Every so often in the vast cosmos, something exciting happens in one of the few places that humans happen to watch closely. So it was with a recent supernova in the Whirlpool galaxy, a photogenic swirl some 30 million light-years away. Shortly after the light from an exploding star there reached Earth at the end of May 2011, amateur reports and images of the cataclysm began pouring in from around the globe.
Astronomers quickly determined that the supernova, SN 2011dh, resulted from the collapse of a massive star, but just what kind of star had met its end remained a mystery. As researchers sought to uncover exactly what had happened, the fact that the Whirlpool galaxy has long captured astronomers' attention once again came in handy. The Hubble Space Telescope had scanned the galaxy in detail in 2005, and a comparison of those images with the 2011 images revealed that at the very spot of the supernova, an unremarkable yellow supergiant star once sat.
Yet some researchers found that the supernova appeared much cooler than would be expected from the collapse of such an enormous star. Instead their early data pointed to the demise of a smaller, bluer star—perhaps a close neighbor of the yellow supergiant. “The yellow star was hiding the bluer star that actually exploded—that was our conjecture,” says astronomer Schuyler Van Dyk of the California Institute of Technology.
A competing team, however, had arrived at a different conclusion. Justyn Maund, now at Queen's University Belfast, and his colleagues hypothesized that the yellow star Hubble had spotted was indeed the one that exploded. But in 2011 no one could say for certain who was right—the brilliant glow of the supernova obscured the area in question.
By this past March the supernova had faded significantly, and Van Dyk and his colleagues commandeered Hubble to take another look. To their surprise, the yellow supergiant star had vanished, indicating that it had produced the supernova after all. “The other team was actually correct,” says Van Dyk, lead author of a study reporting the findings in Astrophysical Journal Letters.
Yet the saga of the supernova will not end there. SN 2011dh turned out to be a rare type IIb supernova, which results from the collapse of a massive star that has been stripped of most of its hydrogen shell, perhaps caused by the pull of a companion star. If that explanation is correct, the supergiant's surviving partner should still exist. And as the bright blemish of the supernova remnant continues to fade, that survivor should reveal itself to Hubble toward the end of the year.