My oh my. It looks like one of the biggest questions still left after nearly 40 years of supernova observations has finally been answered: Astronomers have found excellent evidence of a neutron star at the center of the Supernova 1987A maelstrom!

This is A Big Deal™.

In late February 1987 — in fact, last week was the anniversary; the release of this news was well timed — the light from brightest supernova in four centuries reached Earth. Dubbed Supernova 1987A, or just 87A to its friends, it was actually spotted by astronomer Ian Shelton by naked eye! The stellar explosion occurred in the Large Magellanic Cloud, a dwarf galaxy very close to the Milky Way, just about 170,000 light-years away from the Sun. That was why it could be seen by eye, and became the best-studied event of its kind in history.

I wrote a series of short essays about the explosion and aftermath on my old site (like, the really old site, badastronomy.com) if you’re interested. Start here. 

In 1990 Hubble Space Telescope was launched into space, and 87A was one of the very first objects it observed. I used those and later Hubble images to get my PhD, studying a bizarre ring of gas circling the exploded star, material expelled by the star tens of thousands of years before it blew up, with the light from the explosion illuminating the ring and allowing us to study it.

The star itself was called Sanduleak -69 202, a massive blue supergiant. That was one of many shocks astronomers got about this event; it was thought only red supergiants could explode. As massive stars die, they undergo a lot of changes in their cores that propagate out to their surfaces, changing the size, temperature, and luminosity (how much energy is emitted) of the star. Apparently the core detonated while the star was still blue, which had never before been seen. Pretty cool. It’s always nice to have your hypotheses challenged by observations!

I talk more about how massive stars die in an episode of Crash Course Astronomy, so give that a watch to get up to speed:

 In a nutshell, the core goes through various stages of fusing ever-heavier atomic nuclei together, from hydrogen to helium, carbon, oxygen, magnesium, and more, until it starts fusing silicon. This makes iron, and iron is weird: it takes more energy to fuse it then you get out of the reaction. The immense mass of the core of the star — more than twice that of the Sun, and sometimes a lot more — is held up in part by the energy inside it, and if you sap that energy away the core loses support. Iron fusion also removes electrons that play a big part in supporting the core as well. When iron fusion starts, the core suddenly finds itself without anything to hold it up against its own fierce gravity. It collapses.

If the core is more than about 1.4 times the mass of the Sun it’ll collapse down to a neutron star, a weird object that’s incredibly dense, hellishly hot, and possessing a magnetic field that can be a quadrillion times stronger than Earth’s. More, sometimes. If the core is more than 2.8 times the Sun’s mass, it’ll become a black hole. Either way, a huge amount of energy is released, enough to literally blast away the outer layers of the star at an appreciable fraction of the speed of light. Mind you, we’re talking several octillion tons of mater screaming away at maybe 5,000 kilometer per second, so the energy involved is fantastic. The newly born supernova can release as much energy in a few weeks as the Sun will over its entire 12-billion-year-long lifespan.

Ka — and I cannot stress this enough — BOOM. 

87A is far enough away that it still took some time for the expanding explosion debris to be seen clearly; Hubble images started showing it clearly within a few years. The fastest material started hitting the ring around the year 2000, lighting it up again. That was fun to see.

Still, the question remained: What was left in the center, a neutron star or a black hole? Some early observations indicated a neutron star, but they turned out to be spurious. In 2019 and 2020 more observations seemed to show evidence of a neutron star, but they weren’t conclusive. Still, pretty much everyone expected it would be a neutron star, because at the time of the core collapse the star emitted a whole lot of neutrinos, wee subatomic particles that carry away 99% of the supernova’s energy. They were so spread out by the time they travelled all those light-years that only a couple of dozen were detected at Earth, but enough to be pretty sure a neutron star formed.

But here we are now, with JWST examining the Universe. Observations of 87A in the infrared have finally provided the best evidence of a neutron star to date. We don’t see the star itself, but it’s damned hot, and zapping the material around it — material that used to be part of the original star — with intense ultraviolet radiation. The material is a mix of elements like argon, sulfur, silicon, calcium, and oxygen. When hit by UV, these atoms respond by glowing at very specific wavelengths of light, and it so happens that a couple of them are in the infrared where JWST could see it. 

Singly ionized argon — atoms with a single electron stripped away — glows at a wavelength of around 7 microns (roughly nine times longer than what the human eye can see), and argon atoms with 5 electrons stripped off glows at 4.5 microns, and are predicted to be telltale signs of a neutron star in the supernova. And whaddyaknow, right smack dab in the center of the explosion light at those wavelengths were seen! They were also Doppler shifted due to the debris’ expansion speed, at just the rate you’d expect. Weaker sulfur emission is also seen, which is also predicted [link to research paper].

A black hole wouldn’t produce argon and sulfur emission like this, so it’s pretty likely what we’re seeing is the effect of a super-hot neutron star irradiating the gas. Exactly how it’s doing it isn’t clear; it might just be through the intense UV radiation bombarding the gas. Another possibility is it’s being heated by a pulsar wind. A rapidly spinning highly magnetized neutron star sweeps up material around it and flings it away at high speed, creating two lighthouse-like beams of energy emanating from it. When these beams pass over Earth we see a very short blip of light every time the neutron star spins, which can be many dozens of times per second. Hence the term pulsar, from the pulses of light. The pulsar winds up blowing out a stream of material from it called a pulsar wind (we see this in the famous Crab Nebula, for example) and this could also be the energy source of the infrared emission JWST saw. 

But either way: Neutron star.

That’s super cool. This has been an outstanding mystery for nearly four decades now, and to see it very likely solved, or at least the first big piece of the puzzle laid into place, is very exciting, and, I’ll add, very satisfying. 87A was nothing but a big pile of enigmas when I was studying it, so getting some answers, even after all this time, is pretty nice. 

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