<P> Four days after the event was recorded, the progenitor star was tentatively identified as Sanduleak − 69 ° 202, a blue supergiant . After the supernova faded, that identification was definitely confirmed by Sanduleak − 69 ° 202 having disappeared . This was an unexpected identification, because models of high mass stellar evolution at the time did not predict that blue supergiants are susceptible to a supernova event . Many models of the progenitor have since attributed the color to its chemical composition, particularly the low levels of heavy elements, among other factors . 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 supernovae, although there is still speculation that the evolution of such stars requires mass loss involving a binary companion . It is of note that the supernova of the blue giant Sanduleak − 69 ° 202 was about one - tenth as luminous as the average observed type II supernova, which is associated with the denser makeup of the star . Because blue supergiant supernovae are not as bright as those generated by red supergiants, we cannot see them in as large a volume . We would thus not expect to see as many of them, and so they might not be as rare or unusual as previously thought . </P> <P> 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 is likely due to neutrino emission, which occurs simultaneously with core collapse, but before visible light was emitted . Visible light is transmitted only after the shock wave reaches the stellar surface . At 07: 35 UT, Kamiokande II detected 12 antineutrinos; IMB, 8 antineutrinos; and Baksan, 5 antineutrinos; 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 not believed to be associated with SN 1987A . </P> <P> The Kamiokande II detection, which at 12 neutrinos had the largest sample population, showed that the neutrinos arrived in two distinct pulses . The first pulse, which started at 07: 35: 35 comprised 9 neutrinos, all of which arrived over a period of 1.915 seconds . A second pulse of three neutrinos arrived between 9.219 and 12.439 seconds after the first neutrino was detected, for a pulse duration of 3.220 seconds . </P> <P> 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 10 with a total energy of 10 joules . </P>

Which of the following is not true for supernova sn1987a