From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Jan 22 2004 - 07:50:29 PST
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From: NASA Jet Propulsion Laboratory
Sent: Wednesday, January 21, 2004 6:29 PM
To: ljk4_at_msn.com
Subject: Astronomers Measure Distance to Well-Known Star
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NEWS RELEASE: 2004-026 January 21, 2004
Astronomers Measure Distance to Well-Known Star
The cluster of stars known as the Pleiades is one of the most
recognizable objects in the night sky, and for millennia has been
celebrated in literature and legend. Now, a group of astronomers has
obtained a highly accurate distance to one of the stars of the
Pleiades known since antiquity as Atlas. The new results will be
useful in the longstanding effort to improve the cosmic distance
scale, and to conduct research on the stellar life-cycle.
In the January 22 issue of the journal Nature, astronomers from the
California Institute of Technology and NASA's Jet Propulsion
Laboratory, both in Pasadena, Calif., report the best-ever distance to
the double-star Atlas. The star, along with "wife" Pleione and their
daughters, the "seven sisters," are the principal stars of the
Pleiades that are visible to the unaided eye, although there are
actually thousands of stars in the cluster. Atlas, according to the
team's decade of careful interferometric measurements, is somewhere
between 434 and 446 light-years from Earth.
The range of distance to the Pleiades cluster may seem somewhat
imprecise, but in fact is accurate by astronomical standards. The
traditional method of measuring distance is by noting the precise
position of a star and then measuring its slight change in position
when Earth itself has moved to the other side of the sun. This
approach can also be used to find distance on Earth: If you carefully
record the position of a tree an unknown distance away, move a
specific distance to your side, and measure how far the tree has
apparently "moved," then it's possible to calculate the actual
distance to the tree by using trigonometry.
However, this procedure gives only a rough distance estimate to even
the nearest stars, due to the gigantic distances involved and the
subtle changes in stellar position that must be measured.
The team's new measurement settles a controversy that arose when the
European satellite Hipparcos provided a much shorter distance
measurement to the Pleiades than expected and contradicted theoretical
models of the life cycles of stars.
This contradiction was due to the physical laws of luminosity and its
relationship to distance. A 100-watt light bulb one mile away looks
exactly as bright as a 25-watt light bulb half a mile away. So to
figure out the wattage of a distant light bulb, we have to know how
far away it is. Similarly, to figure out the "wattage" (luminosity)
of observed stars, we have to measure how far away they are.
Theoretical models of the internal structure and nuclear reactions of
stars of known mass also predict their luminosities. So the theory and
measurements can be compared.
However, the Hipparcos data provided a distance lower than that
assumed from the theoretical models, thereby suggesting either that
the Hipparcos distance measurements themselves were off, or else that
there was something wrong with the models of the life cycles of stars.
The new results show that the Hipparcos data was in error, and that
the models of stellar evolution are indeed sound.
The new results come from careful observation of the orbit of Atlas
and its companion -- a binary relationship that wasn't conclusively
demonstrated until 1974 and certainly was unknown to ancient watchers
of the sky. Using data from the Mount Wilson stellar interferometer,
next to the historic Mount Wilson Observatory, and the Palomar Testbed
Interferometer at Caltech's Palomar Observatory near San Diego, the
team determined a precise orbit of the binary.
Interferometry is an advanced technique that allows, among other
things, for the "splitting" of two bodies so far away that they
normally appear as a single blur, even in the biggest telescopes.
Knowing the orbital period and combining it with orbital mechanics
allowed the team to infer the distance between the two bodies, and
with this information, to calculate the distance of the binary to
Earth.
"For many months I had a hard time believing our distance estimate was
10 percent larger than that published by the Hipparcos team," said the
lead author, Xiao Pei Pan of JPL. "Finally, after intensive
rechecking, I became confident of our result."
Coauthor Shrinivas Kulkarni, a Caltech astronomy and planetary science
professor, said, "Our distance estimate shows that all is well in the
heavens. Stellar models used by astronomers are vindicated by our
value."
"Interferometry is a young technique in astronomy and our result paves
the way for wonderful returns from the Keck interferometer and the
anticipated Space Interferometry Mission that is expected to be
launched in 2009," said coauthor Michael Shao of JPL, prinicipal
investigator for that planned mission, and for the Keck
Interferometer, which links the two 10-meter telescopes at the Keck
Observatory in Hawaii. The Palomar Testbed Interferometer was
designed and built by a team of researchers from JPL led by Mark
Colavita and Shao. It served as an engineering testbed for the Keck
Interferometer.
-end-
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