In a galaxy far, far away … about 2.5 million light years, in fact, lie approximately 20,000 hot, young stars and dense clusters that comprise the Andromeda Galaxy. The galaxy, known as M31 in the constellation Andromeda, was recently captured by an ultraviolet optical telescope aboard NASA’s Swift satellite and delivers the highest-resolution view of a neighboring spiral galaxy ever attained in the ultraviolet.
NASA's Swift satellite spends most of its time in space searching for distant cosmic explosions but, for two months last year, took a sojourn from its usual duties to capture stunning views of the largest and closest spiral galaxy to our own.
"Swift reveals about 20,000 ultraviolet sources in M31," said Stefan Immler, a research scientist on the Swift team at NASA's Goddard Space Flight Center. "Of particular importance is that we have covered the galaxy in three ultraviolet filters. That will let us study M31's star-formation processes in much greater detail than previously possible."
The Andromeda Galaxy, is more than 220,000 light-years across and lies 2.5 million light-years away. On a clear, dark night, the galaxy is faintly visible as a misty patch to the naked eye.
Between May 25 and July 26, 2008, Swift's Ultraviolet Optical Telescope (UVOT) acquired 330 images of M31 at wavelengths of 192.8, 224.6, and 260 nanometers. The images represent a total exposure time of 24 hours and took up a massive 85GB.
Erin Grand, an undergraduate student at the University of Maryland at College Park, who worked with Immler as an intern this summer, spent ten weeks processing the data to create the images.
The images highlight some new insights into this galaxy. The first is the striking difference between the galaxy's central bulge and its spiral arms. "The bulge is smoother and redder because it's full of older and cooler stars," Immler explained. "Very few new stars form here because most of the materials needed to make them have been depleted."
Beyond the central bulge, dense clusters of hot, young, blue stars sparkle. As in our own galaxy, M31's disk and spiral arms are a breeding ground for stars because they harbor most of the gas and dust needed to produce them. Star clusters are especially plentiful in an enormous ring about 150,000 light-years across.
What triggers the unusually intense star formation in Andromeda's 'ring of fire'? Previous studies have shown that tides raised by the many small satellite galaxies in orbit around M31 help boost the interactions within gas clouds that result in new stars.
In 1885, an exploding star in M31's central bulge became so bright it was visible with the naked eye. This was the first supernova ever recorded in any galaxy beyond our own Milky Way. "We expect an average of about one supernova per century in galaxies like M31," Immler said. "Perhaps we won't have to wait too long for another one."
"Swift is surveying nearby galaxies like M31 so astronomers can better understand star-formation conditions and relate them to conditions in the distant galaxies where we see gamma-ray bursts occurring," said Neil Gehrels, the mission's principal investigator at NASA Goddard.
Since Swift's November 2005 launch, the satellite has detected more than 400 massive, far-off explosions likely associated with the births of black holes.
Swift is managed by NASA Goddard. It was built and is being operated in collaboration with Pennsylvania State University, the Los Alamos National Laboratory in New Mexico, and General Dynamics of Gilbert, Ariz., in the United States.
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