80 percent. Also, the more massive a star is, the faster it consumes its fuel
and the brighter it shines. Thus, the lower estimated luminosities typically lead
to a cut by a third in their estimated masses. A
star's temperature varies at it ages, so knowing a star's temperature also allows
astronomers to estimate its age. A more accurate temperature estimate will therefore
provide a better estimate of stellar ages and an improved understanding of stellar
evolution. The
new stellar atmosphere model estimates how light from O stars would appear as
it passes through their atmospheres on the way to Earth. Elements in stellar atmospheres
selectively block light of certain unique colors, depending on the type of element
and its temperature. Scientists use a special instrument called a spectrograph
to break down light into its component colors, similar in principle to using a
prism to separate white light into a rainbow. The plot of starlight according
to color (wavelength) is called a plot of its spectrum. A spectral plot resembles
a jagged line, with dips representing where elements in a star's atmosphere have
absorbed light of certain colors. Spectra
of different stars will vary depending on differences in temperature, age, mass,
and composition. The researchers compared spectra from their observations to various
predicted spectra derived from the new stellar atmosphere model, and discovered
that the best matches were to ones based on cooler temperatures. Previous
models typically only included the absorption effects from hydrogen and helium,
the most common elements in stellar atmospheres. However, many more elements are
present, but including them greatly increases the complexity of the calculations.
Lanz and Hubeny were able to include more elements because of advances in numerical
techniques and computing power, and the team believes their model therefore provides
a more accurate fit. Additionally, NASA's Hubble and FUSE allowed the astronomers
to obtain the most detailed spectra of O stars in the Small Magellanic Cloud to
date, increasing the team's confidence in their result. O
stars contribute to galaxy evolution by creating and redistributing heavy elements,
and they also help astronomers study galaxy evolution. Since these stars don't
live long, they don't move very far from their birthplaces. Since they are so
bright, they can be seen over great distances. When astronomers detect a concentration
of O stars in a remote galaxy, they know that it is a region where star formation
is occurring, even if they can't make out the other fledgling stars that happen
to be less massive and fainter. The
astronomers plan to combine their observations of the Magellanic O stars to predict
the integrated spectrum of clusters of young, massive stars in distant galaxies.
Using the Small Magellanic Cloud data as a calibration, the team will compare
it to spectra of other galaxies in order to learn more about star formation and
galaxy evolution in remote regions of the Universe, where it is impossible to
resolve individual stars. The
National Optical Astronomy Observatory (NOAO) is operated by the Association of
Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement
with the National Science Foundation. The Hubble Space Telescope is a project
of international cooperation between NASA and the European Space Agency (ESA).
FUSE is a NASA-supported astronomy mission that was launched on June 24, 1999,
to explore the Universe using the technique of high-resolution spectroscopy in
the far-ultraviolet spectral region. The Johns Hopkins University has the lead
role in developing and now operating the mission, in collaboration with The University
of Colorado at Boulder, The University of California at Berkeley, international
partners the Canadian Space Agency (CSA) and the French Space Agency (CNES), and
corporate partners. Back
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