Using the James Webb Space Telescope (JWST), astronomers have ended a nearly decade-long game of hide-and-seek in the sky after discovering a neutron star in the wreckage of a stellar explosion.
Supernova 1987A represents the remains of an exploded star that once had a mass about 8 to 10 times that of the Sun. It is located about 170,000 light-years away in the Large Magellanic Cloud, a dwarf galaxy that neighbors the Milky Way. Supernova 1987A was first noticed by astronomers 37 years ago in 1987, hence the numerical aspect of the name. When it exploded, Supernova 1987A first showered Earth with ghostly particles called neutrinos and then became visible in bright light. This made it the closest and brightest supernova seen in the night sky above Earth for about 400 years.
Supernova explosions like this one are responsible for seeding the cosmos with elements like carbon, oxygen, silicon and iron. These elements ultimately become the building blocks of the next generation of stars and planets, and can even form molecules that could one day become an integral part of life as we know it. These explosions also create compact stellar remnants, either in the form of neutron stars or black holes; For 37 years, astronomers have not known which of these lurk at the heart of Supernova 1987A.
“We have been looking for evidence of a neutron star in the gas and dust of Supernova 1987A for a long time,” Mike Barlow, professor emeritus of physics and astronomy and part of the team behind this discovery, told Space.com. “We finally have the evidence we were looking for.”
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How does a neutron star hide for four decades?
Neutron stars are born when massive stars deplete the fuel supplies needed for nuclear fusion in their cores. This cuts off the outward energy flowing from the cores of these stars and protecting them from collapsing under their own gravity.
As a star’s core collapses, massive supernova explosions rip through the star’s outer layers, blowing them away. This leaves behind a ‘dead’ star as wide as the average city here on Earth, but with a mass about one or two times that of the Sun; the star is ultimately composed of a fluid of neutron particles, the densest known matter in the universe.
However, neutron stars are protected from complete collapse by quantum effects that occur between neutrons in their interiors. These effects prevent the neutrons from accumulating. This so-called ‘neutron degeneracy pressure’ can be overcome if a stellar core has sufficient mass – or if a neutron star gains more mass after its creation. This would result in the birth of a black hole (however, if the mass minimum is not reached, this will not happen).
Scientists are fairly certain that the object in Supernova 1987A is a neutron star, but they couldn’t rule out the possibility that this recently deceased star, at least as we see it about 170,000 years ago, hadn’t accumulated the mass to transform itself into a black hole.
‘Another possibility was that the invading matter could have settled on the neutron star and caused it to collapse into a black hole. So a black hole was a possible alternative scenario,” Barlow said. “However, the spectrum that incident material produces is not the right type of spectrum to explain the emission we see.”
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The newly identified neutron star had not been discovered for 37 years because, as a newborn, it was still surrounded by a thick layer of gas and dust launched during the supernova explosion that heralded the death throes of its progenitor star.
“Detection was hampered by the fact that the supernova condensed about half a solar mass of dust in the subsequent years after the explosion,” Barlow said. “This dust acted as a screen-obscuring radion from the center of Supernova 1987A.”
The dust is much less effective at blocking infrared light than at blocking visible light. So to peer through this shroud of death and into the heart of Supernova 1987A, Barlow and colleagues turned to the JWST’s highly sensitive infrared eye, specifically the telescope’s mid-infrared instrument and near-infrared spectrograph.
The evidence for this hidden neutron star involved emissions of the elements argon and sulfur from the center of Supernova 1987A. These elements are ionized, which means that their atoms have had electrons stripped from them. Barlow said that this ionization could only have occurred due to radiation from a neutron star.
The emissions allowed the team to put a limit on the brightness, or luminosity, of the once hidden neutron star. They determined it to be about one-tenth the brightness of the Sun.
The team may have determined that Supernova 1987A produced a neutron star, but not all of the neutron star’s mysteries have been solved.
That’s because the ionization of argon and sulfur, which served as a smoking gun, could have been caused by a neutron star in two ways. Winds of charged particles swept up by a rapidly rotating neutron star and accelerated to near the speed of light may have interacted with surrounding supernova material, causing the ionization. Or the ultraviolet and X-ray light emitted from the hot neutron star’s million-degree surface could have taken electrons from the atoms at the heart of this stellar wreck.
If the first scenario is correct, then the neutron star at the heart of Supernova 1987A is actually a pulsar surrounded by a pulsar wind nebula. Pulsars are actually spinning neutron stars. However, if the latter scenario is the correct recipe for these emissions, this nearby supernova would have produced a ‘bare’ or ‘naked’ neutron star, whose surface would be directly exposed to space.
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Barlow suggested that researchers might be able to distinguish between a naked neutron star and a star covered in a pulsar wind nebula by making further infrared observations of the heart of Supernova 1987A with the JWST’s NIRSpec instrument.
“We have a program now collecting data that will obtain data at three to four times higher resolution in the near-infrared,” he concluded. “So by obtaining this new data we may be able to distinguish the two models that have been proposed to explain neutron star emission.”
The team’s research was published Thursday (Feb. 22) in the journal Science.