Astronomers initially pegged the explosion as a Type II-P supernova; the “P” means that the brightness of the light plateaus for some time after the explosion, as opposed to Type II-L stars that have a more variable brightness. But then things got weird. While most Type II-P supernovas fade after 100 days, iPTF14hls stayed shining for more than 600, and in fact had variable brightness. Astronomers also uncovered a 1954 explosion in archival data in the very same spot.
In other words, the star exploded six decades ago and somehow survived. More weirdly, modeling suggested that the star exploded several times during the last few decades of its life, and that it was much larger when it was first formed — somewhere between 95 and 130 times the mass of the sun, according to the paper.
The astronomers, however, remain puzzled as to how the star survived such a violent history, and suggest that we need to revisit our understanding of how massive stars evolve and explode.
iPTF14hls grew bright and dim again at least five times over two years. This behavior has never been seen in previous supernovae, which typically remain bright for approximately 100 days and then fade. Adapted from Arcavi et al. 2017, Nature. |
Credit: LCO/S. Wilkinson
“This supernova breaks everything we thought we knew about how they work,” said lead author Iair Arcavi of the University of California Santa Barbara and Las Cumbres Observatory, in a statement. The research was published in the journal Nature.
In an accompanying “news and views” article in Nature written by astronomer Stan Woosley, who did not participate in the research, he pointed to several other unusual aspects of this star that struck him from reading the research. The explosion varied in brightness. The spectrum of the light had consistent lines, indicating that material from the star was not slowing down as expected. The explosion also stayed at about the same temperature through its peak, instead of cooling down as it expanded (a basic of radiation theory).
Astronomers uncovered an explosion in 1954 (at left) at the exact same location that iPTF14hls exploded in 2014. The supernova observed in 1954 wasn’t seen in follow-up images, such as one shown at right taken in 1993. This means that iPTF14hls experienced at least two supernova explosions in its lifetime instead of being utterly destroyed as astronomers expect. Scientists are still seeking a full explanation for this “zombie star” surviving. |
The most plausible scenario the authors put forward is a model in which a huge star has “supernova-like outbursts” near the end of its life. In other words, instabilities as nuclear fusion ceases would cause a series of explosions that shoots out material from the star without utterly destroying it.
More stellar mass would also be lost to stellar winds and other pulses earlier in the star’s lifetime. But the model has a few limitations, Woosley pointed out, such as not accounting for how the supernova kept a constant temperature.
Additional study of iPTF14hls will be needed to better formulate a model of its later days. Astronomers will also keep an eye on the sky to see if other strange supernovas of its type pop up.
Bonus watch for the interested ones: Why do stars explode?