Supernova with a split personality.

When it was discovered by amateur astronomers on March 28, the supernova known as SN 1993J ranked as the brightest seen in the northern sky in 56 years (SN: 4/17/93, p. 246). But the luminosity of this exploded star isn't the only reason it has captured the attention of researchers. It may represent the suspected link between two kinds of supernovas.

Supernovas are thought to come in two varieties. Type I supernovas lack hydrogen and are believed to arise when a small, compact star, called a white dwarf, steals so much matter from a companion that it explodes. Type II supernovas emit lots of hydrogen and are thought to form when the core of a supermassive star collapses, generating a shock wave that triggers an explosion.

Researchers at first found that the bulk of emission from SN 1993J came from the glow of hydrogen, making this exploded star a type lI supernova. But in May, Alexei V. Filippenko and Thomas D. Matheson of the University of California, Berkeley, found that SN 1993J had undergone a remarkable transformation. The hydrogen emission had been replaced by helium, a characteristic of a type I supernova known as Ib.

According to the Berkeley astronomers, these observations suggest that the star that became SN 1993J had only a thin outer layer of hydrogen; the rest was stolen by an unidentified companion. Soon after the explosion, the hydrogen layer would still be dense enough to be visible; later on, helium emissions could easily penetrate the thinning hydrogen envelope. This scenario suggests that type Ib actually has more in common with type 11 supernovas than with type I. If Filippenko and Matheson are right, both type Ib and type Il supernovas result from the collapse of supermassive stars.