The Webb Telescope Just Solved a 37-Year-Old Space Explosion Mystery

SN 1987a is the only supernova that's been visible with the unaided eye in the last 400 years, and we're only now figuring out what's at its center.

David Malin Anglo Australian Telescope.

Astronomers now know what became of a star that exploded in 1987, close enough for the bright light of the supernova to be visible in the night sky.

With Betelgeuse stubbornly refusing to explode anytime soon, our best chance to study the explosive death of a massive star is SN 1987a, a rapidly expanding cloud of gassy, dusty cosmic debris 160,000 light years away. SN 1987a is the only supernova that’s been visible with the unaided eye in the last 400 years, and astronomers have spent decades watching its aftermath unfold. But so far, astronomers haven’t been able to spot the burned-out, collapsed core of the dead star that should be slowly cooling in the center of the spreading debris cloud.

It turns out that the dying star left behind a densely-packed ball of neutrons called a neutron star, and it’s blasting powerful UV and X-ray radiation out into the surrounding space, stripping electrons from the last atoms the dead star fused in its core. The infrared light emitted from those atoms revealed the neutron star’s presence to Stockholm University astrophysicist Claes Fransson and his colleagues, who published their findings in the journal Science.

This image combines a photo from the Hubble Space Telescope with data from JWST’s NIRSpec instrument (the blue dot in the center is highly ionized argon gas spotted by JWST).

HST, JWST/NIRSpec, J. Larsson

Live Fast, Die Young, Leave an Extremely Dense Corpse

Fransson and his colleagues used the James Webb Space Telescope’s (JWST) Mid-Infrared Instrument (MIRI) and Near Infrared Spectrometer (NIRSpec) to measure the spectrum of light coming from the center of the supernova’s debris cloud, near where the star would have once been. (A spectrum is a rainbow-like map of all the individual wavelengths of light coming from an object; each chemical element emits and absorbs different wavelengths of light, so an object’s spectrum can tell scientists what it’s made of.) Based on that data, the researchers found that high-energy radiation had stripped away electrons from atoms of sulfur and argon, creating positively charged ions that shone brightly in infrared.

It was as if the astronomers were looking at the glowing fingerprint of a neutron star.

In the 37 years since SN 1987a appeared in the night sky, astronomers have watched and measured how the bright flare of the explosion slowly dimmed and the debris cloud spread outward from the site of the cosmic cataclysm. But so far, no one has been able to see into its center to learn exactly what kind of stellar corpse the massive star left behind.

Like forensics on a galactic scale, astronomers used the size and contents of the debris cloud to reconstruct some basic information about the star. It was once a blue supergiant: a bright, hot star about 15 to 20 times the mass of our Sun. During the supernova, what was left of the star’s core should have collapsed in a neutron star: a ball of matter packed so tightly that its original atoms don't even have room to be atoms any more, only neutrons. Neutron stars are some of the densest objects in the universe; a single tablespoon scooped from one would contain as much matter as Mount Everest.

But the details are harder to predict. It's possible that, in the wake of the supernova, some of the gas from the old star fell back onto the still-hot surface of the neutron star, adding just enough mass to make the neutron star collapse even further under its gravity, spawning a black hole.

Astronomers haven’t been able to piece together the details of the supernova’s aftermath because dust from the explosion has shrouded the area around the stellar corpse, making it impossible to get a clear look at the neutron star or black hole lurking at the heart of the debris cloud. So although astronomers expected to find a neutron star at the center of SN 1987a, this is the first direct confirmation that their predictions were right.

On the right is the doomed blue giant star just before the February 1987 supernova, and on the left is the star just afterward.

David Malin Anglo Australian Telescope.

Cosmic Crime Scene Investigation

Sulfur and argon would have been among the last elements that the dead star created before its final explosive end. Both elements are formed by fusing silicon and oxygen atoms deep in the heart of a massive star, so their presence was a clue that astronomers were close to the heart of the cosmic crime scene. And the fact that these elements had been stripped of their electrons was a clue about what lurked within.

It takes powerful ultraviolet and X-ray radiation to strip away electrons and turn ordinary argon and sulfur atoms into irradiated ions. When Fransson and his colleagues used computer simulations to see what sort of object could produce enough radiation to create the ions JWST saw, only two explanations fit. And both of them required a neutron star, not a black hole, whose effect on the nearby gas would be “much weaker at this time,” Fransson tells Inverse.

One option is a neutron star that’s still very hot, but slowly cooling. Its surface would be about a million degrees Fahrenheit (which is blazing hot but also much cooler than the billion degrees it would have boasted in the moments after the star collapsed). The hot neutron star would glow brightly with ultraviolet light and X-rays — enough to ionize nearby argon and sulfur gas.

On the other hand, the neutron star might be a pulsar: a neutron star that spins wildly on its axis, blasting beams of radiation and electrically charged particles from its poles like a lighthouse. Those charged particles get fired off into space at nearly the speed of light, accelerated by the pulsar’s powerful magnetic field, and they could also ionize the surrounding gas.

That lines up well with other evidence that points to a neutron star: a burst of tiny, fast-moving particles called neutrinos that reached Earth shortly before the supernova’s light. Previous studies predicted that a neutron star should create the right kind of radiation to ionize heavy elements like argon and sulfur. More data from JWST and other telescopes could help narrow down exactly what’s going on at the heart of SN 1987a.

At the same time, our view of the dead star should get clearer as time passes and the debris keeps moving away from the site of the stellar explosion.

“It will become less dense and less absorbing,” says Fransson, though it’s hard to predict exactly how long that will take or how clear the view will be. “It depends on how clumpy the ejecta is and if there may be directions which are free from absorbing dust and gas.”

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