From California's high, peaceful Mt. Wilson Observatory last week Dr. Fritz Zwicky reported a tremendous celestial cataclysm which happened 3,000,000 years ago. This was a supernova, a star exploding with suicidal violence. So distant was it that long before the first creatures describable as human beings appeared, the light of the supernova's outburst began flashing toward Earth at 186,000 mi. per sec.
Astronomers have not decided what causes stars to explode. Something seems to go wrong with the delicate mechanism that keeps the outward force of radiation pressure balanced against the inward force of gravity. Then the star throws off rapidly expanding shells of hot gas, increases greatly in brightness. Ordinary novae attain a maximum brightness about 25,000 times that of the sun. Therefore, since the difference between actual brightness and apparent brightness depends on distance, novae have provided important clues to the distances of the nebulae in which they occur. About 130 novae have been recorded—most of them found by chance on photographic plates.
Supernovae are millions of times brighter than the sun—usually shedding more light than the millions of other stars in their nebula. About 15 have been recorded. Three years ago Dr. Zwicky, distinguished young Bulgarian-born astrophysicist who believes exploding stars may be a source of cosmic rays, brought the matter of supernovae to the attention of the National Academy of Sciences. He said then that supernovae probably cease to exist as ordinary stars; that protons and electrons coalesce on the surface into neutrons which, having no electric charges to repel one another, "rain" down toward the centre, pack sluggishly together, creating a heavy, lifeless "neutron star." With the possible exception of one 19th Century supernova, the supernova reported by Dr. Zwicky last week was the brightest ever studied by modern astronomers. It was ten times brighter than the average supernova, 100 times brighter than the whole island universe to which it belonged, 500,000,000 times brighter than the sun. Distant as it was, it reached a magnitude of 8.5, which is only two magnitudes below the limit of naked-eye visibility.