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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 appearance of a star that appeared of a sudden in the night sky, did non movement like a comet, and was visible for eight months before fading again. Over 2,000 years the Chinese recorded roughly twenty supernovae, with corroborating sources from Islamic, European, and Indian sources in some cases.

While the modern history of supernovae observation is much shorter, nosotros've trained telescopes on the areas of heaven where the ancient "guest stars" appeared and, in some cases, found probable candidates for the historical effect. In all our observations, there'south been one steady assumption–that a supernova is the last cataclysmic death of a star, in which the outer crush of material effectually the core is blown outwards at up to 10 percent the speed of calorie-free. Stars, in other words, don't get supernova more once. Except… we've found i that has. Repeatedly.

Writing in Nature, an international research squad discusses the highly unusual instance of iPTF14hls, first classified equally a Blazon II-P supernova on Jan eight, 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 2-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 M) equally it makes its mode inward in mass through the expanding ejecta (that is, the photosphere is moving from material that is farther out from the exploding star towards cloth that is further in, but the cloth within the photo-sphere is expanding in the meantime). This leads to the approximately 100-mean solar day 'plateau' stage of roughly abiding luminosity in the lite bend 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 than 600, with five singled-out peaks in its lite bend over that fourth dimension.

IPTF14hls

In the image above, there's an implicit peak to the far left of the graph (since the lite emission continued to decrease afterward the star was get-go observed, it must have been college in the past). Nosotros then see information technology rise, dip, and ascension once more. Then the star moved behind our sun (that'due south the gap in the data), only to re-sally 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 past as much as 50 per centum.

What'due south fifty-fifty stranger–and this is already plenty strange–is that we observed a similar phenomenon over 50 years ago. In 1954, a star in the aforementioned position as iPTF14hls, equally shown in the plate beneath. 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 milky way nearby. Merely 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 exist the first ane we've ever seen (they've been predicted, but nosotros've never found one). In theory, the star could be creating antimatter in its core, which would pb to "pair instability" betwixt positrons and electrons. In a pair-instability supernova, the production of antimatter in the core reduces its internal force per unit area, which leads to a partial collapse, which kicks-off an explosion and then massive that not even a black hole or stellar remnant is left behind. Pulsational pair-instability supernova theory predicts that a star could accident off a substantial percent of its total mass without completely exploding.

The trouble with this explanation is that it doesn't explain why large amounts of hydrogen go on to be detected effectually the star decades later on the 1954 burst. In curt, nosotros don't have a neat explanation for this star's behavior, nevertheless–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.

At present read: What is a supernova?