WASHINGTON (Reuters) - A gigantic explosion of a star halfway across the universe long ago aimed a burst of gamma rays directly at Earth, an international team of scientists said on Wednesday.
For 40 seconds on March 19, the gamma ray burst could be seen by the naked eye on Earth, they said. It became the most distant object known to have been visible from this planet without the use of a telescope, they said.
Gamma rays, the highest energy form of light, are produced by events that generate enormous amounts of energy such as star explosions and nuclear bomb detonations.
Gamma ray bursts, the universe's most luminous explosions, occur when massive stars, perhaps 20 to 30 times the mass of the sun, burn out their nuclear fuel. As a star's core collapses, it creates a black hole that drives powerful gas jets outward.
The dying star's explosion occurred 7.5 billion light-years away and 7.5 billion years ago. A light-year is 6 trillion miles, the distance light travels in a year.
The burst allowed scientists to look back in time to when the universe was about 6 billion years old, less than half as old as it is today and before the sun and Earth were formed.
If a similar gamma ray burst occurred within our own Milky Way galaxy a few thousand light-years away instead of billions of light years, it might have disrupted Earth's atmosphere and caused a "nuclear winter" effect that could have threatened life on our planet, the researchers said.
"We were not in danger," Paul O'Brien of the University of Leicester in Britain, one of the scientists, said by e-mail. "Although it is extremely powerful, only a tiny fraction of the radiation reached the Earth."
In findings published in the journal Nature, the scientists described data on the gamma ray burst collected by satellites and observatories around world. NASA's Swift satellite and other instruments detected the explosion and determined that it originated in the constellation Bootes.
"I think the thing that's really astonishing here is just the possibility that one could go out into a dark location and look up at the sky and, for a few seconds, see light that was coming from 7 1/2 billion years ago," David Burrows of Penn State University and the Swift X-ray Telescope team said in a telephone interview.
Editing by Maggie Fox