Astronomers using NASA's Swift satellite said yesterday that they have detected the most distant explosion ever discovered in the universe, the collapse into a black hole of a massive star about 12.6 billion light-years from Earth.

The event occurred about 1.1 billion years after the big bang, the explosion that created the universe an estimated 13.7 billion years ago, the scientists calculated. The only more distant objects ever detected are a quasar and a single galaxy, both about 12.7 billion light-years away.

"This is what we've all been hoping and waiting for," said University of Chicago astrophysicist Donald Lamb. "This breaks the record for most distant explosion by a huge amount [500 million light-years], and I don't think we'll have to wait long to break it again."

The massive blast, known as a gamma ray burst, lasted 200 seconds and was detected Sept. 4 by the Earth-orbiting Swift, which relayed the sighting to astronomers who used ground-based telescopes to observe the burst and its afterglow. Because the explosion was so distant, light from it is only now reaching Earth.

Gamma ray bursts, lasting from a fraction of a second to several minutes, occur frequently and are the brightest and most powerful events in the universe since the big bang itself. Swift has detected 71 bursts since its November 2004 launch.

Gamma ray bursts are brighter than galaxies or even quasars, distant, bright objects that scientists theorize are massive black holes that project energy by devouring stars. And with Swift, the bursts are much easier to detect than anything at comparable distances.

For this reason they may offer the best tool yet for studying the dawn of the universe, when stars were first created and then collapsed, sending bursts of matter on long journeys to form all the objects in the heavens.

University of North Carolina astronomer Daniel Reichart, who led the team that tracked the explosion, said Swift relayed news of the burst about 10 p.m. Eastern time Sept. 4. Working with undergraduate Josh Haislip, Reichart called on the Southern Observatory for Astrophysical Research in Cerro Pachon, Chile, to examine the event.

"Swift detects and localizes and sends the information instantaneously to everybody," Reichart said in a telephone news conference. "Every time a burst goes off, we check the resources we have available."

Scientists say gamma ray bursts most likely occur when a star runs out of hydrogen fuel and starts to burn heavier elements produced by nuclear fusion. Eventually the star is left with only iron, which will not burn. The star collapses and, if it is big enough, creates a black hole with gravity so intense that nothing can escape from it. The event is accompanied by a spectacular gamma ray explosion.

Reichart's team, working with observatories and astronomers around the world, quickly established that the burst could not be seen in visible light but only in the infrared -- an indication of great distance. Within three days, astronomers had fixed the explosion as the most distant ever recorded.

Swift lead scientist Neil Gehrels, of NASA's Goddard Space Flight Center said the satellite has revolutionized the science of spotting bursts. It is more sensitive than Earth-based instruments, and its software enables it to announce discoveries almost instantly. He predicted that Swift will see bursts "much further back than this."

If so, Lamb said, astronomers will approach "the moment when the stars formed 600 million years after the big bang." Those early stars eventually blew up, beginning a cycle of destruction and creation. "Over time the stars built up again and formed everything," he added. "The bursts can tell us the complete history from when there were no heavy elements to the present time when life can form."