An invisible ring of superheated gas larger than anything else in our galaxy has been discovered circling the Northern Cross constellation 6,000 light years from Earth.

The halo of gas is so large that it extends 1,200 light years across space, a distance of 72 quadrillion miles. The temperature of the ring averages 3.5 million degrees Fahrenheit, enough superheated gas to create 10,000 new stars like our sun. Its output of energy at any given instant is 10 times as much as our sun has put forth since it was created almost five billion years ago.

"There is no known phenomenon in the galaxy that could be responsible for releasing that much energy," said the University of Colorado's Dr. Webster Cash, who together with the University of California's Dr. Philip Charles made the discovery using the orbiting High Energy Astronomical Observatory. "The only thing we can think of is a chain reaction of exploding stars, one going off every 50,000 or 100,000 years."

A satellite that was put into Earth orbit in August 1977 and burned up in the earth's atmosphere last March, the HEAO carried telescopes that observed x-rays instead of visible, infrared or ultraviolet light. In the 19 months it spent scanning the entire sky, the HEAO discovered a huge halo of X-rays extending around most of the constellation Northern Cross, which is the most prominent summer feature in the nightime skies of the Northern Hemisphere.

Inside the halo is Deneb, the bright northermost star of the Northern Cross and one third of the triple star formation astronomers call the Summer Triangle. The other two stars are Vega and Altair.

Despite its size, the halo is invisible from Earth and is mostly invisible to any telescope but an X-ray one. The halo is so hot that its light is as pale as the sun's corona, and is so surrounded by bright stars and the background light of the galaxy that its light is washed out by the time it reached Earth.

"Observers have scanned portions of this ring before with X-ray satellites," Cash said by telephone from San Francisco, where he presented his findings at a meeting of the American Astronomical Society, "but until we saw the whole ring with the HEAO satellite there was no chance to explain it."

Even though he was first to observe it in its entirety, Cash said he is hard pressed to explain it. He said he feels the explanation for the "superbubble" lies in the fact that it is near a huge, cool gas cloud known as the Great Rift of Cygnus that has been observed for hundreds of years.

"Out of the Rift come the exploding stars that created the bubble and keep it hot," Cash said. "The bubble may be no older than 10 million years and it may disappear with time, but the Rift may be a permanent feature of the universe."

He speculates that a star exploded between 3 million and 10 million years ago with such force that it blew up against the Rift, compressing the cool gas in the Rift to such densities that it formed as many as 1,000 new stars. Among the new stars were at least 10 supernova, which themselves blew up at later dates.

"We know the bubble is no older than 10 million years based on the physics of the bubble's expansion," Cash said. "We can also put a lower limit of 3 million years on its age based on its temperature, which is about 3.5 million degrees."

The halo is expanding at the rate of 30 kilometers (about 20 miles) per second and could go on expanding to three or four times its current size until it starts to cool and then collapse. Cash said the bubble will stay as hot as it is today as long as supernova in the neighboring Rift continue to explode and supply it with energy.

What does it all mean? "We think it's a major mechanism for star formation," Cash said. "A large fraction of the stars in the heavens could have formed this way, including our own sun."