Maximum size for stars pinpointed, scientists announce

With Hubble's help, limit set at 150 times as large as the sun

March 10, 2005|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

Scientists say they've determined an upper limit on the size of stars in the universe - about 150 times the size of the sun.

Researchers at the Space Telescope Science Institute announced yesterday that they had pinpointed the size - something astronomers have speculated about for decades - by training the Hubble Space Telescope on the densest known cluster of stars in our galaxy and measuring the stars it contained.

Separately yesterday, Sen. Barbara A. Mikulski, Rep. Steny H. Hoyer and the American Astronomical Society issued statements calling for the National Aeronautics and Space Administration to extend Hubble's life span by funding an additional servicing mission. Otherwise, experts say, Hubble's instruments will probably begin to fail in the next two to five years.

The findings on the size of stars, being published today in the journal Nature, are expected to help astronomers understand how stars are formed, how they age and how they die.

"To really understand star formation and what's going on in the universe, we need to know as much as we can about the largest stars," said James Kaler, a professor of astronomy at the University of Illinois at Urbana-Champaign who was not a part of the study.

Kaler said that star size has been the focus of intense study by astronomers since the 1950s. But he said that determining star size has always been difficult because of a lack of direct observations.

"It's an area of study that's been a long haul. I'd say this is a really significant paper," Kaler said.

Previous studies have determined that the smallest stars are about one-twelfth the size of our sun. The sun is 300,000 times the size of the Earth.

For the study, researchers spent several years analyzing and measuring stars in the Arches cluster, a hotbed of star formation where huge clouds of gas collide to form some of the largest stars known.

Researchers measured about 1,000 stars and found they ranged in size from six times the size of the sun to 130 times its size. The findings are consistent with star measurements taken of nine other clusters in earlier studies.

"We strictly estimate 130 [times the size of the sun] to be the limit, but to be conservative we're saying the cutoff is 150," said Donald Figer, lead author of the paper and an associate astronomer at the space telescope institute in Baltimore.

He said the Arches cluster turned out to be an ideal study site because it's made up of stars about 2.5 million years old. Stars younger than that are still shrouded in dust, making them difficult to see. In clusters of stars older than 2.5 million years, many of the stars would have exploded or died off, researchers say.

"It was the only area where we could make these kinds of observations," Figer said.

The researchers used Hubble's near infrared camera and multi-object spectrometer to study about a thousand stars in the targeted cluster. At a news briefing yesterday, Figer and other experts said the Arches cluster, which lies 25,000 light-years away, was about the right distance to be observed.

Figer said the findings bolster support for launching a servicing mission to repair the Hubble Space Telescope. He said he would like to see Hubble's life extended as long as possible.

"As an astronomer I want to use the best telescopes in the world, and for me, that's Hubble," he said.

The universe's biggest stars - those that are 100 times the size of the sun - have been the focus of particular interest in recent years because of what happens when they die. Black holes and gamma ray bursts are believed to be created with the collapse of the most massive stars.

"Stars like that live short lives and go out with a big bang," said Stan Woosley, a professor of astronomy and astrophysics at the University of California at Santa Cruz.

Bigger stars are believed to die faster than smaller stars, Woosley said.

Stars 100 times the size of our sun may burn out in 3 million years, Woosley said.

The sun, which is estimated to be about 4.5 billion years old, is expected to have a life span of about 10.4 billion years, experts say.

Figer said the findings are derived from measurements of the amount of light detected from the stars and calculations based on their age and distance.

Figer and other experts said it's possible future observations will detect stars larger than 150 times the size of the sun.

Much of what is known about star formation and star size is based on theories and not direct observations, experts say.

"You're dealing with a theoretical realm where nobody really knows exactly what's going on," Kaler said.

A 1997 report, also based on Hubble images, estimated the size of a star near the center of our galaxy, known as the Pistol star, to be 200 times the size of the sun.

But Figer said that he and other experts are reviewing that estimate and that it may be reduced.

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