Talk about going out with a bang -- and then another bang. Astronomers at Las Cumbres Observatory have discovered a supernova that has been shining for years instead of mere months, surviving far beyond its expected lifespan.
The strange and still-going stellar explosion, described in the journal Nature, defies scientists' understanding of dying stars and may force them to rethink their ideas of how stars evolve.
"The supernova offers astronomers their greatest thrill: something they do not understand," Stan Woosley of the University of California, Santa Cruz, who was not involved in the study, wrote in a commentary.
The supernova known as iPTF14hls didn't seem like anything out of the ordinary when it was picked up in September 2014 by the Intermediate Palomar Transient Factory telescope near San Diego. The supernova sits some 500 million light-years away in the constellation Ursa Major. Astronomers checked in on it every so often, eventually classifying the bright object as a type II-P supernova that was already beginning to grow dim.
Supernovas are powerful explosions, putting out the brightness of around 100 million suns, but that beacon in the night sky lasts only 100 days or so before fading away. There are a few that might last more than 130 days, but those are very rare. So after a while, astronomers largely ignored iPTF14hls, expecting it would ultimately disappear into the night.
That changed after UC Santa Barbara undergraduate student Zheng Chuen Wong, who was interning at the observatory, was asked to go through old data and see whether anything unusual stuck out. He noticed iPTF14hls still shining some 135 to 140 days since being discovered -- and it wasn't fading. It was getting brighter.
"When he showed it to me, my first reaction was, 'Well that can't be a supernova -- it must be something else,'" lead author Iair Arcavi, an astrophysicist with UC Santa Barbara and Las Cumbres, a Goleta, Calif.-based global observatory network.
Arcavi figured they'd get a spectrum of the star, splitting its light into a rainbow to read its chemical fingerprint, and they'd figure out what it really was.
"And I was really shocked when it did look like a supernova in the spectrum," he said. "In fact, it looked like the most common type of supernova. It was the last thing I was expecting to see."
They kept watching. The supernova ultimately stayed bright for more than 600 days, blowing away the competition. And yet, in spite of its advanced age, it looked like a supernova that was just two months or so old.