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In September 2014, astronomers saw a dimming point of light in a small galaxy half a billion light-years away. It looked like an ordinary supernova — a dying star that exploded and whose light was now petering out. But the following January, Zheng “Andrew” Wong, a student intern at Las Cumbres Observatory in Goleta, California, noticed that the light was getting brighter again.

When he showed this odd turnaround to his supervisor, the astronomer Iair Arcavi, “his eyes grew,” Wong recalled. Arcavi soon convinced himself that the light source, designated iPTF14hls, must not be a supernova after all but rather a nearby pulsating (or “variable”) star superimposed by coincidence on the distant galaxy. Arcavi was so sure, he said, “I would have bet my car that that was a variable star.” Still, he helped Wong take a spectrum of iPTF14hls, measuring its color to reveal its chemical makeup. To their astonishment, the spectrum came out to be exactly that of a Type II-P supernova — the most common and well-understood kind of exploding, dying, massive star. When a Type II-P supernova explodes, its brightness rises, plateaus (hence the “P”) for about 100 days and then declines until it’s over. “We had never seen such a supernova decline and go up again,” said Arcavi, who, along with 52 collaborators, reported the discovery of iPTF14hls today in the journal Nature. “That’s when we understood we had something very interesting.”

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Category: Science