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Astronomers have spotted the most distant known star in the Milky Way, and say the accidental discovery could help determine the total mass of the galaxy.

That includes mysterious invisible matter that astronomers know exists but are unable to identify, an astronomer with the Space Telescope Science Institute said this week."Astronomers suspect that there is a lot of additional material surrounding our galaxy that doesn't give off light," said Howard Bond, who along with two others discovered the distant star last spring.

"We don't know the mass of the Milky Way, but if we can find objects that are far out and measure their speed, it will help us determine the galaxy's total mass - the visible areas and the dark areas."

Astronomers use the speed of stars to determine gravitational pull upon them, and from its gravity, the mass of the galaxy.

Astronomers theorize the galaxy's dark matter could be dim stars that are too faint to be seen, black hole remnants of massive stars that exploded early in the galaxy's history or some unknown type of subatomic particle, Bond said.

"We used to think that the majority of the mass of the Milky Way was stars, but now we suspect stars may only be 10 percent of all the mass," he said. Once the mystery of the dark matter is solved, he said, "we may find something completely unexpected that could revolutionize our thinking about physics."

The scientists stumbled upon the star while studying a distant galaxy. The star is located 160,000 light years from Earth in the direction of the constellation Virgo.

The Milky Way was once thought to be only about 100,000 lights year across. Since the Earth is about 30,000 light years from the center of the Milky Way, the discovery means the galaxy is larger than even more recent estimates indicated.

"It was just dumb luck that this star was in the way of what they were looking at. It was totally serendipitous," said Ray Villard, a spokesman for the institute. "It just shows that there are a lot of wonderful surprises out there that we may stumble upon when we're looking for something else."

The star is an isolated member of an immense but thinly populated halo of old, faint stars surrounding the Milky Way. It was spotted while astronomers Robin Ciardullo and George Jacoby of the National Optical Astronomy Observatories in Tucson, Ariz., were surveying the giant elliptical galaxy M49, which is 50 million light-years from Earth.

Ciardullo, Jacoby and Bond have observed the star using telescopes in Hawaii and Arizona, and measurements of its brightness have been made at the Cerro Tololo Interamerican Observatory in Chile.

The star is a member of a well-known class of stars called the RR Lyrae variables, Bond said. Because the brightness of those stars already has been documented, astronomers can use the brightness of the new star, known only as the "Virgo variable," to measure its distance, he said.