Magazine article Science News

Speedy Galactic Aluminum Vexes Astronomers

Magazine article Science News

Speedy Galactic Aluminum Vexes Astronomers

Article excerpt

Astronomers rarely make discoveries that blast a hole in their understanding of the galaxy. Now though, explosive new findings are forcing them to rethink the structure of the Milky Way.

Using gamma-ray spectrometers mounted on high-flying balloons, some researchers have turned their attention to radioactive aluminum associated with clouds of gas near the center of the galaxy. Astronomers believe that this material was ejected by either a supernova or a giant star in the region.

Instead of seeing gamma rays with aluminum's characteristic energy, which would have indicated that the metal moves at the same speed as neighboring galactic material, they saw emissions with a broad distribution of energies around the expected value. This smearing indicated that the cloud of aluminum was expanding at 450 kilometers per second, three times faster than anticipated. The astronomers, from NASA's Goddard Space Flight Center in Greenbelt, Md., and the University of Maryland in College Park, report their findings in the Nov. 7 Nature.

"This raises important questions about what our galaxy is made of," said Goddard's Jack Tueller, a study coauthor. Astronomers had thought that material ejected by supernovas or giant stars would come to rest after about 100,000 years, slowed down by shock waves and collisions with other interstellar material. Instead, they calculate from the size of the cloud and its rate of expansion that it has rocketed around for 750,000 years as if little stood in its way. "Don't worry," says Tueller. "It's nowhere near us."

The researchers guess that the aluminum may be racing through an unusually low density pocket of the interstellar medium (SN: 4/20/96, p. …

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