A star that goes nova is only supposed to explode once… right?
That long-held assumption was upended when astronomers spotted a Type II-P supernova in progress in a star 509 million light-years away on September 8, 2014. The exploding star in question, iPTF14hls, was predicted to fade within 100 days, but its luminosity not only persisted for the next 600 days, it also flared to an even greater brightness at least five more times, implying that this single star had experienced a supernova at least six times.
Supernovae occur when a star that’s at least eight times the mass of the Sun comes to the end of its life: with its nuclear fusion processes winding down, the star’s core is no longer able to maintain outward pressure, and due to its own gravity it violently collapses inward on itself. The resulting temperature and density spike causes a massive explosion, blowing the star’s outer layers outward at speeds of up to 10 percent of the speed of light. This is also where the majority of the Universe’s alchemical processes happen, with the core’s collapse converting lighter atomic elements into heavier ones. And in stars with sufficient mass, the collapsed core is left behind as either a neutron star or a black hole.
Needless to say, this should be a one-time event for each star: one singular cataclysmic event where the star’s mass is either consumed in the collapse or strewn across the cosmos. But the supernova (supernovae?) resulting from iPTF14hls’ eruption just seemed to keep exploding: when it was first spotted, iPTF14hls’ apparent magnitude was nearly 18, and it did indeed begin to taper off over the next few months (apparent magnitude is a measure of a star’s brightness, with higher numbers being dimmer, and anything over magnitude 7 is not visible to the naked eye).
But after this brief drop in luminosity, iPTF14hls’ magnitude jumped to 17.2, and shortly afterward it spiked to 16.8 — far brighter than the first two explosions. The star’s brightness has been tapering off ever since, but it has still managed to produce at least three more spikes since its recorded peak, although the extent of spike #4 is unknown, as iPTF14hls was behind the Sun when it occurred.
Further complicating the matter is that a search through that stellar region’s history revealed that another nova occurred in the exact same spot in 1954, but by 1993 the explosion had entirely vanished. So far, no theories that have been put forward have been able to explain such an odd phenomenon, although this event (or events?) certainly serves to underscore how much we still have to learn regarding how our universe works.