[Archives] [Image] [Navigation Bar] ASTRONOMERS DETECT MUCH OF UNIVERSE'S MISSING MATTER MILKY WAY IS ENVELOPED BY DEAD STARS, TEAM SAYS By Kathy Sawyer Washington Post Staff Writer Wednesday, January 17, 1996 ; Page A01 SAN ANTONIO, JAN. 16 -- SAN ANTONIO, JAN. 16 -- Earth's home galaxy, the Milky Way, is enveloped in a vast sphere of unseen "dark matter" in the form of dead stars, according to new evidence reported today by an international team of astronomers. The newly detected objects, known by the acronym MACHOS (Massive Compact Halo Objects), are most likely burned-out stars called "white dwarfs," the researchers said. They could also include a mix of other stellar remnants including black holes, objects with a gravitational field so strong that not even light can escape it. Scientists called the findings a major step forward in astronomers' long struggle to locate and identify the "missing mass" in the universe. The new evidence suggests that much of the long-sought missing matter is made of the same materials as the stars and planets, but simply could not be detected until astronomers tried a new technique. Only 10 percent of the cosmic mass exists in visible, "shiny" celestial objects such as stars and galaxies. The other 90 percent has been detected only through its gravitational influence on nearby visible objects. The mystery of the dark matter is bound up with questions of how the universe formed and has evolved, and what will be its fate. A debate has raged over whether the missing mass was made up of protons and neutrons like ordinary matter or whether it could be composed of exotic, undetectable elementary particles known as WIMPs (Weakly Interacting Massive Particles). The new observations tilt the playing field strongly toward the "ordinary matter" advocates. If the early evidence holds up, said John Bahcall, of the Institute for Advanced Studies at Princeton, "this is the most important piece of science I've heard about in the last few years." The findings strongly indicate, he said, that "the right stuff' is ordinary stuff." Bahcall said the findings are "bad news" for particle physicists because they don't lend support for the existence of exotic particles required by some theories of how the universe has formed and evolved. MACHOs are lightless bodies such as sub-stellar objects too small to ignite the thermonuclear fusion that gives stars their shine, dead stars and black holes. To search for such objects, the research team spent two years observing 10 million stars a night, using the latest in high-powered detectors. The findings of the international consortium of MACHO hunters, presented here at the annual winter meeting of the American Astronomical Society, represent a revision of the team's earlier report, which had showed a puzzling shortage of MACHOs. Their initial results, presented last April, were hailed as the first "significant" detection of galactic dark matter. But those findings also indicated that the total mass of MACHOs could account for only about 20 percent of the unseen matter that theorists say should be present in the galaxy. Today's revised results indicate that MACHOs in the mass range of white dwarfs could account for at least 50 percent of the galactic dark matter -- and possibly all of it. The difference in the two sets of results is a combination of improved analysis of the original data and the fact that "we now have twice as much data," said David Bennett of the Lawrence Livermore Laboratory and the Center for Particle Astrophysics of the University of California at Berkeley. Those factors made it possible to extend the search to a wider range of objects, he said. The original search had focused on objects ranging from about the mass of Jupiter to about one-tenth that of the sun. The subsequent search included objects less massive than Earth and as massive as the sun. In order to detect the presence of the invisible MACHOs, the team uses a technique called microlensing. This refers to the apparent brightening of a background star caused by the gravitational field of a massive object (a MACHO) passing almost directly in front of it. (The brightening effect is caused by the bending of light rays by the foreground object's gravity.) The team uses the 1.27-meter telescope at Mount Stromlo Observatory in Australia to monitor 10 million or more stars nightly in the Large Magellanic Cloud, a satellite galaxy of the Milky Way located on the verge of, or possibly within, the Milky Way's theoretical halo of dark matter. The Cloud is visible only in the Southern Hemisphere. Its stars provide a backdrop against which the halo objects can be detected as they pass. While the initial search turned up only three microlensing "events," indicating the passage of a MACHO, the new analysis reveals a total of seven such events over two years, representing, on average, more massive objects than those detected initially. The greater the mass of the MACHO, the longer the lensing event, Bennett said. The estimated mass of the lensing objects is "in the range of white dwarfs -- stars which long ago exhausted their nuclear fuel," he said. "Probably our likelihood of seeing {a given lensing event} is one in a million," Bennett added. That's why the team must monitor millions of stars. Earth and the the rest of the sun's family reside near the edge of the Milky Way, a pinwheeling disk of stars more than 100,000 light years across. Just as the planets revolve around the sun, the Milky Way rotates around its center. But astronomers have found that the visible stars, dust and other material account for only a small fraction of the gravity required to keep the Milky Way and other galaxies from flying apart. Those who study the evolution of the universe see no reason to believe things are different in other corners of the cosmos. Cosmologist Neta Bahcall of Princeton earlier in the day presented evidence of unexpectedly enormous halos around far-flung galactic clusters. Colleague Alex Filippenko of the University of California at Berkeley, hearing of Bahcall's report and then the MACHO findings, said, "It seems they're beginning to find some of the material that makes up these giant halos Neta was talking about." 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