Brown dwarf stars make up invisible 'dark matter,' astronomers say


PHILADELPHIA -- A team of astronomers claimed yesterday the first definitive detection of objects making up the elusive "dark matter," long believed to lurk between the stars.

The speculative dark matter is, by definition, invisible. Therefore, the astronomers from Livermore (Calif.) National Laboratory, along with collaborators from Britain and Australia, used an indirect technique: looking for distorting effects that dark matter would have on the light coming to earth from more distant stars.

What they found is evidence of four objects they believe are "brown dwarfs" -- spherical bodies too small to light up with the nuclear fusion that powers the sun and other stars. The finding, they say, is just an indication of many more such objects in our galaxy.

"We are confident we are seeing the effects of dark matter," said Charles Alcock, who leads the team from Livermore. A colleague, Kem Cook, announced the finding at the April meeting of the American Physical Society in Washington, D.C.

At the same time, they announced another surprising finding to come from their unusual observing technique: a distribution of matter that changes our picture of the shape of our own galaxy. The Milky Way may not be a spinning pinwheel shape, as previously thought, but may instead take the form of an oblong spiral, with a bar-shaped concentration of stars in the center.

Astronomers came to believe in dark matter in the 1960s when they noticed that our galaxy and neighboring ones were rotating strangely -- the outer parts going around too fast, based on estimates of the mass of stars they could see.

It appeared that the galaxies were embedded in a much greater, unseen mass.

In fact, the rotations of many galaxies indicated that they were about 90 percent dark matter and only 10 percent visible stars. Astronomers came up with several possible candidates for dark matter, including black holes and exotic particles yet to be found.

Princeton astrophysicist John Bahcall said the objects announced yesterday were the most plausible explanation for dark matter.

Astronomers speculated that if such dud stars indeed existed, they would occasionally drift into the line of sight to more distant stars and distort their images.

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