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January 12, 2000
Michael Purdy

FUSE Spacecraft Observes Interstellar
Lifeblood of Galaxies

The extended halo of half-million-degree gas that surrounds the Milky Way was generated by thousands of exploding stars, or supernovae, as our galaxy evolved, according to new observations by NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) spacecraft.

The spacecraft has nearly completed its shakedown phase, and its first results are already providing a wealth of new information to astronomers about the material that becomes stars, planets, and ourselves.

The new findings confirming the nature of the Milky Way halo are being presented today in Atlanta at the 195th meeting of the American Astronomical Society (AAS).

The roughly football-shaped hot gas halo which surrounds our galaxy extends about 5,000-10,000 light years above and below the galactic plane and thins with distance. One light year is almost 6 trillion miles.

"The hot gas halo has been known for some time, but we weren't sure how it got there or stayed hot," said FUSE co-investigator Blair Savage of the University of Wisconsin in Madison. "The new FUSE observations reveal an extensive amount of oxygen VI (oxygen atoms that have had five of their eight surrounding electrons stripped away) in the halo. Some scientists thought that ultraviolet radiation from hot stars could produce the halo, but the only way to make the observed amount of oxygen VI is through collision with the blast waves from exploding stars, called supernovae."

"Stars destined to explode don't live long, compared to stars like our Sun, so star explosions are actually a record of star formation," said George Sonneborn, FUSE project scientist at NASA's Goddard Space Flight Center, Greenbelt, Md. "By comparing supernova generated halos among galaxies, we may be able to compare their star formation histories."

"FUSE measures the pulse of the lifeblood of our galaxy, the thin gas between stars," said Warren Moos (pictured at right), FUSE principal investigator at Johns Hopkins University in Baltimore. "This interstellar gas courses through our veins, because dense clouds of it collapsed to form new stars and planets, including our solar system."

The FUSE observatory is now "open for business," Moos said. "After an extended on-orbit checkout and debugging period, common for complex space observatories, we are now performing observations on a routine basis for both members of the Principal Investigator Team and the 62 guest investigators from around the world selected by NASA for the first year of operations.

"We are continuing to tune the instrument," Moos added. "In the spring we expect to begin a comprehensive study of the abundance of deuterium, a fossil atom left over from the Big Bang. As our team becomes more practiced, we need less time to optimize the instrument, and the amount of time we can spend on scientific observations will go up. This means higher scientific productivity."

FUSE is able to detect interstellar gas and determine its composition, velocity and distance by viewing bright celestial objects further away. The intervening gas selectively absorbs the light from these objects in a unique pattern of colors, depending on the composition of the gas. The spectrograph on FUSE separates the light into its component colors, similar to the way a prism separates white light into a rainbow. The resulting patterns identify the gas like optical fingerprints. When the patterns shift to different colors, velocity and distance measurements can be inferred.

The FUSE spectrograph is at least 100 times more powerful than previous instruments, helping it reveal a large number of new atomic and molecular features in interstellar gas that could only be guessed at before. The ultraviolet light analyzed by FUSE is invisible to the human eye.

FUSE scientists are also reporting early results at the AAS meeting about investigations into two other components of the galactic "circulatory system:" cold clouds of molecular hydrogen where new stars are born, presented by Michael Shull of the University of Colorado, and hot gas "winds" from stars so bright they nearly blow themselves apart, presented by John Hutchings of the National Research Council of Canada.

Editor's Note: New images related to this science, and more information about FUSE, can be found on the internet at: http://fuse.pha.jhu.edu/.

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