Scientists discover source of gamma ray bursts

CHICAGO — The cause of split-second explosions that release 100 thousand trillion times more energy than the sun, and are second in enormity only to the Big Bang itself, has been pinned down by an international team of hundreds of scientists.

Using three star-gazing satellites and four ground-based telescopes, the scientists concluded that the brief but powerful explosions are caused either when two neutron stars collide or when a black hole gobbles up a neutron star, Neil Gehrels of NASA's Goddard Space Flight Center in Greenbelt, Md., said at a Washington news briefing.

Called gamma ray bursts, the explosions have been one of astronomy's biggest mysteries ever since they were first detected 35 years ago during the Cold War. Some scientists thought at that time that the bursts might be caused by new space weapons from the Soviet Union, but they quickly realized they originated far away in the universe.

The new findings, which appear in Thursday's issue of the British journal Nature, involve gamma ray bursts captured on May 9 and July 9, including images of a neutron star being pulled apart as it was sucked into a black hole.

"It really opens a completely new window on the universe," said University of Chicago astrophysicist Don Lamb, who played a key role in analyzing the data. "It tells us that we're going to be seeing gravitational waves in the near future, in addition to knowing how stars die and how the material that we are made of and the earth is made of came to be and how life came to be possible."

Albert Einstein predicted that stupendous explosions in the universe would produce gravitational waves. Scientists hope to detect them because they may lead to a better understanding of how gravity fits in with the other three forces of nature — electromagnetic, strong force and weak force.

A neutron star is an old star that has burned off most of its fuel and collapsed under gravity to about 10 miles in diameter with the mass of 1 1/2 suns. The star's atoms are squeezed together so tightly that their protons and electrons merge to form neutrons. A thimbleful weighs as much as Mount Everest.

Most stars exist as pairs orbiting each other. When they become neutron stars their rotational speed picks up, reaching half the speed of light when they collide. Light travels at 186,000 miles per second.

Such collisions produce a jet of energy that is short, lasting milliseconds, but travels at more than 99 percent of the speed of light. The glow it produces from ramming through surrounding gas outshines its host galaxy by 10 million billion times, Lamb said.

Two years ago astronomers discovered the cause of longer gamma ray bursts typically lasting 20 to 50 seconds: old stars that explode and become black holes if they are big enough, neutron stars if they are medium sized and white dwarfs if they are small. Black holes are also formed when neutron stars collide.

By studying the collision of neutron stars, scientists may get their first chance to see how black holes are born, said Edward Kolb, director of Fermilab's Particle Astrophysics Center.

Short and long gamma ray bursts occur all the time in the universe, and the violent explosions could doom any nearby planets. In our Milky Way galaxy such bursts occur about every 300,000 years, astronomers estimate. A burst occurring close enough to affect the Earth is estimated to occur once in a billion years.

The two gamma ray bursts reported in Nature occurred in distant galaxies. The burst whose visible glow was captured by the Hubble orbiting space telescope on July 9 occurred one to two billion years ago. It took that long for the energy to arrive, traveling at the speed of light.