Astronomers Find Star That Has Exploded Six Times

Joel Hruska

Supernova are some of the oldest recorded astronomical phenomena in human history. In 185 AD, Chinese astronomers recorded the appearance of a star that appeared suddenly in the night sky, did not move like a comet, and was visible for eight months before fading again. Over 2,000 years the Chinese recorded roughly 20 supernovae, with corroborating sources from Islamic, European, and Indian sources in some cases.

While the modern history of supernovae observation is much shorter, we’ve trained telescopes on the areas of sky where the ancient “guest stars” appeared and, in some cases, found likely candidates for the historical event. In all our observations, there’s been one steady assumption–that a supernova is the final cataclysmic death of a star, in which the outer shell of material around the core is blown outwards at up to 10 percent the speed of light. Stars, in other words, don’t go supernova more than once. Except… we’ve found one that has. Repeatedly.

Writing in Nature, an international research team discusses the highly unusual case of iPTF14hls, first classified as a Type II-P supernova on January 8, 2015. At first, this appears to have been an open-and-shut designation (II-P supernovas are the only known phenomena that produce the spectra observed for iPTF14hls). The team writes:

In a type II-P supernova, the core of a massive star collapses to create a neutron star, sending a shock wave through the outer hydrogen-rich envelope and ejecting the envelope. The shock ionizes the ejecta, which later expand, cool and recombine. The photosphere follows the recombination front, which is at a roughly constant temperature (T ≈ 6,000 K) as it makes its way inward in mass through the expanding ejecta (that is, the photosphere is moving from material that is further out from the exploding star towards material that is further in, but the material inside the photo-sphere is expanding in the meantime). This leads to the approximately 100-day ‘plateau’ phase of roughly constant luminosity in the light curve and prominent hydrogen P Cygni features in the spectrum.

But iPTF14hls didn’t play nice. Instead of plateauing over 100 days, it lasted more than 600, with five distinct peaks in its light curve over that time.

In the image above, there’s an implicit peak to the far left of the graph (since the light emission continued to decrease after the star was first observed, it must have been higher in the past). We then see it rise, dip, and rise again. Then the star moved behind our sun (that’s the gap in the data), only to re-emerge at a higher apparent magnitude than it had previously. The light plateau of a standard II-P supernova, SN1999em, is shown in the bottom left. Moreover, the temperature has stayed fairly constant, while its brightness varied by as much as 50 percent.

What’s even stranger–and this is already plenty strange–is that we observed a similar phenomenon over 50 years ago. In 1954, a star in the same position as iPTF14hls, as shown in the plate below. By 1993, the explosion had vanished, but now, it’s back again. Supernova are incredibly bright; our feature image above shows a supernova that’s literally outshining the galaxy nearby. But a star that repeatedly explodes? That’s something new.

One potential explanation, the BBC notes, is that this star is actually pulsational pair-instability supernova. If true, it would be the first one we’ve ever seen (they’ve been predicted, but we’ve never found one). In theory, the star could be creating antimatter in its core, which would lead to “pair instability” between positrons and electrons. In a pair-instability supernova, the production of antimatter in the core reduces its internal pressure, which leads to a partial collapse, which kicks-off an explosion so massive that not even a black hole or stellar remnant is left behind. Pulsational pair-instability supernova theory predicts that a star could blow off a substantial percentage of its total mass without completely exploding.

The trouble with this explanation is that it doesn’t explain why large amounts of hydrogen continue to be detected around the star decades after the 1954 burst. In short, we don’t have a great explanation for this star’s behavior, yet–and it’s an excellent example of how, even after millennia of watching the sky, we’re still learning how much we don’t know.