McDonald Observatory
University of Texas

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


9 December 2003

Astronomers Re-measure the Universe with Hubble Space Telescope

AUSTIN, Texas -- University of Texas at Austin astronomers are using Hubble
Space Telescope (HST) to improve measurements of vast distances in space, which
could greatly increase the accuracy of knowledge in all areas of astronomy from
understanding how stars evolve to the size and age of the universe itself.

Fritz Benedict, Barbara McArthur, Tom Barnes and colleagues are shoring up the
wobbly "extra-galactic distance ladder" by measuring the tiny apparent motions,
or "parallax," of a particular kind of star called "Cepheid variables."

"HST is the only telescope on Earth or in space that can do this with the
required precision right now," Benedict said. "Obtaining these parallaxes is
extremely difficult, equivalent to measuring the size of a quarter seen from
3,000 miles away."

The project, which ranked first among 1,100 proposals by astronomers for use of
HST this year, continues later this month with more Hubble observations.

Cepheid variable stars are one tool that astronomers use to measure vast
astronomical distances. They work well for this because the rapidity with which
their light-output varies tells scientists their intrinsic brightness. This
interdependence is called the "period-luminosity (PL) relationship." So
astronomers can measure the period of variation for a Cepheid variable star in a
galaxy and deduce that galaxy's distance from knowledge of the luminosity of a
Cepheid with that period.

Cepheids make up one rung on the extra-galactic distance ladder that astronomers
use to measure distance to objects outside our own Milky Way galaxy. In this
ladder, each rung is a type of distance measurement that is the basis for the
next rung above it, to measure out to farther distances.

One of the lower rungs is knowledge of the distance to the Large Magellenic
Cloud (LMC) -- one of the satellite galaxies of the Milky Way. Astronomers'
knowledge of the LMC's distance is based in large measure on Cepheids inside
that galaxy. The problem is, those Cepheids are not made up of the same stuff as
the ones in our galaxy. So astronomers aren't sure if the P-L relationship
really works right on them.

The team is working to eliminate the LMC rung from the distance ladder and to
replace it with something sturdier. They're using HST to directly measure the
distance to 10 Cepheid variable stars inside our own Milky Way galaxy.

"By doing this we can compare the direct distance measurement with the one
predicted by astronomers' best calculation of the Cepheid P-L relationship --
revealing any discrepancies and allowing for necessary adjustments in that
calculation," said Barnes. McArthur added, "Cepheids will then become a better
yardstick."

For this study, the team is using HST to make extremely precise measurements of
the location of each of the 10 Cepheids at various times over two years. In
comparing earlier observations to those taken later, each star appears to have
moved. This apparent motion is called "parallax."

"Trigonometric parallax -- watching a star seeming to move from side to side
because the Earth orbits around the Sun -- is the only fundamental method of
getting Cepheid distances and luminosities free from complicating assumptions,"
Benedict said.

His team is making the measurements using HST's Fine Guidance Sensors (FGS) --
the instruments whose primary reason for being is to enable HST's cameras and
spectrographs to lock onto their targets. However, this "bonus science" with FGS
was planned from the start. Benedict helped ensure that the FGS could be used
for parallax work, and has helped in planning their use for more than two decades.

"Because of the great demand for HST time, we can do these measurements only for
a small number of stars in the Milky Way, fewer than a dozen," Benedict said.
"In the future, SIM can do this for a lot more stars."

SIM, the Space Interferometry Mission, is a future NASA space observatory, but
one whose final results will not be available until 2015.

With this HST project, Bendict says, "I'm happy that we'll have good results in
two years instead of 12."