The first bombshell was that the wrong kind of star had blown up. In the textbook view, only red supergiants—the largest stars—could go supernova. But the progenitor of SN 1987A turned out to be a smaller, hotter kind of star called a blue supergiant. “Supernova 1987A was quite different from other supernovas because it was a compact star when it exploded,” says Harvard astronomer Robert Kirshner. “And of course there are other objects just like it that we now recognize.” That means supernovas may be more common than previously thought. After Hubble was launched, it spotted huge, faint ruby loops framing the supernova; their origin is still a mystery. Theoretical models predicted that the supernova should leave behind a neutron star (an ultradense stellar residue), but so far there is no sign of it.
The latest Hubble images of SN 1987A further boost astronomers’ appreciation of what a complex beast a supernova is. Remnants of the star’s outer atmosphere show up as lustrous red rings. “That’s an interesting detail because it tells you what the star was doing 10,000 or 20,000 years ago,” Kirshner notes. Before it died, the convulsing star apparently shed its gaseous aura, which then lit up after the explosion. First visible to Hubble 13 years ago, the rings continue to brighten as more of the blast hits them. Soon they will be bright enough to illuminate the star’s surrounding area like a campfire in the woods. The new Hubble pictures also reveal that the debris in the middle—the shredded star itself—is asymmetrical, confounding expectations. “This is a clue to the actual guts of the explosion,” Kirshner says. There’s also a peculiar glow in the center, believed to be energy released by a radioactive isotope of titanium created in the first second of the explosion. From this chemical signature, theorists can work backward to dissect the underlying physics of the blast.