In September 2014, astronomers observed an interesting supernova, which they called iPTF14hls. What they didn’t know was that they were observing something that shouldn’t have happened in the recent universe. Not only was the object brighter for a lot longer than regular supernovae, but it had also already exploded in 1954.
As reported in Nature, when first observed, iPTF14hls looked like a regular Type II-P supernova. These objects tend to remain bright for about 100 days, but iPTF14hls persisted in its brightness for more than 600 days, going from bright to dim and back again over five times. The team estimate that the general evolution of the supernova is about 10 times slower than that of the standard Type II-P.
This alone would make the supernova a record-breaker, but apparently this wasn’t enough. To try and work out a potential explanation for the unusually long-lasting event, researchers looked through archival data in the hopes of finding a hint about this object’s progenitor. They got more than they bargained for. They discovered that in the same location 50 years previously, there was another massive explosion. This star apparently exploded in 1954, survived, and then exploded again in 2014.
“This supernova breaks everything we thought we knew about how they work," lead author Dr Iair Arcavi, from University of California Santa Barbara and Las Cumbres Observatory, said in a statement. "It’s the biggest puzzle I’ve encountered in almost a decade of studying stellar explosions."
The team have proposed an explanation for this never-before-seen behavior: It is a pulsational pair-instability supernova, an extreme event that is supposed to happen when stars are so massive that they create electrons and positrons (anti-electrons) in their cores. This leads to a matter-antimatter annihilation that blows up part of the star.
If the pulsational pair-instability scenario is correct, then when it exploded in 1954, the star could have been between 95 and 130 times the mass of our Sun. Estimates suggest that at the time of its last explosion, the star was at least 50 times the mass of our Sun.
“These explosions were only expected to be seen in the early universe and should be extinct today," co-author Dr Andy Howell, leader of the supernova group at Las Cumbres, explained. "This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur."
While the proposed solution explains several features, it is not completely satisfactory. The star seems to have released more energy than the theory expects. Astronomers are still continuing to study this object, as its brightness persists to this day.