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Space

Hubble sees stars in red, white and blue

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July 6, 1999
Web posted at: 3:00 p.m. EDT (1900 GMT)

BALTIMORE, Maryland (CNN) -- More than a swarm of stars, the cluster known as M80 is a galactic demolition derby whose colorful pieces could shed light on some deep, dark stellar mysteries.

Located about 28,000 light years from Earth, M80 contains hundreds of thousands of stars, making it one of the densest of the 147 known globular clusters in the galaxy, according to NASA scientists. NASA released a composite image of the cluster last week.

Such clusters offer scientists a unique opportunity to study stellar evolution. While stars in each cluster have the same age, they have a wide range of masses.

The stars in M80 are about 15 billion years old. The stars visible in the image, pieced from snapshots taken by the Hubble Space Telescope, are either more evolved or more massive than the sun. Some of the particularly bright ones are red giants, which put on spectacular displays near the end of their lives.

The Hubble Heritage Team, based in Baltimore, Maryland, created the composite image for the public, drawing from the extensive Hubble archives.

"Some of the images may not be complete," Hubble spokeswoman Cheryl Gundy said. "We put them together so that they may be artistically pleasing."

The group may often have an eye on aesthetic values, but "this one was picked because it's one of the densest clusters, so it's scientifically interesting as well," she said.

Besides providing scientists with a wealth of information, the Hubble snapshots have stirred up an abundance of questions.

Scientist had previously thought that the stellar swarm had no "blue stragglers," unusually young and massive blue stars. But NASA astronomers determined that M80 boasts more than twice as many as any other globular cluster surveyed by Hubble.

How can a cluster with same-age stars have some much younger than others? Collisions can take place in the core, resulting in the merger of two stars into an unusually massive one that mimics a normal young star. Based on the number of blue stragglers in the heart of M80, many collisions seem to have taken place.

But another observation clouds the issue. While some Hubble astronomers studied the blue stragglers, a second team looked at M80 because a rare nova explosion took place there in 1860.

Nova outbursts take place when a companion star transfers fresh hydrogen to a white dwarf. Eventually the light gas ignites a thermonuclear explosion on the burned-out dwarf, giving rise to a nova.

Images taken through an ultraviolet filter revealed the cinder of the exploding star, named T Scorpii in the 19th century. But curiously, only two other nova-like binary stars were detected, far fewer than thought based on the rate of stellar collisions.

Blue straggler stars suggest many stellar fender benders in M80, but white-dwarf novas only a few. Scientists are trying to unravel the mystery.

"The formation mechanism for blue stragglers is not 100 percent," said Mike Shara, curator in astrophysics at the American Museum of Natural History in New York.

Direct stellar collision and the merger of binary stars are two common explanations for how stars join.

But inside cluster cores, where gravitational forces are intense, they can combine after one star captures another.

One possible theory is that mergers create the blue stragglers, and captures lead to the novas, Shara said.


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RELATED SITES:
American Museum of Natural History
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