Every so often in the vast cosmos something exciting happens in one of the relatively few places that humans happen to watch closely. Like a rare bird touching down for a bath in the Trevi Fountain, such serendipitously placed exotica produces a wealth of witnesses and plenty of photographic documentation.
So it was with a recent supernova in the spiral galaxy M51—better known to casual stargazers as the Whirlpool Galaxy, a photogenic swirl some 25 million light-years away. Shortly after the light from an exploding star there reached Earth at the end of May 2011, amateur reports of the cataclysm began pouring in to the Central Bureau for Astronomical Telegrams, a clearinghouse for new telescope data. Soon the explosion was assigned the official designation supernova 2011dh.
The Whirlpool Galaxy has plenty of admirers, so a brand-new bright spot on the edge of the spiral was sure to catch the attention of many observers. “It’s really one of the nearest galaxies, and it’s a galaxy that’s beautiful and very famous,” says astronomer Schuyler Van Dyk of the California Institute of Technology.
Even better, the well-documented supernova in the Whirlpool Galaxy turned out to be a rare variety known as a type IIb supernova. Those explosions result from the collapse of a massive star that has been stripped of most of its outlying hydrogen shell, possibly by the pull of a binary stellar companion. Of all the stars that end their lives in a catastrophic collapse—just one of two ways to produce a supernova—only about one in 10 produces a type IIb.
Astronomers have some general explanations for type IIb explosions, but uncovering the exact chain of events leading up to a supernova is a difficult task. Because astronomers never know that a star is about to go supernova until it has already exploded, it is usually impossible to determine which star, exactly, met its violent end. Only in rare cases can astronomers turn up sufficiently detailed pre-explosion images of the region in question to identify the culprit. In 2011, however, the famousness of the Whirlpool Galaxy once again came in handy. “Within days of discovery of the supernova we went to the Hubble Space Telescope data archive, and it turned out that one of the former directors of the HST had orchestrated this beautiful mosaic of M51—this glorious picture in various colors,” Van Dyk says. In the Hubble images, at the very spot where the supernova appeared without warning in 2011, there had been in 2005 an unremarkable yellow supergiant star.
But many researchers found that the profile of the explosion did not fit what would be expected from the collapse of a supergiant. Instead their data for 2011dh pointed to the explosion of a more shrunken star—perhaps a binary companion of the yellow supergiant that had been stripped down, nearly to its core, by the gravitational pull of its neighbor. “We thought initially that the progenitor was essentially this very stripped star, very blue, and so it was unseen” in the Hubble images, Van Dyk says. “The yellow star was hiding the bluer star that actually exploded—that was our conjecture.”
A competing team, however, had arrived at a different conclusion. An early analysis by Justyn Maund, now of Queen's University Belfast, and his colleagues found that the giant star that Hubble had spotted at the site of the explosion had indeed been the progenitor. “They said that the yellow star was the star that exploded,” Van Dyk says. “They had other data that was more consistent with a more extended progenitor. So there we were.”