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EMBARGOED UNTIL: 11:00 A.M. (EDT) May 25, 1999

PHOTO NO.: STScI-PRC99-19


HUBBLE FINDS VARIABLE STARS IN DISTANT SPIRAL GALAXY

A NASA Hubble Space Telescope (HST) view of the magnificent spiral
galaxy NGC 4603, the most distant galaxy in which a special class of
pulsating stars called Cepheid variables have been found. It is
associated with the Centaurus cluster, one of the most massive
assemblages of galaxies in the nearby universe. The Local Group of
galaxies, of which the Milky Way is a member, is moving in the direction
of Centaurus at a speed of more than a million miles an hour under the
influence of the gravitational pull of the matter in that direction.

Clusters of young bright blue stars highlight the galaxy's spiral arms.
In contrast, red giant stars in the process of dying are also found.
Only the very brightest stars in NGC 4603 can be seen individually, even
with the unmatched ability of the Hubble Space Telescope to obtain
detailed images of distant objects. Much of the diffuse glow comes from
fainter stars that cannot be individually distinguished by Hubble. The
reddish filaments are regions where clouds of dust obscure blue light
from the stars behind them.

This galaxy was observed by a team affiliated with the HST Key Project
on the Extragalactic Distance Scale. Because NGC 4603 is much farther
away than the other galaxies studied with Hubble by the Key Project
team, 108 million light-years, its stars appear very faint from the
Earth, and so accurately measuring their brightness, as is required for
distinguishing the characteristic variations of Cepheids, is extremely
difficult. At this distance some non-variable stars may by chance appear
to grow brighter and fainter in the same fashion as Cepheids due to the
physical impossibility of perfect measurements of such dim objects.
Determining the distance to the galaxy required an unprecedented
statistical analysis based on extensive computer simulations.

Researchers found 36-50 Cepheids and used their observed properties to
securely determine the distance to NGC 4603. These measurements indicate
that when the expansion of the universe and the motion of the Local
Group are accounted for, the Centaurus cluster is very nearly at rest
compared with the surrounding regions. It is part of the cause of the
rapid motions in the nearby universe, rather than being strongly pulled
by other concentrations of matter. Observations of distant Cepheids
such as those in NGC 4603 also help astronomers to precisely measure the
expansion rate of the universe.

Credit: Jeffrey Newman (Univ. of California at Berkeley) and NASA