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UA astronomer grapples with gamma rays

By Sarah Spivack
Arizona Daily Wildcat
September 29, 1998
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Leigh-Anne Brown
Arizona Daily Wildcat

John Cocke, associate astronomer for Steward Observatory, shows how to tell the size of a galaxy during his speech on gamma-ray bursters, gravitation theory and the nature of redshift. Cocke spoke as part of the astronomy department's weekly evening lecture series.

Some UA students probably associate gamma rays with the strange radiation that turned Dr. Bruce Banner into the Incredible Hulk.

But one University of Arizona scientist said the true gamma ray mystery is determining where in the universe the powerful bursts of energy originate.

John Cocke, a UA astronomy professor, spoke to a crowd of about 40 in the Steward Observatory last night about the discovery and study of gamma-ray bursts.

Cocke said satellites launched into the Earth's orbit in 1972 to look for gamma ray emissions produced by Russian nuclear weapons tests, instead found 16 mysterious bursts of photons scattered across the sky.

When the bursts were first sighted, a debate arose among astronomers regarding the source of those events: whether they originate in the Milky Way galaxy or elsewhere in the universe, he said.

In order to determine the origin of the energy bursts, scientists measured their distance from Earth using "redshift" data.

The universe is expanding. As galaxies, stars and other cosmological objects move away from Earth, light emanating from them appears red. That phenomenon is known as redshift.

By measuring to what degree astronomical objects appear red, physicists and astronomers can determine how far away they are.

"It's like the universe consists of a whole lot of little light bulbs and they're all a little red," Cocke said.

In 1997, a satellite helped scientists to find the position of some gamma-ray bursts. Using redshift information, it was determined that the bursts did not originate in the Milky Way, but were located all over the universe - some as far away as five billion light-years.

Scientists found that gamma-ray bursts can be viewed all over the sky, concluding that they emanate from outside the galaxy.

Once astronomers made the discovery, they developed math formulas to calculate the rays' points of origin.

Only the biggest and brightest objects can be seen from such great distances, Cocke said.

Energy emitted with the bursts is usually equal to or greater than the amount of energy put out by the sun over the course of its 10 billion-year lifetime, he said.

Astronomers have developed several theories to explain what could cause such immense bursts of energy, Cocke said.

Some researchers think gamma rays are emitted when neutron stars collide or are sucked up by massive black holes, he said.

Neutron stars are very dense balls of energy - they can be as small as 10 miles across while being as heavy as the sun, Cocke said.

"The merging process (when two neutron stars collide) exposes the cores of these stars and masses of energy pour out," Cocke said. "A black hole will eat a neutron star and in the process of being eaten, the extremely high energy core will be exposed."

Cocke described the star-collision theory as "exotic astronomy" but said it explains how such enormous bursts of gamma-ray photons could be produced.

Several gamma-ray bursts can be observed from Earth each day. Cocke estimated that one burst originates from the Milky Way every 10 to 100 million years.

If stars collided on the other side of the galaxy, the resulting burst of energy would probably not affect Earth.

But if a burst of that immensity happens too near to Earth, "we are toast," Cocke said.

Sarah Spivack can be reached via e-mail at Sarah.Spivack@wildcat.arizona.edu.