TLANTA, Jan. 5 — Astronomers have spotted what they believe is the biggest, brightest star they have ever seen — and indeed bigger than can be explained by the usual theories of how stars form.

The star, LBV 1806-20, is 5 million to 40 million times as bright as the Sun, at least 150 times as massive and at least 200 times as wide, astronomers reported on Monday at a meeting of the American Astronomical Society here.

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Next to LBV 1806-20, the Sun would appear just as minuscule as Mercury does next to the Sun.

Dr. Steven Eikenberry, a professor of astronomy at the University of Florida, who led the research, said it was surprising that even after decades of observing the Milky Way, "we're still finding these big monsters out there."

The findings have been submitted to The Astrophysical Journal.

The brightest star known until now is one called the Pistol Star, five million to six million times as bright as the Sun. The brightness of a star, or its power output, is proportional to the cube of its mass; a star 10 times the mass of the Sun would be 1,000 times as bright, and a star 100 times the mass would be one million times as bright.

LBV 1806-20, despite its brightness, is not easily seen. Forty-five thousand light-years away on the other side of the Milky Way galaxy, it is blocked from view by dust clouds. But about 10 percent of its infrared light does make it to Earth.

When LBV 1806-20 was discovered in the 1990's, astronomers categorized it as a bright, short-lived blue star and estimated that it was at least a million times the mass of the Sun. But with new observations from the Palomar Observatory in California and the Cerro Tololo Inter-American Observatory in Chile, astronomers produced better images and new estimates of mass and brightness. Because the star is so bright, it will burn out quickly, within a few million years, compared with the 10 billion-year lifespan estimated for the Sun.

Dr. Eikenberry said the high-resolution images ruled out the possibility that fooled astronomers previously into erroneous claims: that the supposed supermassive star was actually a cluster of smaller ones. Still, he said it might be a binary or triple-star system.

If it is a single star, it probably did not form the usual way: a cloud of hydrogen gas collapsing under gravity, heating up and turning on when the hydrogen fuses into helium. But calculations indicate that stars should be at most 120 times as massive as the Sun, because heat and radiation from the collapsing cloud would blow away any more gas.

A clue to LBV 1806-20's history can be found in its neighbors, Dr. Eikenberry said. Nearby are other stars, which, while not quite reaching LBV 1806-20's girth, are still very large; there are also a proto-star and a neutron star, the burned-out core of a star that exploded in a supernova.

"Nothing like this has ever been seen before," Dr. Eikenberry said.

Perhaps when the star exploded, the shock wave caused nearby gas clouds to collapse and form the large stars, including LBV 1806-20, he said. When these stars exhaust their hydrogen fuel in a few million years, they, too, will explode in a succession of supernovas, he went on, "like a fireworks display."

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