Hubble Space Telescope


Researchers Claim First Proof That Stars Collide
By Jack Lucentini
Special to
posted: 07:00 am ET
01 June 2000

Scientists claimed Wednesday to have found hard evidence of stellar collisions, something researchers had sought for decades.

The announcement came at the outset of a conference entitled Stellar Collisions and Mergers, held at the American Museum of Natural History in New York. The three-day event is being held to kick off the department’s new astrophysics program.

Calculations and computer modeling have convinced physicists that stellar crashes are a much more common event than they believed a few years ago. None have been seen though.

This computer-generated image shows two stars about to collide.

Physicists said they had found the next best thing: the stars created as a result of these crashes. "Our results strongly suggest this is that Holy Grail," said Rex Saffer, an assistant professor of astronomy and astrophysics at Villanova University.
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Animation of Stellar Collision

"I’d call it 99-percent irrefutable," added Mike Shara, chairman of the museum’s astrophysics department, which participated in the study along with Saffer and the European Space Agency. The team claimed to have discovered five stars created from these great collisions deep within a blob of stars called a globular cluster.

Watch video animation of a stellar collision.

Globular clusters are areas where many stars, sometimes hundreds of thousands, crowd together. Many of the clusters lie around the sides of our galaxy like crumbs on a plate. And they are clogged with enough stars, physicists say, to make stellar crashes a common event.

Stars within a given cluster are all about the same age. They also don’t exceed a certain weight, because the most massive stars, which burn energy much faster, die off first. For this reason, the cluster observed by Saffer and his colleagues contain almost no stars heavier than 80 percent of our sun’s weight.

But a handful of stars in that cluster appear to be much heavier. Saffer reasons that these couldn’t exist if they were not a product of stars that collided and became a single star, their masses combined.

These stars, five in all, "are hot, blue, very massive stars that should not exist" -- barring stellar collisions, Saffer said. The group only came to this conclusion on Sunday, May 28. They plan on submitting their findings to the academic publication Astrophysical Journal.

One of these stars is three times heavier than the most massive ordinary stars in the cluster, said Saffer, meaning it must have stemmed from a three-star crash. This probably happened when one star flew into a binary star system, in which two stars orbit each other, he explained; the three stars would have done a complex dance for a while, then plunged together, sucked in by each other’s gravity.

The cluster in question is the nearest one to our solar system. Called NGC 6397, it is about one-fourth the distance between the center of our Milky Way Galaxy and us, near the fringes.

The results of the study will have to go through a rigorous peer-review process. However, other scientists at the conference didn’t give any serious immediate criticism of the work, at least not publicly.

There is little chance of a similar crash involving our sun, physicists said, because our cosmic neighborhood is less crowded. By one calculation, the sun is likely to have one crash per 10,000 trillion, trillion years (that’s 28 zeros), and it will burn out on its own accord much sooner than that.

As dramatic as the announcement of the new evidence for crashes was, scientists said it is probably the first of many much bigger finds to come in the next few years.

The collisions Saffer's group identified were between relatively light stars. Physicists think that far more violent crashes occur involving heavier stars, as well as neutron stars and black holes – both of which are types of super-compact dead stars.

Such collisions, scientists speculate, may account for as-yet unexplained gamma-ray bursts. These are blasts of energy at the furthest reaches of our universe, so bright that they outshine galaxies with billions of stars.

These explosions are nasty. If one occurred even as far as 100 light-years from Earth – meaning its light would take a century to reach us – it would still obliterate us.

New observatories to be built in the next few years will help explain whether these are stellar crashes by measuring gravitational waves, which these great collisions produce, said Vicky Kalogera, a research postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics.

"You can think of these as ripples in space-time that propagate at the speed of light" away from the crash, she said. "We’re just a few years away from very significant developments."

The biggest crashes, physicists said, would be between black holes. These are the densest objects known, which suck in everything around them including light. Scientists haven’t found such collisions either, but they’re looking.

"These types of black hole-black hole collisions, they’re simply too beautiful not to exist," Shara said.

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