Astronomers find new way to measure distance to other galaxies (Forwarded)

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FOR RELEASE: 2:30 p.m. Hawaiian Standard Time, May 28, 2007

ASTRONOMERS FIND NEW WAY TO MEASURE DISTANCE TO OTHER GALAXIES

Dr. Rolf-Peter Kudritzki of the Institute for Astronomy (IfA) at the
University of Hawaii at Manoa will report today on a new method used to
measure extragalactic distances accurately based on the stellar gravities
and effective temperatures of blue supergiant stars in galaxies beyond the
Local Group. The paper will be presented at the meeting of the American
Astronomical Society (AAS) in Honolulu. His collaborators for this project
are Drs. Fabio Bresolin and Miguel Urbaneja (IfA) and a group of
astronomers of the Universidad de Concepcion, Chile, led by Dr. Wolfgang
Gieren.

Kudritzki and his collaborators observed the galaxy NGC 300, which is
about 6 million light-years away and located in the constellation
Sculptor, to test this new method of measuring distances between galaxies.
They took spectra to measure the atmospheric temperature of the stars and
their surface gravity. With these measurements, they were able to
determine the intrinsic brightness of each star and then to compare the
apparent brightness with intrinsic brightness to find the distance.

They then compared their distance measurements to those made using Cepheid
variable stars, the traditional way to measure the distance to nearby
galaxies. "This new method works excitingly well," says Kudritzki. "It
seems that we can determine distances to other galaxies with an accuracy
of five percent, which opens a complementary way to constrain the Hubble
constant more precisely." The Hubble constant denotes the current rate of
expansion of the universe.

Their observations consisted of medium- and low-resolution spectra taken
with the Focal Reducer Spectrograph (FORS) on the European Southern
Observatory's Very Large Telescope in Paranal, Chile. Kudritzki was
instrumental in designing FORS while at the University of Munich before
coming to Hawaii in 2000. In its most recent work on other galaxies, the
project team has also used the Low Resolution Imaging Spectrometer (LRIS)
on the Keck I telescope at Mauna Kea Observatories in Hawaii. Both FORS
and LRIS are very efficient spectrographs, which allows the team to take
spectra of many stars in a galaxy simultaneously (in a single exposure).
These stars can then be studied individually later, when the data are
analyzed to determine atmospheric temperatures and surface gravities.

The talk at the AAS meeting will describe their analysis method. It will
also discuss the determination of the chemical composition of the
individual stars in distant galaxies and how the chemical composition
changes from the center of the galaxies to their outskirts.

Blue supergiant stars are the largest and brightest stars in the universe
at visual wavelengths, with an intrinsic brightness between 10,000 and
1000,000 times that of the sun. They appear blue in color because the
temperature of their atmospheres is 150 percent to 500 percent times than
that of the sun. Because of their blue color and their enormous
brightness, they stand out among the hundred billion stars in a galaxy,
and so are easy to find and identify. This makes them ideal for studying
stellar populations in galaxies beyond the Local Group (the group of
galaxies that contains our own Milky Way galaxy) to learn about distances,
chemical composition, star formation in galaxies, and the absorption of
starlight by gas and dust between stars. In the Milky Way, Deneb in the
constellation Cygnus (the Swan) and Rigel in Orion are two well-known blue
supergiants.

The title of the paper being given at the AAS meeting is "Extragalactic
Stellar Astronomy -- Quantitative Spectroscopy of A Supergiants in
Galaxies beyond the Local Group."

The Institute for Astronomy at the University of Hawaii conducts research
into galaxies, cosmology, stars, planets, and the sun. Its faculty and
staff are also involved in astronomy education, deep space missions, and
in the development and management of the observatories on Haleakala and
Mauna Kea.

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IMAGE CAPTION:
[http://www.ifa.hawaii.edu/info/press-releases/AAS/kud_ass07-630.jpg
(398KB)]
The spiral galaxy NGC 300. The green circles and squares indicate the blue
supergiant stars measured in this experiment. The objects within the
circles are 50 to 100 percent hotter than the sun, whereas the objects in
the squares have a temperature at least 300 percent that of the sun.


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