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SN1994D

Supernova are some of the oldest recorded astronomical phenomena in human being history. In 185 AD, Chinese astronomers recorded the advent of a star that appeared suddenly in the dark heaven, did not movement similar a comet, and was visible for eight months before fading again. Over ii,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, plant probable candidates for the historical event. In all our observations, in that location's been one steady assumption–that a supernova is the final cataclysmic death of a star, in which the outer shell of fabric around the core is diddled outwards at up to 10 percent the speed of light. Stars, in other words, don't go supernova more than one time. Except… we've found one that has. Repeatedly.

Writing in Nature, an international research team discusses the highly unusual instance of iPTF14hls, first classified every bit a Type II-P supernova on Jan 8, 2022. 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 end, which is at a roughly abiding temperature (T ≈ 6,000 K) as it makes its manner inward in mass through the expanding ejecta (that is, the photosphere is moving from cloth that is further out from the exploding star towards material that is farther in, but the fabric 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 bend and prominent hydrogen P Cygni features in the spectrum.

But iPTF14hls didn't play overnice. Instead of plateauing over 100 days, it lasted more than 600, with five distinct peaks in its calorie-free curve over that time.

IPTF14hls

In the image above, there'south an implicit elevation to the far left of the graph (since the light emission continued to subtract afterward the star was commencement observed, it must have been higher in the past). We so come across it rising, dip, and rise again. Then the star moved behind our sunday (that's the gap in the information), only to re-emerge at a higher apparent magnitude than it had previously. The light plateau of a standard 2-P supernova, SN1999em, is shown in the bottom left. Moreover, the temperature has stayed fairly constant, while its effulgence varied by every bit much as l percent.

What'southward even stranger–and this is already enough strange–is that we observed a similar phenomenon over l years agone. In 1954, a star in the same position as iPTF14hls, as shown in the plate below. Past 1993, the explosion had vanished, but now, it'south dorsum again. Supernova are incredibly bright; our feature image above shows a supernova that's literally outshining the galaxy nearby. Merely a star that repeatedly explodes? That's something new.

One potential caption, the BBC notes, is that this star is actually pulsational pair-instability supernova. If truthful, information technology would be the first one we've always seen (they've been predicted, but nosotros've never found one). In theory, the star could exist creating antimatter in its cadre, 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 level, which leads to a partial collapse, which kicks-off an explosion so massive that not fifty-fifty a black pigsty or stellar remnant is left behind. Pulsational pair-instability supernova theory predicts that a star could blow off a substantial percent of its total mass without completely exploding.

The problem with this explanation is that it doesn't explain why large amounts of hydrogen go on to be detected effectually the star decades after the 1954 flare-up. In short, we don't have a dandy explanation for this star'due south beliefs, yet–and information technology's an splendid example of how, fifty-fifty after millennia of watching the sky, nosotros're notwithstanding learning how much we don't know.

Now read: What is a supernova?