Supernova 1987A exploded on February 23, 1987 in the Large Magellanic Cloud. Because of its relative proximity to us (a mere 168,000 light years) SN 1987A is by far the best-studied supernova of all time. Immediately after the discovery was announced, literally every telescope in the southern hemisphere started observing this exciting new object.
Both the slow speeds and the unusual composition show that the rings were expelled from the progenitor star when it was a red supergiant, more than 20,000 years before that star exploded as a supernova. However, one would have expected such a star to eject material in a more regular fashion, steadily expelling material in all directions.
Another puzzle is that the observations of the star just prior to the explosion show that it was a blue supergiant. This was a puzzle in 1987, because up to that time theorists had believed that only red supergiants could explode as a supernova. Apparently the star was, until relatively recently, indeed a red supergiant, but over the millennia before the explosion, it shrank in size and its surface heated up gradually.
In addition to light, particle emission was detected from the supernova. "Kamiokande II" is a neutrino telescope whose heart is a huge cylindrical tub, 52 feet in diameter and 53 feet high, containing about 3000 metric tons of water; it is located in the Kamioka mine in Japan, 3,300 feet underground. On February 23, around 7:36 am Greenwich time, the Kamiokande II recorded the arrival of 9 neutrinos within an interval of 2 seconds, followed by 3 more neutrinos 9 to 13 seconds later.
Simultaneously, the same event was revealed by the IMB detector (located in the Morton-Thiokol salt mine near Faiport, Ohio), counted 8 neutrinos within about 6 seconds. A third neutrino telescope (the "Baksan" telescope, located in the North Caucasus Mountains of Russia, under Mount Andyrchi) also recorded the arrival of 5 neutrinos within 5 seconds from each other.
This made a total of 25 neutrinos detected on Earth, out of the 10 billions of billions of billions of billions of billions of billions of them produced in the explosion! Neutrinos are elusive particles of very small (possibly zero) mass and very high energy, which are produced in huge quantities in the supernova explosion of a massive star. They interact so infrequently with ordinary matter that almost all of them of them can travel through the entire diameter of the Earth without being stopped; so they are extremely difficult to detect.
Nevertheless, a little more than two dozen neutrinos was more than enough to understand what was going on. And, in fact, the detection of those neutrinos was a perfect confirmation of the theoretical expectations for the core collapse of a massive star. The core-collapse process is believed to be the cause of the explosions of massive stars at the end of their lives, and SN 1987A provided strong experimental confirmation of this idea.
Unfortunately, the Hubble Space Telescope was not yet in operation when the supernova exploded, since it was not launced until April 1990. The first images of SN 1987A, taken with the ESA Faint Object Camera on August 23-24, 1990, revealed the inner circumstellar ring.
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