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It was not until the 1930s that Caltech Astronomer Fritz Zwicky recognized supernovas (he coined the name) as a class of exploding star fundamentally different from ordinary novas. With Colleague Walter Baade, he began formulating the modern theory about how supernovas explode and launched the first systematic search for them. While the average galaxy has only an occasional supernova, Zwicky reasoned, there are so many distant galaxies visible through large telescopes, astronomers should have no trouble finding the great explosions popping out all over the universe. At first Zwicky's colleagues thought the idea ridiculous, but over the four decades that followed, he and his team found nearly 300 supernovas, about 30 times as many as appear in all of recorded history prior to 1885; the contributions of other astronomers have pushed the total to more than 600.
Armed with a growing number of examples, theorists refined their views of stellar evolution in general and of how, for some stars, an inevitable violent death occurs. The basic theme: a star performs a continual balancing act between its own immense gravity, which tries to pull all of its matter in toward the center, and the intense thermonuclear energy radiating from its core, which pushes the matter outward, keeping the star in the form of a distended ball of hot gases. For most of a star's lifetime, these forces are in equilibrium.
When the nuclear fuel is exhausted and the fusion reactions stop, however, gravity takes over. Without the outward pressure needed to keep it "inflated," the core of the star begins to collapse like a deflating balloon, its matter crushing down toward the center. For a star about the size of the sun, the collapse stops after several intermediate steps when the stellar material is compressed so much that its atoms virtually touch, forming what physicists call degenerate matter; what prevents further collapse is the tendency of the atoms' negatively charged electrons to repel one another. The star has become a white dwarf. Says David Branch, an astrophysicist at the University of Oklahoma: "It's the size of the earth but has the mass of the sun."
Degenerate matter is so resistant to further compression that nothing much can happen to a white dwarf unless, as is common in the Milky Way, it is part of a binary star system. If it is, the white dwarf's powerful gravity can draw gaseous matter away from its companion. In some cases, as the dwarf becomes bloated with its companion's substance, gravitational pressure triggers a fusion reaction in the captured gases, which are blown off in the explosion, resulting in a garden-variety (nonsuper) nova. According to Astrophysicist Branch, about 50 novas are observed flaring up each year in the Milky Way.