It was not a slow death — it lasted a few hours at most. The casualty was a star in a spiral galaxy some 160 million light-years away. Its core collapsed in on itself, triggering a supernova explosion as bright as 100 billion suns. On a cosmic scale, this star was rather ordinary, probably a red supergiant some 10 times more massive than our sun. But on October 6, 2013, when the light from the explosion finally reached Earth, its death made history. The glow from SN 2013fs, as it’s now called, hit the right robotic witness, and then astronomers caught a cosmically lucky break.

Humans have recorded supernovas for centuries. In A.D. 185, observers in China (and possibly Rome) recorded a bright “guest star” that shone for eight months. To Renaissance astronomers like Tycho Brahe and Johannes Kepler, these sudden heavenly bursts proved that the sky was not immutable. Modern astronomers study supernovas to understand how they seed the universe with heavy elements that can’t be made otherwise. But despite all the attention, supernovas are still riddles wrapped in mysteries.

Scientists know that the most common type of supernovas — so-called “Type II” supernovas, of which SN 2013fs was one — happen when a large star runs out of fuel and its core collapses in on itself. But the details of this process are still opaque. Does a dead star show any signs of imminent explosion? How does the supernova eject so much energy in such a short period of time? And might some events that we think of as supernovas really be other astrophysical cataclysms in disguise?

The questions are compounded by the fact that supernovas are hard to find and even harder to study in detail. Supernovas only happen about once or twice per galaxy per century — our Milky Way is now hundreds of years overdue for one — and appear to flash at random. Astronomers have to cast a wide celestial net and wait to see what they catch. Then, as soon as a promising blast has been spotted, they try to secure time on a large, powerful telescope to study the event’s spectrum of light, its astrophysical fingerprint. Often this means a wait of weeks, if not months. By then the fireworks may have flickered out. “This is like stellar forensics,” said Matteo Cantiello, an astrophysicist at the Simons Foundation’s Flatiron Institute and at Princeton University. “We know a star died, we want to understand exactly why and who did it. We can look at the corpse, but even better if we can have footage of its last hours of life.”

This time, astronomers got that final look. Three hours after the first light from SN 2013fs reached Earth, an automated sky survey, the Palomar Transient Factory in Southern California, spotted the bright flash. By sheer luck, a team of supernova hunters had already pre-booked time on the massive Keck Telescope in Hawaii for that same night. They had been planning to make a completely different set of observations, but now Ofer Yaron, an astrophysicist at the Weizmann Institute of Science in Israel, and his collaborator Dan Perley, a researcher at the California Institute of Technology, pointed Keck at the flare. Yaron’s team also lined up the space-based Swift telescope to take measurements in X-ray and ultraviolet light.

What they found is challenging long-held beliefs about how supernovas behave. “Until recently, everyone would say that there is no way to tell if a star is going to explode to within about 10,000 years,” said Stan Woosley, an astrophysicist at the University of California, Santa Cruz.

Yet SN 2013fs may have shown signs that it was about to die just several months before the eruption. “Stars sometimes give you a warning sign that they are about to explode,” said Jim Fuller, an astrophysicist at Caltech. And once we understand how to spot these signs early, maybe we can predict which stars are about to blow up.

The First Burst

When Yaron and his team examined the early data from SN 2013fs, they saw that the exploding star was surrounded by a dense shell of gas. This raised a question: Had that gas been there for dozens or perhaps hundreds of years — a long-ago cosmic burp unrelated to the supernova itself? Or was it somehow a consequence of the same processes that gave rise to the supernova — a ghostly premonition of the imminent astrophysical violence?

The traditional theory of core-collapse supernovas implies that a star should look outwardly quiet while its insides obliterate. It’s the core of the star that collapses, after all — the outer part of the star, called the mantle, only bursts once the collapsing core has rebounded against itself. “We expect that the surface will show nothing special even though dramatic changes are occurring in the core in the last years,” said Anders Jerkstrand, an astrophysicist at the Max Planck Institute for Astrophysics in Germany.

Over the past decade, however, a number of research teams have suggested that dying stars slough off their outer layers of gas before exploding. One team, who sifted through Palomar images of 16 older supernovas, spotted minor flashes at five of them during the months immediately before detonation.