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SN 1987A
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PrecursorSoon after the event was recorded, the progenitor star was identified as Sanduleak -69° 202a, a blue supergiant. This was an unexpected identification, because at the time a blue supergiant was not considered a possibility for a supernova event in existing models of high mass stellar evolution. Current understanding is that the progenitor was a binary system, the stars of which merged about 20,000 years before the explosion, producing a blue supergiant. Difficulties persist with this interpretation.
Neutrino emissionsApproximately three hours before the visible light from SN 1987A reached the Earth, a burst of neutrinos was observed at three separate neutrino observatories. This is due to the neutrino emission (which occurs simultaneously with core collapse) preceding the emission of visible light (which occurs only after the shock wave reaches the stellar surface). At 7:35am Universal time, Kamiokande II detected 11 antineutrinos, IMB 8 antineutrinos and Baksan 5 neutrinos, in a burst lasting less than 13 seconds. Water-based instruments detect only antineutrinos of thermal origin, while a gallium-71-based instrument detects only neutrinos of either thermal or electron-capture origin.
Although the actual neutrino count was only 24, it was a significant rise from the previously-observed background level. This was the first time neutrinos emitted from a supernova had been observed directly, and the observations were consistent with theoretical supernova models in which 99% of the energy of the collapse is radiated away in neutrinos. The observations are also consistent with the models' estimates of a total neutrino count of with a total energy of joules.
One highly significant result was obtained from the data regarding gravity. It appeared that the neutrinos and antineutrinos both took the same amount of time to arrive at earth, about 168,000 years. The difference in their arrival times was less than 12 seconds. This was the first empirical evidence that matter, antimatter, and photons all react similarly to gravity, which had been widely predicted by standard theories of gravity but had not been previously shown from direct empirical data.
Missing neutron star?SN 1987A appears to be a core-collapse supernova, which should result in a neutron star. Since the supernova first became visible, astronomers have been searching for the collapsed core but have not detected it.
The Hubble Space Telescope takes sharpest images of the supernova regularly since August 1990. The images show no evidence of a neutron star. Two possibilities for the 'missing' neutron star are being considered. The first is that the neutron star is enshrouded in dense dust clouds so that it cannot be seen. The second is that large amounts of material fell back on the neutron star, so that it further collapsed into a black hole.
SN1987A distance and the speed of lightThe three bright rings around SN 1987A are material from the stellar wind of the progenitor. These rings were ionized by the ultraviolet flash from the supernova explosion, and consequently began emitting in various emission lines. These rings did not "turn on" until several months after the supernova, and the turn-on process can be very accurately studied through spectroscopy. The rings are large enough for their angular size to be measured accurately: the inner ring is 0.808 arcseconds in radius. Using the distance light must have traveled to light up the inner ring as the base of a right angle triangle, and the angular size as seen from the Earth for the local angle, one can use basic trigonometry to calculate the distance to SN1987A, which is about 168,000 light-years.
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