Biggest, Brightest Star
Yet Puzzles Astronomers

By Tariq Malik
Staff Writer -

ATLANTA -- A team of researchers has found what appears to be the most luminous known star around, one so massive that it shouldnt have formed in the first place.
The star, known as LBV 1806-20, tips the scales of stellar masses at about 150 times the heft of the Sun. It shines up to 40 million times brighter than the Sun. The previous title-holder called the Pistol Star, is a mere six million times brighter than the Sun and weighs about 100 solar masses.
LBV 1806-20 was known before, but just as a bright blue object in high-powered telescopes. Now it has been examined more closely. Even if it proves to be a binary or triple-star system, and therefore all the mass is not its own, it would still be behemoth, astronomers said here yesterday at a meeting of the American Astronomical Society.
"Its definitely not a cluster of stars, we can rule that out completely," explained Stephen Eikenberry, who led a team of researchers that studied the star. "It could be part of a binary or triple system, though it seems unlikely."
Light from LBV 1806-20 undergoes periodic variations that seem specific to a one-body object, Eikenberry said. If it is eventually found to be a binary or triple-star system, it would be even more confusing, since astronomers would have to explain how these massive stars manage to exist so close together, he said.
Doesn't make sense
Current theories of star formation fail to explain the existence of big, bright LBV 1806-20, since it should have destroyed itself before it ever ignited. Astronomers long believed that as young stars grew to 120 solar masses, their energy output would burn off any excess. That is, the heat and pressure within the still-forming star would be so great, it would shear off any additional material from the stars surface.
Eikenberry is a professor of astronomy at the University of Florida.
One possible answer to the mystery of LBV 1806-20s continued existence could be the neighborhood beyond the star's immediate surroundings. The star resides in a cluster populated by extremely rare star or unusual stars, including a intensely powerful magnetic neutron star and a massive protostar, one yet to be born.
"So its part of a giant cluster of freakishly massive stars," Eikenberry said.
LBV-1806-20 may have formed in what Eikenberry called "violent, triggered star formation." In the process, a huge, massive star reaches the end of its lifespan and explodes in an intense supernova. The shockwave from that supernova then hits a young star just as its forming, compressing gas around it quickly -- over a period of 100,000 years or so -- at forces greater than the star is able to blow off on its own.
Twinkle, twinkle massive star
Despite LBV 1806-20s luminosity, it is all but hidden to researchers on the ground. From Earth, it has a magnitude of 8.4, beyond the limit of about 6.5 on an astronomer's scale in which larger numbers denote dimmer objects. It's relatively dim terrestrial eyes because it sits about 45,000 light-years away, on the other side of the Milky Way, and is almost completely obstructed by interstellar gas and dust. One light year is the distance light travels in one year, or about six trillion miles (9.7 trillion kilometers).
"Almost 90 percent of its light, visible and infrared, is absorbed by the interstellar medium," Eikenberry said.
But that doesnt mean the star is invisible.
Eikenberry and his team were able to sharpen infrared images of LBV 1806-20 taken by the eight-foot (2.5 meter) telescope at the Palomar Observatory in California using a camera equipped with "speckle imaging," technology designed to mitigate the interfering effect of the Earths atmosphere on star observations. The team also used data collected with the four-meter Blanco telescope at the National Optical Astronomy Observatorys Cerro Tololo Inter-American Observatory in Chile.
At its dimmest, LBV 1806-20 would be just as bright as its closest competitor, Pistol Star, with a temperature range of between 18,000 to 36,000 degrees Kelvin. Because it burns so bright, the star should have only a limited lifespan. Bright, massive stars tend to burn themselves out rather fast.
"These types of stars only run a short, violent, time, about two million years," Eikenberry said, noting that Earths Sun is about five billion years old. "They tend to erupt and do really bad things to themselves before they blow themselves to bits."
Stars like LBV 1806-20, which Eikenberry estimates is only middle-aged at about one million years old, can shed huge amounts of material in a wind similar to solar wind of the Sun. He said the huge star could end in a hypernova explosion with a powerful burst of gamma rays.
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