Death of a Star

When Caltech Astronomer Charles Kowal first examined the photographic plate that he had exposed atop Mount Palomar last month, he was openly skeptical. At the edge of the small, irregular galaxy that he was studying in the constellation Centaurus, he saw a large burst of light brighter than the entire galaxy itself that had not been there before. Had a stray asteroid wandered into the telescope's field of view? Closer inspection quickly revealed that the light came not from a nearby asteroid, but from a far more awesome heavenly phenomenon: a supernova, the explosive death of a giant star.

No supernova has been seen in the earth's own Milky Way galaxy for 368 years. But astronomers constantly search for, and frequently observe similar stellar explosions in the universe's myriad other galaxies, or islands of stars. Since the 1930s, for example, astronomers at California's Hale Observatories have photographed some 200 extragalactic supernovae. What makes Kowal's supernova significant to astronomers is that it occurred in a relatively nearby galaxy--only 10 million light-years away. * It is the brightest exploding star sighted in 35 years. Moreover, it seems to have been spotted only days after it reached maximum intensity, or at the height of the star's death throes. Thus astronomers have a rare opportunity to study at close range the mechanisms of stellar death.

According to theory, a supernova occurs after a giantstar-- substantially more massive than the sun--has exhausted its thermonuclear fuel. The star's distended gases begin to collapse toward its center of gravity, crush together and reheat to incredible temperatures of 100 billion degrees, and then explode in a fiery outburst as bright as a billion suns. Left at the center of the supernova is a tiny (about ten miles across) star consisting of tightly packed neutrons, or a smaller "black hole"--a star so dense that its tremendous gravity prevents even light from escaping. The 1967 discovery of pulsars, since identified as neutron stars, seemed to support this explanation of how stars die. Now, observations of Kowal's supernova may help to confirm it.

* Which means that the explosion actually took place 10 million years ago, or the time it took light from the flare-up to reach earth.

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