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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 in Baltimore said 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.

The findings on the size of stars, published 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, Champagne-Urbana 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 past 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.

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 already 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. 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 span extended for 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. Both black holes and gamma ray bursts are believed to be created with the collapse of the most massive of 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, Santa Cruz.