January 13, 2000, Thursday
National Desk Spacecraft Demystifying Halo of Gas In Milky Way
By JOHN NOBLE WILFORD
From the first observations by a new spacecraft, astronomers have found evidence that they say not only confirms the existence of a halo of hot gas surrounding the Milky Way galaxy but also seems to reveal how the gas got there, stays hot and contributes recycled material for the formation of new stars.
The findings from the Far Ultra violet Spectroscopic Explorer, or FUSE, were reported here today at a meeting of the American Astronomical Society. The small 300-pound satellite was launched into Earth orbit last June to conduct a planned three-year investigation of the evolution of galaxies and formation of stars and planets.
The new results show that ''FUSE is open for business and is already producing groundbreaking science,'' said Dr. George Sonneborn, the project's chief scientist at NASA's Goddard Space Flight Center in Greenbelt, Md.
The most telling discovery so far, scientists said, was the detection a vast abundance of a certain type of oxygen atom, known as oxygen VI, that appears in almost all directions and extends as much as 5,000 to 10,000 light-years away from the plane of the Milky Way, the region in Earth's home galaxy where most stars reside. Oxygen VI is an oxygen atom that has five of its eight electrons stripped away.
Noting that the presence of these atoms indicates the existence of gas with temperatures approaching half a million degrees Fahrenheit, Dr. Blair D. Savage of the University of Wisconsin at Madison said this was strong evidence linking the dynamics of the galactic halo to stars that exploded long ago. Some scientists had thought the halo gases were much cooler. Others had suggested that the gas was heated by ultraviolet radiation from hot stars.
''But the only way to make the observed amount of oxygen VI is through collision with the blast waves'' from exploding stars, or supernovas, Dr. Savage said.
This idea of a galactic halo of hot gas was postulated in 1956 by Dr. Lyman Spitzer, an astrophysicist at Princeton University. And 40 years ago, Dutch astronomers discovered clouds of gas falling into the plane of the Milky Way at extremely high velocities.
The FUSE spacecraft has also gathered detailed data on the composition of these infalling clouds. Metals in the clouds could link them to supernovas, revealing how many times the cloud's contents have been processed by the nuclear furnaces inside stars. This is how stars transmute light elements from the beginning of the universe into heavier elements, the stuff of planets, minerals and people.
Analyzing the early results, Dr. Edward Murphy, a research scientist at Johns Hopkins University, said most of the clouds appeared to have metals that had been processed by stars at least once. It was too early to tell if that applied to all the clouds, Dr. Murphy said.
In any event, the findings offered a plausible view of one important aspect of galactic dynamics. When dying stars explode, they heat the gas around them, creating pressures that push the gas up and away like a colossal bubble. They rise to the halo region, which is continuously heated and replenished by more rising gases. As some of the gases cool, though, they fall back into the galaxy in clouds, bringing with them the gas out of which new stars form and the supernova debris that winds up in planetary systems.
''It may be a bit like the water cycle on Earth,'' Dr. Murphy said. And just as the movement of water into the atmosphere and back to Earth again explains climate and weather, discoveries by the spacecraft could yield insights into the nature of the galactic halo.
The craft was designed to sample ultraviolet light in a nearly unexplored region of the electromagnetic spectrum. Its spectrograph, which breaks down light like a prism, is at least 100 times more powerful than such instruments on previous spacecraft. The ultraviolet light is invisible to the human eye, and the kind of most interest to astronomers is screened out by Earth's atmosphere.
In examining the halo, astronomers looked at the ultraviolet light coming from a dozen bright objects in the distant universe. As the light passed through the halo gas, some of it was absorbed at certain known wavelengths. These absorptions were like signatures of the elements within the halo, like the oxygen VI.
Other early findings by the spacecraft included detailed observations of the molecular hydrogen clouds out of which stars form in the Milky Way and neighboring galaxies. The spacecraft also tracked hot gas ''winds'' from huge bright stars that blow away their mass in 10 million years, brief lifetimes in stellar terms.
The spacecraft was built and is operated by Johns Hopkins for the National Aeronautics and Space Administration in collaboration with Canada, France, the University of Colorado and the University of California at Berkeley.
Organizations mentioned in this article:
American Astronomical Society
Space; Far Ultraviolet Spectroscopic Explorer; Milky Way Galaxy; Stars and Galaxies
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