[FPSPACE] FW: McDonald Obs.: Major Improvement in Cosmic Distance Scale withHubble Space Telescope
ljk4 at msn.com
Fri Apr 6 12:29:36 EDT 2007
>From: "AAS Press Officer Dr. Steve Maran" <Steve.Maran at aas.org>
>To: "AAS Press Officer Dr. Steve Maran" <steve.maran at aas.org>
>Subject: McDonald Obs.: Major Improvement in Cosmic Distance Scale
>withHubble Space Telescope
>Date: Thu, 5 Apr 2007 16:17:34 -0400
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(FORWARDING DOES NOT IMPLY ENDORSEMENT BY THE AMERICAN ASTRONOMICAL
SOCIETY.) Steve Maran, American Astronomical Society maran at aas.org
rjohnson at astro.as.utexas.edu
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FOR IMMEDIATE RELEASE
April 5, 2007
UNIVERSITY OF TEXAS ASTRONOMERS ACHIEVE MAJOR IMPROVEMENT
IN COSMIC DISTANCE SCALE WITH HUBBLE SPACE TELESCOPE
AUSTIN, Texas -- An international team of astronomers led by
Fritz Benedict and Barbara McArthur of The University of
Texas at Austin has used Hubble Space Telescope to solve one
of the biggest problems in measuring the universe's
expansion. The results of their in-depth studies of Cepheid
variable stars with HST is published in the April issue of
the Astronomical Journal.
?We took a classic approach to measuring cosmic distances,
made significant improvements, and carried out a successful
test,? Benedict said. "The result is a new, improved
distance measuring tool."
The universe's rate of expansion, "the Hubble constant," has
been hotly debated for decades. To calculate it, astronomers
must be able to measure precise distances to galaxies
billions of light-years away. That capacity, in turn, is
built on a series of measurement techniques in the so-called
"cosmic distance ladder" - each of which allow astronomers
to measure distances a little farther out into the universe.
One rung in the distance ladder is called a "Cepheid
variable star." Nearly 100 years ago, astronomers noticed
that the light output from intrinsically brighter Cepheids
varied more slowly than that from intrinsically fainter
Cepheids. But that "period-luminosity relationship" was not
known exactly. Benedict's team set out to precisely
determine this relationship for Cepheids in our own galaxy.
To accomplish the calibration, they directly measured the
distance to 10 Milky Way Cepheids. They followed these stars
for two years, measuring their apparent motion on the sky,
"When we measure a parallax, we?re looking at the little
circle that the star makes on the sky because the Earth goes
around the Sun," Benedict said. "The size of that circle
gives the absolute distance to the star." That circle is so
small for these distant stars (equivalent to a quarter seen
from 1,500 miles away) that it takes the Fine Guidance
Sensors on HST to make the measurements.
Once a star's precise distance is known from parallax, its
intrinsic brightness can be established. "We established the
intrinsic brightnesses of Cepheids whose light varied by
different amounts, and came up with an accurate
period-luminosity relationship," said Tom Barnes of The
University of Texas, the team's resident Cepheid expert.
"Knowing the period with which the brightness of a Cepheid
varies now accurately indicates its intrinsic brightness."
According to Benedict, "With this calibration, astronomers
can deduce the distance to any galaxy in which a Cepheid can
McArthur added, "We tested our newly derived Cepheid
period-luminosity relations on other galaxies with Cepheids
and found our results to be consistent with distances
derived from other methods." (This includes the galaxy NGC
4258, whose absolute distance has been measured by tracking
the motion of water masers around its center.)
Applying this relationship to many and more distant galaxies
should improve the accuracy of the Hubble constant. "A
precise Hubble constant is the top rung in the distance
scale ladder. With it you know the distance of any galaxy
with a measured velocity," McArthur said.
Measuring parallaxes sounds simple, but "success is in the
details," Benedict said. McArthur explains that "not only do
we take into account the motions of stars near our target
Cepheids, but we also look at how the minute motions of the
telescope itself can effect our measurements." She puts
these many corrections into the model when she derives the
parallaxes. "The journal paper includes the fine print," she
said, "so that our methods will be clear to our colleagues
who appreciate the fine art of precise position measurement,
or 'astrometry.' "
The HST Astrometry Team was founded at The University of
Texas at Austin long before the telescope launched in 1990,
and helped design HST's Fine Guidance Sensors and ensure
they would be useful for this kind of study. "We've been
cranking on this since 1977," Benedict said. "and as we tell
our children, 'Practice makes perfect!' "
"This result has excited me more than any in my 35-year
career," Benedict said, "and we will have more and better
over the next five years."
In addition to Benedict, McArthur, and Barnes, the
international team for this research consisted of Michael E.
Feast of The University of Cape Town, Thomas E. Harrison of
New Mexico State University, Richard J. Patterson of The
University of Virginia, John W. Menzies of the South African
Astronomical Observatory, Jacob Bean of The University of
Texas at Austin, and Wendy L. Freedman of the Observatories
of the Carnegie Institution of Washington.
Even with the team's exciting HST results, Benedict and
McArthur are looking toward the future. NASA's Space
Interferometry Mission, SIM, will measure parallaxes in even
greater detail than HST. SIM will measure the parallax of
stars one at a time, with great precision. Another future
mission, GAIA, will also make parallax measurements, but in
greater numbers and with less precision, through an all-sky
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