A pair of stars far from earth is exchanging clouds of gas so hot they are radiating X-rays 10,000 times stronger than all the combined radiation pouring off the sun.

The stars are located in the constellation Ophiuchus adjoining Sagittarius in the northern skies of earth and are so small and distant they are barely visible through even the most powerful optical telescopes on earth. So energetic are their Xrays, they stand out as the second or third brightest X-ray objects in the heaven.

They are generating temperatures we estimate to be about 5 million degrees," said Dr. Herbert Friedman of the Naval Research Laboratory, which together with the Harvard-Smithsonian Observatory discovered the stars. "They are bright in the X-ray region as the Crab Nebula, which is the second brightest star (after Scorpius) X-ray object in the sky."

The star were found by instruments aboard the High Energy Astronomical Observatory, which was put into the earth orbit by the National Aeronautics and Space Administration five weeks ago. The stars were tracked by the spacecraft for 10 days and then lost when they moved out of the spacecraft's field of view.

"The stars grew in X-ray brightness until they were at their brightest at the end of the 10 days," Friedman said. "We believe that we've seen a very bright X-ray nova, only the second one of its kind recorded."

A noca is a celestial object that suddenly and dramatically increases in brightness, usually because its outer shell explodes. Astronomers think an X-ray nova takes place when a super-dense star - called a neutron star - passes close enough to a normal star to draw in the normal star's gas and trigger a continous explosion of the gas.

Friedman said he believes the companion stars in Ophiuchus move around each other in a highly elliptical orbit, meaning they only trigger an explosion when they brush by one another at their closest points of their orbit.

"It could be that these stars are only that close every once a year or once every 100 years," Friedman said. "We may have been lucky enough just to catch these stars when they were in contact."

Astronomers have no idea how far away the stars are, what their ages are and how often they have been coming together to trigger explosions. The stars are now being tracked by a new 154-inch optical telescope in Australia and by the British Ariel satellite, which has an X-ray instrument to follow it through the sky.