On 23 February 1987, light that had been travelling for roughly 160,000 years reached Earth from the Large Magellanic Cloud. The blue supergiant Sanduleak -69° 202 had torn itself apart, producing the nearest naked-eye supernova seen in centuries. At Las Campanas Observatory in Chile, University of Toronto graduate student Ian Shelton noticed the new object while developing photographic plates and then confirmed it with his own eyes.
The star’s core collapsed in less than a second, shrinking from roughly the diameter of Earth to a compact object comparable in size to a city. Theory indicated that the remnant should be either a neutron star or a black hole. Nearly four decades later, the James Webb Space Telescope has detected the effects of high-energy radiation from what appears to be the neutron star left behind.
A supernova next door
SN 1987A was the closest naked-eye supernova since Johannes Kepler observed one in the constellation Ophiuchus in 1604. The Large Magellanic Cloud is a satellite galaxy of the Milky Way, but astronomers consider it to be outside our galaxy. That is why the Milky Way’s own visible-supernova drought technically continues.
The supernova blazed for months in the far-southern constellation Dorado. A NASA retrospective says it radiated with the power of about 100 million suns for several months after its discovery. Australian astronomer Robert McNaught and New Zealand amateur astronomer Albert Jones independently recorded the event within roughly a day.
Pre-explosion photographs identified the progenitor as Sanduleak -69° 202, a B3 blue supergiant estimated at about 20 solar masses. The identification surprised astronomers because red supergiants had been considered the expected progenitors of such core-collapse explosions. In later images of the same field, Sanduleak -69° 202 was gone.
The ghost particles that arrived first
Roughly two to three hours before the first visible-light observation, neutrino detectors in Japan, Ohio and Russia recorded a brief pulse of particles from the collapsing core. Kamiokande II recorded 12 events, the Irvine-Michigan-Brookhaven detector recorded eight and the Baksan observatory recorded five. That produced a total of 25 recorded neutrino events, not merely about a dozen.
The neutrinos escaped from the collapsing core almost immediately, while the shock wave needed hours to travel through the star’s outer layers. Their detection provided the first observation of neutrinos from beyond the solar system and supported long-standing models of core-collapse supernovae. It also showed that a compact object had formed, although it did not settle whether that object remained a neutron star or subsequently became a black hole.
Why the remnant hid for so long
The explosion left behind a three-ring structure surrounding a turbulent cloud of debris. Hubble began observing SN 1987A in 1990, while Chandra followed in X-rays after its 1999 deployment. ALMA later mapped the cold dust and molecules in the remnant’s obscured centre.
As supernova ejecta cool, newly formed atoms can condense into dust grains. That dust is scientifically valuable because it helps astronomers understand how stellar explosions disperse planet-building material, but it has also blocked a clear view of the compact object. For decades, instruments could study the remnant without directly seeing what lay at its centre.
The first hints, and then Webb
High-resolution ALMA observations provided an important clue when they revealed an unusually warm blob of dust in the core of the remnant. The feature was brighter than its surroundings and appeared close to the expected position of the neutron star. It remained indirect evidence because another energy source could potentially have heated the dust.
In 2021, observations from Chandra and NuSTAR strengthened the case. The telescopes detected high-energy X-ray behaviour consistent with a possible pulsar wind nebula, although particle acceleration by the expanding blast wave remained another explanation. Chandra reported that the combined data supported the possible presence of a pulsar wind nebula at the centre of the ring.
The decisive new evidence came from Webb observations analysed by an international team led by Claes Fransson of Stockholm University and published in 2024. Using MIRI and NIRSpec, the researchers detected emission from strongly ionized argon and other elements in the innermost ejecta. NASA described this as the first detection of the effects of high-energy emission from the probable young neutron star.
The distinction matters. Webb did not resolve a 20-kilometre neutron star as a discrete object in an ordinary image. It detected emission lines from surrounding gas that require a central source of energetic photons, providing a spectral fingerprint of the hidden compact object.
What Webb actually saw
Webb’s observations showed signals from singly and multiply ionized argon near the centre of the ejecta. Producing these ions requires energetic ultraviolet or X-ray photons. The models examined by Fransson’s team found that the most plausible scenarios involved a newly formed neutron star, either radiating as a hot cooling object or powering a pulsar wind nebula.
Open Access Government reported that the NIRSpec observations revealed increasingly ionized elements, indicating highly energetic radiation at the heart of the remnant. The evidence therefore favours a neutron star, but it should not be described as a direct photograph of one.
A separate Webb near-infrared image released in 2023 revealed a central keyhole-shaped structure and two faint crescents that earlier telescopes had not resolved. The equatorial ring contains bright hot spots created as the supernova’s shock wave struck material expelled by the progenitor thousands of years before the explosion. Sci.News described the crescents as probable outer layers of gas expelled by the supernova.
Earlier measurements showed two central debris clumps moving apart at roughly 20 million miles per hour. The equatorial ring is about one light-year across and was present at least 20,000 years before the explosion. Ultraviolet light from the supernova initially energized the ring, while later collisions with the expanding shock wave produced its changing pattern of bright hot spots.
Why 37 years matters
The interval between the 1987 neutrino burst and the 2024 Webb result spans much of an astronomer’s working career. In a NASA retrospective, astrophysicist Robert Kirshner said decades of observations provide insight into the final stages of stellar evolution. Few supernovae have been observed with such a succession of modern instruments from the opening hours onward.
Kepler’s supernova of 1604 was observed centuries before photography. Cassiopeia A probably exploded in the late 17th century, but whether John Flamsteed actually recorded the event remains disputed. SN 1987A is different because instruments were ready to follow its evolution across much of the electromagnetic spectrum.
Hubble began its observations in 1990, Chandra followed after launching in 1999, ALMA added detailed millimetre and submillimetre measurements, and Webb began science operations in 2022. Each observatory encountered the remnant at a different stage. Together they recorded the ring brightening, the blast wave moving into thinner gas and the obscured interior finally revealing evidence of its energy source.
The Nancy Grace Roman Space Telescope has now joined that succession of major observatories. Named for NASA’s first chief of astronomy, Roman launched aboard a SpaceX Falcon Heavy on August 30, 2026 and is travelling toward the Sun-Earth L2 region. Its wide-field infrared surveys will complement the more narrowly targeted observations made by Hubble and Webb.
The city-sized corpse
If the interpretation of the Webb spectra is correct, a neutron star roughly 20 to 25 kilometres across lies inside the ejecta. Such an object can contain more mass than the Sun while occupying a volume smaller than a large city. The observations do not yet establish its exact rotation rate or magnetic-field strength.
One possible interpretation is that the object is a hot, cooling neutron star. Another is that it is a pulsar generating a wind of energetic particles, but the present evidence does not justify describing that scenario as settled. Additional Webb and ground-based observations will be needed to distinguish between the remaining models.
The surrounding remnant is still evolving. The blast wave has moved beyond much of the dense equatorial ring and into thinner material whose structure depends on the progenitor’s earlier winds. Continued monitoring will show how the shock, dust and newly formed elements change as the remnant expands.
The light from Sanduleak -69° 202 travelled for approximately 160,000 years before reaching Earth on that February night in 1987. Since then, successive generations of instruments have extracted information that was present but initially impossible to read. Webb has not photographed the neutron star itself, but it has found the clearest evidence yet of energetic radiation emerging from its hiding place.
Somewhere inside the keyhole of glowing gas, a compact object is influencing the atoms around it. Webb has revealed its spectral signature. Future observations will try to determine precisely what the young neutron star is doing in the dark.
