A type Ia supernova is perhaps the ultimate combination of insult and injury—a star steals material from a companion star, reaches critical mass, becomes unstable, and then unleashes a nuclear blast powerful enough to pulverize its victim.
The culprit in these cases is clear: type Ia supernovae arise from the cataclysmic explosions of small, dense stars known as white dwarfs. But the victim's identity is clouded. Traditionally, scientists believed that the victims were sunlike main-sequence stars or swollen giant stars. Recent studies have pointed to a major role for a lesser-known mechanism—pairings of two white dwarfs in which one cannibalizes its orbital companion before exploding as a supernova.
A study in the September 27 Nature bolsters the latter argument, concluding that only a small minority of type Ia supernovae stem from main-sequence or giant stars. (Scientific American is part of Nature Publishing Group.) Jonay González Hernández of the Astrophysics Institute of the Canary Islands and his colleagues looked for remnants of a victim star that precipitated the type Ia supernovae seen from Earth in the year 1006. They came up empty. The lack of a surviving companion seems to rule out any large star as a partner because the core of such a star should have weathered the blast and should remain visible today. But a white dwarf would have left no trace. In conjunction with other, mostly fruitless searches for supernova survivors, the researchers estimate that fewer than 20 percent of type Ia supernovae originate from the classically assumed scenario.
Astronomer Andrew Howell of Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., calls the 20 percent figure a “vast overstatement.” He notes that a normal star somewhat smaller than the sun also would not leave any detectable traces and would fit the bill for the companion to supernova 1006.