Astronomers have discovered a new kind of supernova, the cataclysmic explosion that marks the death of certain types of stars. While the better-known forms of supernovae shine with the brilliance of an entire galaxy for a few months, the newly found form is dim.

The finding is forcing astronomers to realize that some stars die as the result of processes other than those already supposed.

The best-known form of supernova occurs when a large star has burned most of its hydrogen in a fusion reaction that welds light hydrogen atoms to heavier atoms. These, in turn, are fused into heavier atoms, such as oxygen, sodium, calcium and, finally, iron. Iron does not combine readily to make heavier atoms, so as the amount of iron grows in a star's core, its thermonuclear reaction slows.

As a result, there is less energy pushing outward; the accumulating iron eventually outweighs the outward push and the star's core collapses on itself. In less than a second, the remaining mass slams toward the star's center, producing a massive shock wave that rebounds, pushing outward. The energy of the rebound triggers one final paroxysm of fusion, burning the hydrogen that remains in the star's outer shell. This is the supernova. For a few months it burns brightly, fading as the remaining unburnable mass cools and shrinks into a spinning dark cinder, called a pulsar. In even more massive stars, the collapse may continue indefinitely, producing a black hole.

There is a second previously known type of supernova in which a smaller star pulls in enough mass in the form of gas from a companion star to trigger the collapse.

The newly discovered supernova appears to be of a third type in which the rebound from the collapse causes no final flare-up. The reason seems to be that the star has somehow lost its outer shell of hydrogen before the collapse, leaving nothing to be burned by the energy of the rebound.

The new supernova was discovered by Wallace L.W. Sargent of the California Institute of Technology and Alexei V. Filippenko of the University of California at Berkeley. They found it while examining a distant galaxy in the course of an entirely different study with the 200-inch telescope at Mt. Palomar.

An analysis of light from the supernova lacked the usual spectra that would indicate the light was coming from burning hydrogen. Instead, the light, though dim, was being emitted by atoms of oxygen, magnesium, sodium and calcium -- atoms that are normally deep inside the star's core.

"We just jumped out of our chairs when we saw this," Filippenko said. "It was the craziest thing we had ever seen. This supernova had no hydrogen around it, and we don't know where it went."