This news starts with a massive explosion. No, I’m not talking about Betelgeuse, it’s still hanging out in Orion refusing to go boom. I’m talking about SN 2006gy. Located in the spiral galaxy NGC 1260 at a distance of about 250 million lightyears, this supernova was one of the brightest ever observed, and for the past 13 years, astronomers have been trying to figure out why. Adding extra intrigue were the 2009 observations from a Japanese team that spotted unidentifiable elements in the spectrum of the supernova remnant. Let me be clear – they didn’t see an unknown element, they saw an element they could not completely identify, and like students doing the standard mystery chemical identification lab, astronomers around the world have been trying to find an explanation for this overly-bright explosion and those exceptional spectral lines. 

Recently, a team at the Max Planck Institute for Astrophysics joined forces with the Japanese observers to try and figure out this system … and it looks like they may have succeeded! 

Let’s start with those unknown spectral lines. Spectra consist of three basic components: there is background continuum emission – a nice rainbow of light that is brighter in some colors than others based on the temperature of the system. This is the black-body radiation of a star, and in the case of a supernova, is the combined light from any hot remnant and the light from the surrounding nebula. In addition to this rainbow of light are absorption and emission lines from atoms and molecules that are getting excited by that continuum emissions and energetic processes. In stars, identifying lines can be a challenge, but you can at least assume all the elements are at basically the same temperature, and guess what kind of atoms and molecules can exist at that temperature. In a supernova remnant, things are a lot more complicated because you have that surrounding nebula with unknown temperatures and composition. 

In trying to figure out what elements were in SN 2006gy’s spectra, they started from the assumption that it was a hot environment, where complicated elements are ionized and missing electrons. This simple assumption turned out to be just plain wrong and led to 14 years of confusion. It turns out those unknown lines were from neutral iron. According to researcher Anders Jerkstrand, “This low-energy state of iron is typically not seen in supernovae, where the high energies involved tend to strip one or several electrons from the atoms.” With those spectral lines identified, the team could calculate that at least ⅓ a Solar Mass of Iron is present, and that kind of iron content only comes from one kind of a supernova – a type 1a supernova where a white dwarf star is overwhelmed by mass from another star falling onto its surface and exploding. 

But here is the thing, type 1a supernova are supposed to be standard candles that are all the exact same mass and exact same brightness, so this raises the question: how is one of the brightest supernovae known also this standard type 1a kind of a supernova? Well, this is where it gets awesome. They think that a white dwarf star was orbiting a normal star that expanded into a giant, and engulfed the white dwarf in the process. This kind of an expansion is a normal part of stellar evolution, and eventually our own Sun will expand and consume Mercury and Venus. Once that white dwarf was engulfed into the giant star, it began a slow spiral into the star’s core. While the white dwarf journeyed inward, the star shed its outer atmosphere – puffing off this envelope of material to create a close-in nebula. When the white dwarf reached the star’s core, the combined mass exploded, and the shock wave collided with the surrounding, iron-rich envelope creating the weird lines that were observed. In this case, we have a white dwarf going nova inside another star, with the combined mass of two stellar cores.

This kind of a supernova is evidence that pretty much anything that you can imagine probably will happen somewhere in the universe. It’s also a reminder that creativity is required to figure out weird systems, and to think outside of the box to solve unusual mysteries. This may be the coolest supernova I have ever read about.

You can read more about this supernova here: 

Progress in understanding the brightest explosions in the Universe (Max Planck)