A supernova may be one of the most extraordinary events in the Universe, but the Universe is a very big place, and the extraordinary happens with great regularity. We've now observed a huge number of these events and have managed to break them down into categories based on patterns in the light they produce. Astrophysicists have built models of exploding stars that explain these properties, matching them to the mass of the original star and the process by which it exploded. We're at the point where, after just a few observations, we can understand exactly what we're looking at.

Except when we can't.

Today in Nature, a team of researchers is announcing observations of a supernova that it simply can't explain. In some ways, the event looks like a prosaic stellar explosion. Except it has stayed bright more than six times longer than it should have and experienced five periods of enhanced brightness that we can't explain. Different features of the supernova appear to be arising from physically distinct locations in space. And even the best model for what triggered this—something that involves a type of explosion we haven't definitively observed previously—doesn't account for all the observations.

Curiouser and curiouser

This whole confused mess started rather mundanely back in late 2014. A project called the Intermediate Palomar Transient Factory uses an automated telescope to scan the skies repeatedly, comparing each new image with previous ones of the same location so it can identify cases where something has changed (meaning there has been a transient event). It's a great way to identify supernovae, since they result in a dramatic brightening of the star that explodes.

In September of 2014, the survey covered an area of sky that it had not imaged in 100 days, and it found a telltale brightening. By January, followup observations of the event (termed iPTF14hls) showed that its luminosity was similar to that at its first discovery and dominated by hydrogen emissions. This led to its classification as a Type-IIp supernova. A Type-IIp's steady production of light, which typically lasts 100 days, is caused by ionized hydrogen cooling off enough to recombine with electrons, emitting light at a specific wavelength in the process. The critical temperature is typically reached at a set distance from the site of the explosion, meaning there's a steady flow of debris through this point that keeps things lit for 100 days.

Before too long, however, it became clear that this wasn't what was happening with iPTF14hls, which remained bright well past the 100-day mark. In fact, by the time a general dimming was apparent, it was 600 days after the supernova was first spotted. Obviously, that's hard to explain by a steady flow of debris spreading out and cooling off.

The other problem here is that there's no sign of an actual flow of debris. Obviously, material that gets far enough away from the site of the explosion to cool off later is moving more slowly than the material that manages that process quickly. As such, the velocity of the material that's emitting light should drop over time. (Another way of looking at this is that, as the fast moving material stops emitting light, it lets us see the slower-moving material underneath it.) iPTF14hls again is refusing to cooperate here, as there is little sign of slower-moving materials in the debris. Over the entire 600 days, hydrogen-rich material seems to only be slowing by 25 percent; iron-rich material doesn't slow at all.

Complicating matters further, the followup observations identified five points where the supernova brightened by as much as 50 percent. Typically, these post-explosion brightenings occur as specific isotopes created in the supernova undergo radioactive decay. The peaks occur at around the first half-life of that isotope. The 100-day uncertainty about when the explosion actually took place means that there are several candidates that could account for some of the peaks in brightness. The problem is that there's no sign of any of these elements in the debris. So we're at a loss for how these peaks are generated.

Boom today and boom tomorrow

All of this is, to put it mildly, pretty confusing. The one hint the researchers have to explain iPTF14hls comes from their estimates of where the material is. If you just look at the broad-spectrum light glowing debris, you get one value for its distance from the site of the explosion. If you look at the bright lines produced by specific elements, however, you get a value double that. This implies that there's more than one shell of debris involved in producing some of the phenomena seen here. In essence, it looks like the star has exploded twice.

How in the world does that work? In describing the fates of stars, we'll typically say that big stars explode and leave behind a neutron star, while really big stars explode and leave behind a black hole. That, as it turns out, is a simplification. Really, really big stars—objects with more than 130 times the mass of the Sun—explode in a way that leaves nothing behind. That's because the reactions in their core eventually produce so many high-energy photons that some of them will spontaneously convert to matter. Removing that energy takes away some of the pressure that keeps the star's core from collapsing in on itself. The change is so sudden that it produces a massive fusion explosion, blowing the star to pieces.

Black-hole-forming explosions top out at about 100 solar masses; complete-destruction supernovae start at about 130 solar masses. What happens in between? The same spontaneous conversion to matter occurs in the core of the stars, but it's at somewhat lower levels, so the core collapses more slowly. This allows time for a specific fusion reaction (oxygen-oxygen fusion) to occur, creating a sudden outward pulse of energy. This pulse ejects huge amounts of material—about 10 solar masses—but it leaves the core of the star intact. The process can repeat until the star sheds enough mass to undergo a black-hole-forming supernova.

We haven't definitively identified any of these things (called a pulsational pair-instability supernova) happening, but there are a couple of candidates, including the Great Eruption of η Carinae in the 1840s. But an eruption of this sort would put a second sphere of debris out in space ahead of the final supernova debris. And photographic plates indicate there was a transient event in this general location in 1954.

If the star that produced iPTF14hls had a similar eruption shortly before its final supernova, it could explain some of the observations. But definitely not all of them. For example, the earlier eruptions are expected to eject most of the star's remaining hydrogen, but there was clearly plenty of that left in the supernova. In addition, it can't explain how the debris of iPTF14hls appears to have maintained a constant velocity for over a year and a half.

So there's clearly a lot left to do to understand iPTF14hls. If other debris is out there, then we should eventually be able to observe the material ejected by the final supernova interacting with it (I'd expect that the authors of the Nature paper grew frustrated waiting for something like that to occur). There's also the very real possibility that iPTF14hls will throw us yet another curveball, upsetting our models once again.

Nature, 2017. DOI: 10.1038/nature24030 (About DOIs).