SN 1987A was a Type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately 51.4 kiloparsecs (168,000 light-years) from Earth and was the closest observed supernova since Kepler's Supernova in 1604. Light and neutrinos from the explosion reached Earth on February 23, 1987, and it was designated "SN 1987A" as the first supernova discovered that year. Its brightness peaked in May of that year, with an apparent magnitude of about 3, brighter than the constellation's brightest star, Alpha Doradus.
It was the first supernova that modern astronomers were able to study in great detail, and its observations have provided much insight into core-collapse supernovae. SN 1987A provided the first opportunity to confirm by direct observation the radioactive source of the energy for visible light emissions, by detecting predicted gamma-ray line radiation from two of its abundant radioactive nuclei. This proved the radioactive nature of the long-duration post-explosion glow of supernovae.
In 2019, indirect evidence for the presence of a collapsed neutron star within the remnants of SN 1987A was discovered using the Atacama Large Millimeter Array telescope. Further evidence was subsequently uncovered in 2021 through observations conducted by the Chandra and NuSTAR X-ray telescopes.
SN 1987A was discovered independently by Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987, and within the same 24 hours by Albert Jones in New Zealand.
Later investigations found photographs showing the supernova brightening rapidly early on February 23. On March 4–12, 1987, it was observed from space by Astron, the largest ultraviolet space telescope of that time.
Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak −69 202 (Sk −69 202), a blue supergiant.
After the supernova faded, that identification was definitively confirmed, as Sk −69 202 had disappeared. The possibility of a blue supergiant producing a supernova was considered surprising, and the confirmation led to further research which identified an earlier supernova with a blue supergiant progenitor.
Some models of SN 1987A's progenitor attributed the blue color largely to its chemical composition rather than its evolutionary stage, particularly the low levels of heavy elements. There was some speculation that the star might have merged with a companion star before the supernova. However, it is now widely understood that blue supergiants are natural progenitors of some supernovae, although there is still speculation that the evolution of such stars could require mass loss involving a binary companion.
Approximately two to three hours before the visible light from SN 1987A reached Earth, a burst of neutrinos was observed at three neutrino observatories. This was likely due to neutrino emission which occurs simultaneously with core collapse, but before visible light is emitted as the shock wave reaches the stellar surface. At 7:35 UT, 12 antineutrinos were detected by Kamiokande II, 8 by IMB, and 5 by Baksan in a burst lasting less than 13 seconds. Approximately three hours earlier, the Mont Blanc liquid scintillator detected a five-neutrino burst, but this is generally believed to not be associated with SN 1987A.
The Kamiokande II detection, which at 12 neutrinos had the largest sample population, showed the neutrinos arriving in two distinct pulses. The first pulse at 07:35:35 comprised 9 neutrinos over a period of 1.915 seconds. A second pulse of three neutrinos arrived during a 3.220-second interval from 9.219 to 12.439 seconds after the beginning of the first pulse.
Although only 25 neutrinos were detected during the event, it was a significant increase from the previously observed background level. This was the first time neutrinos known to be emitted from a supernova had been observed directly, which marked the beginning of neutrino astronomy. The observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in the form of neutrinos. The observations are also consistent with the models' estimates of a total neutrino count of 1058 with a total energy of 1046 joules, i.e. a mean value of some dozens of MeV per neutrino. Billions of neutrinos passed through a square centimeter on Earth.
The neutrino measurements allowed upper bounds on neutrino mass and charge, as well as the number of flavors of neutrinos and other properties. For example, the data show that the rest mass of the electron neutrino is < 16 eV/c2 at 95% confidence, which is 30,000 times smaller than the mass of an electron. The data suggest that the total number of neutrino flavors is at most 8 but other observations and experiments give tighter estimates. Many of these results have since been confirmed or tightened by other neutrino experiments such as more careful analysis of solar neutrinos and atmospheric neutrinos as well as experiments with artificial neutrino sources.
SN 1987A appears to be a core-collapse supernova, which should result in a neutron star given the size of the original star. The neutrino data indicate that a compact object did form at the star's core, and astronomers immediately began searching for the collapsed core. The Hubble Space Telescope took images of the supernova regularly from August 1990 without a clear detection of a neutron star.
A number of possibilities for the "missing" neutron star were considered. First, that the neutron star may be obscured by surrounding dense dust clouds. Second, that a pulsar was formed, but with either an unusually large or small magnetic field. Third, that large amounts of material fell back on the neutron star, collapsing it further into a black hole. Neutron stars and black holes often give off light as material falls onto them. If there is a compact object in the supernova remnant, but no material to fall onto it, it would be too dim for detection. A fourth hypothesis is that the collapsed core became a quark star.
In 2019, evidence was presented for a neutron star inside one of the brightest dust clumps, close to the expected position of the supernova remnant. In 2021, further evidence was presented of hard X-ray emissions from SN 1987A originating in the pulsar wind nebula. The latter result is supported by a three-dimensional magnetohydrodynamic model, which describes the evolution of SN 1987A from the SN event to the present, and reconstructs the ambient environment, predicting the absorbing power of the dense stellar material around the pulsar.
In 2024, researchers using the James Webb Space Telescope (JWST) identified distinctive emission lines of ionized argon within the central region of the Supernova 1987A remnants. These emission lines, discernible only near the remnant's core, were analyzed using photoionization models. The models indicate that the observed line ratios and velocities can be attributed to ionizing radiation originating from a neutron star illuminating gas from the inner regions of the exploded star.
After an explosion of a Type II supernova such as SN 1987A, much of the light is produced by the energy from radioactive decay. The radioactivity keeps the remnant hot enough to glow, without which the remnant would dim quickly. The decay of 56Ni through its daughters 56Co to 56Fe produces gamma-ray photons that are absorbed and dominate the heating and thus the luminosity of the ejecta at intermediate times (several weeks) to late times (several months). Energy for the peak of the light curve of SN1987A was provided by the decay of 56Ni to 56Co (half life of 6 days) while energy for the later light curve in particular fit very closely with the 77.3-day half-life of 56Co decaying to 56Fe. Later measurements by space gamma-ray telescopes of the small fraction of the 56Co and 57Co gamma rays that escaped the SN1987A remnant without absorption confirmed earlier predictions that those two radioactive nuclei were the power source.