WASHINGTON, March 9 /PRNewswire/ -- Astronomers have taken an important
step toward establishing an upper limit to the masses of stars. Using NASA's
Hubble Space Telescope, they made the first direct measurement within our
Milky Way Galaxy, and concluded stars cannot get any larger than about 150
times the mass of our sun.
The astronomers used the Hubble to probe the Arches cluster, the densest
in our galaxy. The finding takes astronomers closer to understanding the
complex star formation process. It also gives the strongest backing yet to
the notion stars have a weight limit.
"This is an incredible cluster that contains a rich collection of some of
the most massive stars in the galaxy, yet it appears to be missing stars more
massive than 150 times the mass of our sun," said astronomer Donald Figer of
the Space Telescope Science Institute, Baltimore. "Theories predict the more
massive the cluster, the more massive the stars within it. We looked at one
of the most massive clusters in our galaxy and found there is a sharp cutoff
to how large a star can form," he added.
A star's weight ranges from less than one-tenth to more than 100 times the
mass of our sun. Although astronomers know stars come in a variety of masses,
they don't know if the bodies have a weight limit at birth. Knowing how large
a star can form may offer important clues to how the universe makes them.
Astronomers have been uncertain about how large a star can get before it
cannot hold itself together and blows apart. Astronomers don't know enough
about the details of the star-formation process to estimate a star's upper
mass. Consequently, theories have predicted stars can be anywhere between 100
to 1,000 times more massive than the sun.
Figer's finding is consistent with statistical studies of smaller-mass
star clusters in our galaxy and with observations of a massive star cluster
known as R136 in our galactic neighbor, the Large Magellanic Cloud.
Figer used Hubble's Near Infrared Camera and Multi-Object Spectrometer to
study hundreds of stars ranging from six to 130 solar masses. Although Figer
did not find any stars larger than 130 solar masses, he conservatively set the
upper limit at 150 solar masses. The Arches cluster is a youngster about 2 to
2.5 million years old. It resides 25,000 light-years away from Earth in our
galaxy's hub, a hotbed of massive star formation. In this region huge clouds
of gas collide to form behemoth stars.
Hubble's infrared camera is well suited to analyze the cluster, because it
penetrates the dusty core of our galaxy. It produces sharp images, allowing
the telescope to see individual stars in a tightly packed cluster.
Figer estimated the stars' masses by measuring the ages of the cluster and
the brightness of the individual stars. He also collaborated with Francisco
Najarro of the Instituto de Estructura de la Materia in Madrid, Spain.
Najarro produced detailed models to confirm the masses, chemical abundances
and ages of the Arches cluster stars. "Standard theories predict 20 to 30
stars with masses between 130 and 1,000 solar masses," Figer explained. "But
we found none. If they had formed, we would have seen them," he added.
Figer cautions the upper limit does not rule out the existence of stars
larger than 150 solar masses. His next step is to pinpoint more clusters to
test his weight limit. Several telescopes, including NASA's Spitzer Space
Telescope, have been searching for new star clusters in the Milky Way.
The finding is published in the March 10 issue of Nature. For imagery and
more information about the research on the Internet, visit:
http://hubblesite.org/news/2005/05
For information about NASA and agency programs on the Internet, visit:
http://www.nasa.gov/
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