Stars that explode repeatedly, creating what stargazers call a nova, but burn normally between explosions may be 10 times more common in the Milky Way than most astronomers have thought.

This new estimate by University of Illinois astronomers emerged from discovery in its normal phase of an extremely faint star that last flared into a prominent nova more than 300 years ago.

On the night of June 20, 1670, a Carthusian monk in Dijon, France, recorded the sudden appearance of a third-magnitude star near the beak of the constellation of the swan, a star "not to be met with in any catalogue of astronomers." The new, or nova, star burned brightly for about two years and then faded from view.

For three centuries, astronomers searched in vain for any remnant of the nova. Then, using highly sensitive, electronically aided telescopes, Illinois astronomer Ronald F. Webbink found the dim star, shining one forty-millionth as brightly as in 1670.

Because it remains so dim, Webbink concluded that the star must be recycling for another explosion at a rate only about one-tenth as rapid as current theory assumes. Astronomers had thought that nova stars exploded about every 1,000 to 10,000 years. If the 1670 nova is typical, they may actually take 100,000 years.

Because about 30 novae are seen in the Milky Way each year, Webbink suggested that there must be about 10 times as many to produce the observed rate of explosions.

Novae are thought to develop in binary star systems in which one star steadily draws matter from its companion until it accumulates a critical mass that causes a sudden flare-up, exploding like a colossal hydrogen bomb. Then the nova dies down and begins reaccumulating matter.

Supernovae are entirely different. They are dying stars that flare in one last burst of brilliance before collapsing into a relatively cold cinder or, if they have enough mass, into a black hole.