Has Supernova 1987A's elusive neutron star finally been found?

In late February 1987, the light from the closest supernova in four centuries reached Earth. It had traveled for 167,000 years when it was spotted, dimmed to a blip of light in the southern sky. Still, Supernova 1987A was the first naked eye supernova seen in modern times, and would become one of the most studied objects in the sky.

A lot of surprises were in store for astronomers, one of which is that the star that blew up wasn't a red supergiant, which is what everyone assumed it had to have been. Massive stars swell up and cool at the ends of their lives, and models indicated that when the core collapsed, triggering the supernova, the outer layers would still be swollen and red. But when the light from the explosion dimmed, and that area of the sky compared to older maps, the only star missing was a blue supergiant. So right away, this supernova was throwing curveballs.

One of those tricky pitches may finally be caught, though. The star that exploded — Sanduleak -69 202 — was born with a mass of around 19 times that of the Sun, and over its lifetime shed about 5 solar masses into space around it. Stars like that aren't quite massive enough to leave a black hole behind when they explode, so everyone expected to find a neutron star in is wake. These are incredibly dense, hot, and rapidly spinning objects, the mass of more than the entire Sun compressed down into a sphere only a dozen or so kilometers wide. They have powerful magnetic fields when they're born, stirring up material around them, so they announce their presence without much subtlety.

But… no neutron star was seen in the expanding debris from the supernova. There were a couple of false alarms early on, but even after 30 years of intense searching no compact object was seen there.

The possibly location of the neutron star left over from Supernova 1987A is marked by a black square, just to the upper right of the actual center of the ring (white square) where the explosion occurred. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis (annotated by Phil Plait)

This may finally have changed. Using the Atacama Large Millimeter/submillimeter Array (ALMA), astronomers have announced they've found evidence of the elusive beast. There's no "Here be a neutron star" sign or anything so obvious. Instead, what they found was a patch of warm dust near the explosion site. I know, it doesn't sound like much, does it? But by all accounts, that blob shouldn't be there, and its presence is best explained by a hot, young, neutron star warming up material around it.

The expanding debris from the star’s explosion is thick and absorbs almost all the light coming from within it. Some of that is from the heavy elements created in the fierce heat and pressure of the explosion, but a lot of it is from cosmic dust. This material is usually composed of either long carbon-based molecules (essentially soot) or silicaceous (rocky) grains. Both are fairly opaque, making clouds of them difficult if not impossible to see through.

Zooming in on the expanding supernova debris, an ALMA image of warm dust in Supernova 1987A shows the possible neutron star location (cyan loop) to the right of center of the ring (plus symbol). The scale bar denotes 1.5 trillion km, about 1/6th of a light year. Credit: Cigan et al. 

But we can see the dust itself, glowing in very long wavelengths, and the astronomers found the brightish blob pretty close to the center of the nebula. The thing is, looking at the distribution of heavy elements around the explosion site (which can give an indication of where dust should be) a blob of dust at that spot shouldn't be so warm. They went through a handful of other possibilities for what it might be, but conclude that the most likely possibility is that the heat source is the long-sought neutron star, still glowing from its formation 32 years ago (as we see it).

Interestingly it's not at the exact center of the supernova remnant! But that's not too surprising. Sometimes, the explosion of the star's core isn't perfectly symmetric, and instead is off-center in the core. This means there's a force to the side, like a rocket nozzle, that gives a hellacious kick to the compact object formed. Even though the object, in this case a neutron star, might weigh over an octillion tons (!!), it can be shot away from the supernova site at hundreds of kilometers per second.

Supernova explosions are powerful.

So this blob being off-center isn't a deal breaker. It's a little bit farther than expected, but still within the expected range of where it could be after this much time given typical kick velocities. It's moving in the right direction (northwest) as predicted by models, too, given the way the star ejected matter into space.

With all this, does this mean the neutron star has finally been seen for sure? This evidence is pretty dang good, but unfortunately it's not direct and conclusive. It's certainly self-consistent, and quite compelling, though. It does seem likely they've zeroed in on where it's been hiding all these years, but we'll need more direct evidence of it to be sure.

That might not take too long, though. The team provided evidence that the neutron star has entered what's called the pulsar wind nebula phase, where the powerful magnetic fields of the star are whipping up and accelerating particles away from it at nearly the speed of light. This is the kind of subatomic particle wind that powers quite a bit of the energy of the Crab Nebula, the expanding debris from a much closer supernova (6,500 light years away) the light from which reached us in the year 1054. If SN87A's neutron star has reached this stage, then it's possible that will energize the dust or even start to push it away from the neutron star, which will make its existence more obvious.

Three ring circus: The hourglass-shaped structure of gas around Supernova 1987A existed long before the star exploded, and was lit up by the event. The expanding supernova debris is in the center of the inner ring. Credit: X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis

I studied SN87A for my PhD way back when, not long after Hubble Space Telescope launched, back when we still understood very little about the supernova. Heck, my work was just categorizing what we saw in the images, and how the different parts of the gaseous structures around the supernova were behaving. That was difficult, since Hubble didn't focus well back then (subsequent servicing missions installed cameras that corrected for the incorrectly ground primary mirror).

Still, it was exciting to be a small part of the effort to figure out what the heck this weird exploding star did, and it's always fun to see a big paper come out that pushes our knowledge even further. There is still a lot we don't know about this event, so I expect to be reading a more papers about it in the future.