For 30 years, scientists have hunted for elusive quarry in the sizzling ashes of the most famous supernova in recent history. On February 23, 1987, astronomers watched in awe as a star in a nearby galaxy exploded, the closest known instance of a supernova in the last 400 years. Around the world, stargazers immediately trained their telescopes in the dying star’s direction, expecting to see a new object—a neutron star—born in the supernova’s wake. But those early searches were fruitless, as were all those in the decades that followed, leaving experts disappointed and puzzled. Now, however, some observers say they have spotted telltale evidence of the “missing” neutron star, lurking in the still-cooling stellar debris, giving scientists what may be a once-in-many-lifetimes opportunity to study the moments before and after a star’s cataclysmic demise up-close, in breathtaking detail.
In a paper published in the Astrophysical Journal, the team, led by Phil Cigan from Cardiff University in the United Kingdom, peered through the dust to glimpse the object. One of the key impediments to studying the supernova, called SN 1987A, is that it left behind a vast veil of dust about half the mass of our sun, obscuring where the star had once been. But analyzing archival data from 2015 gathered by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, the team spotted emissions from this dust suggesting the presence of something hidden within. “We’re able to see that there’s this little bright region, this little patch of dust, that is consistent with being at a hotter temperature than the stuff around it,” says Cigan. “And that happened to line up exactly with where we thought the neutron star should be.”
SN 1987A, located 163,000 light-years from Earth in the Large Magellanic Cloud dwarf galaxy—an astronomical stone’s throw away—was a core-collapse supernova. This is when a star collapses under its own weight after using up its nuclear fuel—a process that results in a gigantic shock wave as the plummeting material piles up and rebounds outwards. The end results of this catastrophe are the densest astrophysical objects known: a neutron star that squeezes several solar masses into a city-sized orb, or something even more extreme, a black hole. Around this remnant, ejected material expands out into writhing, majestic filaments and sheets. SN 1987A is no exception, with bright and beautiful eccentric rings of dust set aglow by the rippling shockwaves.
While astronomers have studied many supernovae before, SN 1987A is unique due to its proximity. Before this explosion, experts had never observed a supernova’s progenitor star in such detail; even before it was famous, astronomers knew it to be about 20 times the mass of our sun, a size that would presumably result in the formation of a neutron star. Were astronomers to find the remnant, it would be the first time they had glimpsed the moments directly before and after a supernova, says Danny Milisavljevic from Purdue University in Indiana, allowing studies of these events in greater detail than ever before. “We have a cartoon notion that a star goes through successive stages of nuclear fusion, the core collapses down to a neutron star, and there’s a bounce shock that passes through the outer envelope of the star,” he says. “But there’s a lot of details in there we don’t understand.”
Astronomers had long suspected SN 1987A left behind a neutron star after the Nobel Prize–winning discovery of neutrinos—subatomic particles with no charge and very little mass—arriving from the supernova on Earth the same day its light arrived. However, the lack of a direct detection of the object despite efforts to see it with the world’s best telescopes led some to wonder if maybe theoretical models of supernovae were wrong, and some other process had taken place. “It had been suggested it maybe became a black hole,” says Mikako Matsuura from Cardiff University, a co-author on the paper. “There was a lot of discussion as to why we can’t find a neutron star.”
In some respects, however, this “detection” remains as nebulous as SN 1987A’s enveloping shroud. Without being able to see the neutron star directly inside its cocoonlike clump of gas, nicknamed “The Blob” by the team, no one can prove beyond a shadow of a doubt it is there and not just, for example, a mirage produced by a particularly dense collection of dust. To find out for certain, astronomers are hoping that in the coming years and decades the dust will begin to clear, letting them truly peer inside for the first time, but even that remains uncertain. “There’s evidence that dust forms in the center,” says Robert Kirshner from Harvard University, who has time on Hubble to observe SN 1987A again in 2020. “We expect it to clear, but it may be that’s not the only possibility. It could be that the dust will continue to form and continue to obscure our view.”
What is clear, however, is that the fascination with this supernova is unlikely to abate any time soon. Armed now with the best-yet evidence for the existence of a neutron star there, astronomers will be watching in earnest, hoping to observe the aftermath of a supernova from the front row in this vast cosmic theater. “We see many supernova explosions happening every year, but usually they’re much further away,” says Cigan. “This is really the only supernova that we can see in real time.”