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Astronomers Get a Direct Look at 'Dark Matter'

Space: An international team sights elusive white dwarf stars that make up some of the universe's missing mass.


Solving at least part of one of cosmology's great mysteries, an international team of scientists reports that it has found a chunk of the missing "dark matter" that makes up most of our galaxy.

The components of dark matter have eluded scientists for 70 years, prompting an increasingly intense game of cosmic hide-and-seek.

Scientists have determined that the universe must have a certain mass. Anything less than that would not provide enough gravity to hold galaxies together--they would simply fly apart.

But visible matter can account for only a small part of that total mass; hence, the rest resides in the posited dark matter. In our galaxy, less than 10% of the matter is visible in stars that can be detected.

The new findings suggest that between 3% and 35% of that dark matter is made up of white dwarf stars. These are burned-out husks of stars that once shined as brightly as our sun but now glow only feebly, said Ben R. Oppenheimer, a postdoctoral researcher at UC Berkeley who led the study that was published online today in the journal Science.

White dwarfs are a type of MACHO, or massively compact halo object, that astronomers have been trying to detect for more than a decade. Now the Berkeley researchers report they have directly sighted white dwarfs.

At one time, many astronomers thought all of the missing matter in the galaxy might be made up of MACHOs. The leading MACHO candidates were brown dwarfs--huge balls of gas that were not quite big enough to be stars. But the sea of brown dwarfs that would have been required to fill the galaxy never materialized.

By 1995, leading MACHO expert Charles Alcock indirectly detected MACHOs through a technique called gravitational lensing. Though the MACHOs were not visible from Earth, when they passed in front of stars that were visible, the MACHOs distorted the light from those stars and made them shine brighter for a brief time.

Alcock calculated that the MACHOs his team had found must have had about half as much mass as our sun--or the mass of a white dwarf. He has been searching for a direct view of such an object since then, but the Berkeley team beat him to it.

"They've gotten in ahead of us. They've done a good job," said Alcock, a professor of physics and astronomy at the University of Pennsylvania. The new finding supports Alcock's current theory--that about 20% of the galaxy's dark matter is composed of MACHOs.

As for the rest? "There's a lot of stuff out there and we have no idea what it's made of," Alcock said.

The team snared the elusive quarry, in part, because it knew what color of star to search for. Astronomers had long assumed that white dwarfs turn red as they cool down. But a new theory suggests that the atmosphere of most white dwarfs would cause them to appear blue when they turn very cool.

With this in mind, the team started to scan old photographic plates that covered about 12% of the southern sky.

Team member Nigel C. Hambly of the University of Edinburgh picked out 92 suspicious objects.

In October, Oppenheimer, Hambly and colleague Andrew Digby observed the objects with a 4-meter telescope at the Cerro Tololo Interamerican Observatory in Chile. Looking for the characteristic blue spectrum of light, they found 38 of the cool white dwarfs.

"We went through the data very quickly," Oppenheimer said. "We knew after a few nights we had hit pay dirt."

Other teams, including those examining the Hubble Deep Field images, have preliminarily detected a handful of white dwarf stars in recent years. But Oppenheimer's team provided the first really large sample--enough so that the mass of dead stars can no longer be dismissed as an insignificant part of the galactic equation.

The dead stars are relatively close to us--within 450 light-years of Earth. Even more dead stars, the scientists suggest, could populate the spherical halo of old gas and stars that surrounds our Milky Way galaxy and extends for more than 100,000 light-years.

Depending on how thickly these ashen stars are strewn through the halo of our galaxy, they could make up as much as a third of the missing matter, or as little as 3%, Oppenheimer said.

Finding old, large dead stars in such large numbers also raises questions about how stars form. By current equations, many of the galaxy's older stars should be smaller ones. "It's surprising," said Roger Blandford, a theoretical astrophysicist at Caltech. "It's a puzzle for the stellar evolution of our galaxy."

And a puzzle for those seeking the rest of the dark matter. There's still no telling what the rest of the galactic halo is made of.

And the universe, which on average is far less dense than star-filled galaxies, is even stranger. Theories suggest that less than 5% of the universe is made of baryonic, or normal, matter--the type that makes up stars, planets and human beings.

Theories predict that most of the dark matter is made up of exotic particles that interact so weakly with matter that they slip past, or through, even the most sophisticated of detectors without a trace.

One type of particle, called a WIMP, for weakly interacting massive particle, is the subject of a heated international quest. The victors may well determine what makes up the largest component of our universe.

"The answer to that would be the start of a whole new field of science," said Leslie J. Rosenberg, a physicist at the Massachusetts Institute of Technology who is helping lead the search for yet another dark matter candidate, the axion.

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