Supernova 1987a was a supernova in the Large Magellanic Cloud, a nearby dwarf galaxy. It occurred approximately 50 kiloparsecs from Earth, the closest supernova since Supernova 1604, which occurred in the Milky Way itself. The light from the supernova reached Earth on February 23, 1987. As the first supernova discovered in 1987, it was labeled "1987a." Its brightness peaked in May with a apparent magnitude of about 3 and slowly declined in the following months. It was modern astronomers' first opportunity to see a supernova up close.
Since 50 kiloparsecs is approximately 164,000 light-years, the cosmic event itself happened approximately 164,000 years ago.
It is a source of regret to astrophysicists and particle physicists that two particular experiments were not done. First of all, the energy spectra of the neutrinos could have been measured if more sensitive neutrino detectors had been available. Secondly, if the clocks of two neutrino observatories had been synchronised, it would have been possible to measure the time the burst took to travel between them, and thus to determine whether the neutrinos were travelling at the speed of light (like massless particles) or slower than the speed of light (like massive particles). Unfortunately, while the detector in one of the laboratories was synchronized to an atomic clock, the detector in the other laboratory was not, and this experiment was impossible.
The precursor to SN 1987a was a blue supergiant presumed to have a mass of about 20 solar masses. This required some revisions to models of high mass stellar evolution, which had suggested that supernova would result from red supergiants.
The supernova remnant formed by debris from SN 1987a is one of the most-studied astronomical objects today.