What scientists believe was a thermonuclear explosion on the surface of a star was photographed two months ago by an American satellite named the High Energy Astronomy Observatory-2.

"We not only saw it, we also located it," said Dr. Jonathan Grindlay of the Harvard-Smithsonian Center for Astrophysics from Cerro Tololo in Chile, where he is observing the southern skies. "This is the first time we have been able to photograph what astronomers call an X-ray burster and identify the star that is bursting with X-rays."

Grindlay said the X-ray telescope aboard the HEAO satellite observed for 50 seconds what most scientists think was the equivalent of a large hydrogen bomb explosion on the surface of a small star centered in a cluster known as Terzan 2.

He said the explosion released more X-ray energy in 50 seconds than our sun does in five weeks. He said he did not understand why the explosion lasted so long except that it suggests the flash originated deep within the star and took longer to consume the star's accumulated nuclear fuel.

"Most X-ray bursters last only 10 seconds," Grindlay said, "and what I'm really saying about this one is that we don't understand the length of the burst."

While most scientists believe X-ray bursters are hydrogen explosions on stellar surfaces, Grindlay said a few scientists think they're the result of explosions in the flow of hydrogen and helium gas to an invisible black hole that's sucking energy from a companion star.

The X-ray burster was located by Grindlay almost at the center of Terzan 2, where a black hole would most likely be found. Theory predicts that black holes would sink to the center of any star cluster since they would be the heaviest objects in the cluster.

A black hole is a collapsed star so dense that its gravity allows no light to escape, pulling its own light right back into itself before it can escape to space.

The 50-second explosion observed by Grindlay came during a 20-minute observation of the star cluster itself. The sixth X-ray burster to be detected in the heavens, this one lies between 15,000 and 20,000 light years from earth near the center of the Milky Way galaxy.