Solving a 400-year-old supernova riddle.

In 1604, an assistant to the astronomer Johannes Kepler discovered an object that shone brighter than any star in the heavens. It was the exploding star now known as Kepler's supernova, the last one that astronomers have witnessed in the Milky Way (SN: 12/11/04, p. 378). But even as the glowing remnant of that stellar cataclysm endures, so does a fiddle about the supernova's origins.

The abundance of iron in the remnant and the explosion's location, outside the Milky Way's star-forming disk, suggest that it was a type 1a supernova. Such an event occurs when a white dwarf--the burnedout remains of a star similar in mass to the sun--siphons gas onto its surface from a companion star and eventually accumulates a layer of material that causes the white dwarf to explode.

Other features of the remnant, however, especially its dense shell of gas and dust, indicate that it came from a core-collapse supernova. In such an explosion, a single star more massive than a white dwarf hurls its outer layers into space while its core shrinks and becomes a neutron star or black hole.

Analyzing nearly 9 days of observations from NASA's Chandra X-ray Observatory, Stephen Reynolds of North Carolina State University in Raleigh and his colleagues have now determined that Kepler's supernova was indeed type 1a. Chandra found no evidence of a neutron star or a black hole. In addition, the researchers confirm that the ratio of iron to oxygen was high, the value expected from a type 1a explosion.

"The X-ray evidence for [Kepler's supernova] being a type 1a is becoming quite compelling," says astronomer Bill Blair of Johns Hopkins University in Baltimore.

That still leaves astronomers to account for the dense material in the remnant, more typical of a core-collapse explosion. Reynolds and his colleagues suggest that the star that ultimately exploded as a type 1a was more massive than usual, perhaps as much as eight times the sun's mass. During its lifetime, such a heavyweight would have shed a greater amount of material than a lower mass star would have. A supernova explosion occurring in this gas-rich environment would create a denser remnant.

Such a star would take only about 100 million years to reach supernova stage, in contrast to the several billion years it takes for lower-mass stars to reach that point.

Understanding the age and mass of stars that die as type 1a supernovas could be critical to revealing the origin of these explosions, says Reynolds. Astronomers still don't fully understand what drives these violent events, despite routinely relying on them for details of cosmic expansion.