Texas Astronomers Achieve Major Improvement in Cosmic Distance Scale with HST (Forwarded)

McDonald Observatory
University of Texas

Contact: Rebecca Johnson
ph: 512-475-6763 fax: 512-471-5060

5 April 2007

Texas Astronomers Achieve Major Improvement in Cosmic Distance Scale with
Hubble 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 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, called "parallax."

"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 be detected."

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.