It was a young, hot, bright blue star, what they call a blue supergiant, twinkling in the dwarf galaxy known as the Large Magellanic Cloud. But there are many such stars, and there was nothing terribly unusual about it. No reason to give it a second look.
Until it blew.
Supernova 1987A was the brightest exploding star seen in the past 400 years. It was literally a blast from the past: The light from the supernova “blazed with the power of 100 million suns,” as NASA put it, and traveled for 160,000 years before reaching Earth and thrilling astronomers on Feb. 23, 1987. The new “star” in the night sky could be seen with the naked eye for months before fading.
In the decades since, scientists have wondered what, exactly, was left behind in that violent explosion. Theorists advanced two possibilities: a black hole or an ultradense object known as a neutron star.
A new paper published Thursday in the journal Science, based on observations by the James Webb Space Telescope, claims to have resolved the debate, saying there are compelling signs of a neutron star hiding in the explosion’s debris field.
Neutron stars have been identified before, but, if the latest report holds up to further scrutiny, this is by far the youngest, freshest such object even seen. It is still cooling down. For astrophysicists, it’s like being present at the creation of something rare and exotic.
“Neutron stars are common, but we haven’t seen any one being born from a star,” said Claes Fransson, an astronomer at Stockholm University and lead author of the new report.
Fransson remembers the day he heard about Supernova 1987A, initially assuming it was a joke. It turned out to be not only real, but also relatively close and readily observable. Fransson later recalled in an email how he saw it with his own eyes, standing atop Mount Kinabalu in Borneo, high above the rainforest: “It is a different feeling to see the [supernova] live, compared to just seeing images of it!”
Supernova 1987A quickly became the most studied event of its kind, and the repercussions spread beyond just the inquiry into exploding stars. Scientists view this explosion as a natural laboratory for high-energy physics, dense with information about the laws of nature in extreme environments.
No terrestrial laboratory could sustain the kind of temperatures and pressures created by a supernova, said Hans-Thomas Janka, a physicist at the Max Planck Institute for Astrophysics in Germany who was not part of this new research.
That’s why evidence of a neutron star in Supernova 1987 is “super-important for nuclear physics and particle physics,” Janka said.
The top suspect
Astronomers have documented supernovae for nearly a thousand years. The Crab nebula is the remnant of a supernova recorded by Chinese astronomers in 1054. Danish astronomer Tycho Brahe observed a supernova in 1572, and German astronomer Johannes Kepler documented one in 1604 that was so bright it could be seen in daytime.
But the supernova of 1987 was unique, an instant superstar in the field of modern astronomy. “Kepler’s Star” had come out of the cosmic nowhere, but Supernova 1987 happened to a star that had already appeared in a catalogue of stars (listed as “Sanduleak−69 202”).
This supernova gave the science community an unprecedented view of what happens before, during and after a giant star’s violent death.
Even back then, astronomers suspected that Supernova 1987A left behind a neutron star, an object made of the densest matter in the universe. You would not want to put neutron star-stuff into your bread dough because half a cup of it would weigh as much as Mount Everest.
The violent death of the blue supergiant star created a burst of neutrinos that reached Earth in advance of the visible signs of an explosion. Three neutrino detectors registered the burst for several seconds. This observation fit with theoretical models suggesting the supernova resulted from the collapse of the core of a giant star.
Neutrinos are subatomic particles emitted as a neutron star forms. Gravity is the driver of the process, compressing matter into an ultradense core that can’t compress further. As matter continues to fall toward the center of the star, the temperature and pressure keep rising until conditions get so extreme they send a shock wave outward. The compressed center remains intact, but the rest of the star explodes, creating a vast debris field.
Scientists have long thought that the mass of the original star influences what’s left after all this violence. Very large star: black hole. Smaller star: neutron star. But theorists are still debating this, Janka said, and size is not the only thing that matters.
In the case of the blue supergiant, astrophysicists estimate that it was roughly 15 to 20 times the mass of our sun, and, according to their rule of thumb, that’s a bit undersized for the creation of a black hole. Thus a neutron star has always been the favored candidate for a remnant.
The new report leveraged two instruments on the Webb telescope that can characterize different wavelengths of infrared light coming from the debris field. The neutron star emits much of its radiation in the X-ray portion of the spectrum. The authors of the report say that the excitation of elements in the debris field, including argon and sulfur, can only be explained by X-ray and ultraviolet radiation from a neutron star.
Born with a kick
Even this report, though, does not claim a direct detection of the hypothesized neutron star. The existence of the object has only been inferred. If it’s there, it’s hidden in the debris field.
Still, the report is a major advance in understanding the supernova, said Stanford Woosley, an astrophysicist at the University of California at Santa Cruz who was not part of the research team.
Woosley said he was stuck by one finding in the new paper: The source of the X-ray emissions (the neutron star, presumably) is not precisely in the center of the debris field. It’s offset. That suggests that the explosion was not perfectly spherical but had enough asymmetry to give the neutron star a “kick,” sending it careening through space at hundreds of kilometers per second.
“This would be the first time we have actually seen the neutron star born with a kick,” he said, quickly adding that we’ve never actually seen a neutron star born under any circumstances.
He said the new data is compelling, if not quite 100 percent conclusive of the presence of a neutron star. He said the evidence has reached a tipping point, and “astronomers are ready to declare a victory.”
But the case is not yet closed, he said: “It’s going to be hard to declare a decisive observation of the neutron star when you can’t just go there and see it.”
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