The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

In the 1980s, however, the clear distinction between Type I and Type II began to blur. Spectroscopic analysis of three supernovae—one each in 1983, 1984, and 1985—showed that they consisted of huge amounts of calcium and oxygen, consistent with the interiors of massive stars that end their lives as Type II supernovae, but no hydrogen, consistent with white dwarfs that end their lives as Type I supernovae. Some astronomers, including Kirshner, suggested that they were seeing a third type of supernova, essentially a hybrid of the other two. It was the product of a core collapse that had already lost its outer shell: a hydrogen-free implosion. They added this specimen to the Type I column, calling it Type Ib. The old Type I, a thermonuclear explosion with no hydrogen, was now Type Ia. In 1991, even that classification—Type Ia—began to blur. On April 13, five amateur observers in four locations around the world discovered a supernova designated 1991T.* On December 9, an amateur astronomer in Japan discovered a supernova designated 1991bg. Follow-up spectroscopic observations by professional astronomers—including Kirshner, on April 16, for 1991T—showed that they were both Type Ia supernovae. But their luminosities differed widely. Supernova 1991T was much brighter than the usual Type Ia at its particular distance, and 1991bg was much dimmer than the usual Type Ia at its particular distance. Astronomers could rule out the possibility that they were simply miscalculating distance...