Exploding Stars May Be New Type of Supernova

There's more than one way to blow up a star—in fact, scientists know of two. But could there be a third? A team of astronomers has identified two supernovae that don't seem to fit into established categories, though another team claims that there's not much new about them. If the first team is right, it could solve a longstanding mystery about the origins of one of the basic elements needed for life.

The first type of supernova occurs when one star in a binary pair explodes. A white dwarf star, roughly the mass of our sun, keeps snatching excess gas from its close-orbiting stellar companion. At a certain point, the extra gas renders the consuming star unstable, like a balloon filling with too much air, and the accumulated gas triggers a thermonuclear explosion so powerful that the star blows itself completely out of existence. This class of supernovae include what are called Type Ia, the brightness and duration of which are so precise that astronomers use them to gauge distances to other galaxies and to track the rate of acceleration of the universe.

The second general variety occurs when a young, giant star greater than about 10 solar masses collapses under its own weight. The gravitational crunching of the star's core releases a tremendous amount of energy, propelling a good deal of the star's mass out into space as a blinding explosion and leaving behind a massive remnant, either a neutron star or a black hole. Such stars go out like candles in the wind, by stellar standards. Instead of burning for billions of years, like our sun, these giants can go supernova within 30 million years.

But there may be other supernovae that don't quite fit into these two tidy categories. Take the case of a pair of misfit supernovae called SN 2005cz and SN 2005E, discovered 5 years ago. Each is located in a type of shaped galaxy where star-making has virtually ceased—the first in NGC 4589, about 80 million light-years from Earth, and the second is in NGC 1032, about 100 million light-years away. In both cases, astronomers thought at first that they were seeing the explosive collapse of giant stars. But the light was too faint and faded much too quickly to fit the core-collapse category. Adding to the mystery, their host galaxies are very old and contain few new stars. So how could a young, giant star blow up in such a place?

In two papers in tomorrow's issue of Nature, astronomers tackle possible explanations. In the first, which focuses on SN 2005cz, lead author Koji Kawabata of Hiroshima University in Japan and colleagues decided to keep analyzing the explosion's fading light. After 6 months of observations, they accumulated the telltale sign: a strong spectral signature of calcium, an element associated only with core-collapse supernovae. Their studies also showed that, despite NGC 4589's dearth of new stars, the region where SN 2005cz was located showed signs of recent star-making activity. The upshot, says Kawabata, is that "the findings can be explained by predictions from standard [core-collapse] theory."

Case closed? Not quite. In the second paper, lead author Hagai Perets of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues argue that the supernovae represent a variation on the binary pair theory. But in these two cases, both supernovae were generated by a low-mass, helium-rich white dwarf instead of the more common hydrogen-rich white dwarf. Just as in SN 2005cz, the team found a strong calcium signature in the light of SN 2005E. But they argue that the signature could also be produced by the thermonuclear fusion of the added helium with other constituent elements of the exploded star.

The question is more than academic. Massive stars are relatively rare, possibly too rare to have seeded galaxies such as ours with as much calcium as seems to exist. But white dwarfs are commonplace, and even if helium-rich versions comprise only a small portion, the answer to the calcium mystery may have been found.

Astrophysicist Stanford Woosley of the University of California, Santa Cruz, says the Perets team makes a "compelling case" for the idea of a helium-rich, faint supernova—a model that he and colleagues developed in 1986. It means, Woosley adds, that there is probably a wider range of stellar explosions than previously thought. And because "these events are much fainter than a [conventional] supernova," he says, they "are just now turning up."

*This article has been corrected. The previous version erroneously reported that both supernovae were located in elliptical galaxies. SN 2005cz is located in an elliptical galaxy, while SN 2005E occurred in a spiral galaxy dominated by old stars.