The Milky Way, our home galaxy, recycles the raw materials of stars through a violent series of "galactic fountains," according to debut observations from a newly launched NASA spacecraft.
Astronomers using the Far Ultraviolet Spectroscopic Explorer (FUSE), placed into orbit 460 miles above Earth last summer, have determined that the enigmatic halo of super-hot gas that surrounds our galaxy--and that eventually drizzles back down into it to form new stars--is squeezed and heated by supernovae, the catastrophic death-explosions of stars.
The existence of such a halo had been debated for decades before it was finally confirmed in recent years. But no one knew its extent, extreme temperature or what process accounted for it.
The new FUSE data indicate that the halo envelops the galaxy to a distance of 5,000 to 10,000 light-years into intergalactic space. (One light-year is about 6 trillion miles. The spiral disk of the Milky Way is approximately 100,000 light-years across.)
It appears that the halo "is heated by dying stars," FUSE scientist Blair Savage of the University of Wisconsin told the 195th meeting of the American Astronomical Society here. "Some scientists thought that ultraviolet radiation from hot stars could produce the halo," but that would never make it hot enough. "The only way to [do it] is through collision with the blast waves."
The process, Savage said, recurs about twice a century when supernovae propel enormous amounts of gas above the plane of the galaxy, making it so hot that it remains about 500,000 degrees millions of years later. (By comparison, the surface of the sun is about 5,800 degrees.) FUSE scientists measured the halo by using the 3,000-pound satellite's special sensors, which are designed to detect high-energy ultraviolet radiation.
The type of UV radiation absorbed or emitted by an object provides a "fingerprint" of its dimensions, chemical composition and temperature. Those rays are invisible from the surface of the Earth because they are blocked by the atmosphere. And until FUSE, NASA had had no orbiting observatory to examine those wavelengths since the early 1970s.
The first FUSE data also revealed large and widespread concentrations of molecular hydrogen in the Milky Way. That gas, from which new stars are formed, is "the lifeblood of the galaxy," said Warren Moos of Johns Hopkins University. FUSE also showed that different galaxies recycle stellar material in different ways and at different speeds.
Two separate groups also announced at the meeting that they had used Earth-based radio telescopes to detect what seem to be globular star clusters of unprecedented youth.
There are a couple hundred of these foggy blobs, typically containing between 1,000 and a few hundred thousand stars each, in the Milky Way. But the newest are several billion years old. Even outside the galaxy, the youngest globular cluster previously detected is several million years old.
The newly detected clusters, however, are about 500,000 years old--mere newborns by the standards of stellar chronology. Five were found in one galaxy 32 million light-years away by a team from the University of Wisconsin and the University of Colorado.
Another cluster--previously invisible because it is so young that nearly all of its radiation is in radio wavelengths, not visible light--was discovered in a different galaxy 12 million light-years from Earth by Jean Turner of the University of California at Los Angeles and colleagues elsewhere. The scientists used data from the National Science Foundation's Very Large Array radio telescope in New Mexico along with infrared images from the Keck Observatory in Hawaii. Astronomers can't yet explain the phenomenon.