The appearance of a years-long supernova explosion challenges scientists' current understanding of star formation and death, and work is underway to explain the bizarre phenomenon.

Stars more than eight times the mass of the sun end their lives in fantastic explosions called supernovas. These are among the most energetic phenomena in the universe. The brightness of a single dying star can briefly rival that of an entire galaxy. Supernovas that form from supermassive stars typically rise quickly to a peak brightness and then fade over the course of around 100 days as the shock wave loses energy.

In contrast, the newly analyzed supernova iPTF14hls grew dimmer and brighter over the span of more than two years, according to a statement by Las Cumbres Observatory in Goleta, California, which tracked the object. Details of the discovery appeared on Nov. 8 in the journal Nature. [First Supernova Shock Wave Image Snapped by Planet-Hunting Telescope]

An inconspicuous discovery

Supernova iPTF14hls was unremarkable when first detected by a partner telescope in San Diego on Sept. 22, 2014. The light spectrum was a textbook example of a Type II-P supernova, the most common type astronomers see, lead author Iair Arcavi, an astronomer at the University of California, Santa Barbara, told Space.com. And the supernova looked like it was already fading, he said.

The observatory was in the middle of a 7.5-year collaborative survey, so Arcavi focused on more-promising objects. But in February, 2015, Zheng Chuen Wong, a student working for Arcavi that winter, noticed the object had become brighter over the past five months.

"He showed me the data," Arcavi said, "and he [asked], 'Is this normal?' and I said, 'Absolutely not. That is very strange. Supernovae don't do that,'" Arcavi said.

At first, Arcavi thought it might be a local star in our galaxy, which would appear brighter because it was closer, he said. Many stars are also known to have variable brightness. But the light signature revealed that the object was indeed located in a small, irregular galaxy about 500 million light-years from Earth.

And the object only got weirder. After 100 days, the supernova looked just 30 days old. Two years later, the supernova's spectrum still looked the way it would if the explosion were only 60 days old. The supernova recently emerged from behind Earth's sun, and Arcavi said it's still bright, after roughly three years. But at one one-hundredth of its peak brightness, the object appears to finally be fading out.

"Just to be clear, though, there is no existing model or theory that explains all of the observations we have," said Arcavi. The supernova may fade out; it may grow brighter, or it may suddenly disappear.

One reason for Arcavi's uncertainty is that a supernova was seen in the same location in 1954. This means that the event Acavi has been observing, whatever it is, may actually be 60 years running. There's a 1 to 5 percent chance the two events are unrelated, but that would be even more surprising, said Arcavi. Astronomers have never observed unrelated supernova in the same place decades apart. "We are beyond the cutting-edge of models," Arcavi said.