Image of the Day: The Enduring Mysteries of the Great 1987 Supernova Explosion

Sn87a Supernova 1987A exploded on 1987 February 23, 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.

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 makes 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.

The origin and the nature of the beautiful circumstellar rings are still a mystery. They have been measured to expand rather slowly, "only" 70,000-100,000 miles per hour (this is considered slow because the supernova material in the center is expanding outward at speeds that are 100-2000 times higher!). Spectroscopic observations show that the rings are enriched in the element nitrogen. 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, rather than puffing rings like a pipe smoker.

Another mystery 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.

As time goes on, the high-velocity material emerging from the supernova explosion will overtake and crash into the slower-moving surrounding rings. Recent observations with Hubble's Space Telescope Imaging Spectrograph (STIS) show that this collision is starting to occur already.

In less than a decade the full force of the supernova fast material will hit the inner ring, heating and exciting its gas and producing a new series of cosmic fireworks that will offer a spectacular view for several years thereafter. But this will be the "beginning of the end" because in about another century most, if not all, the material in the rings will be swept away and disappear, loosing their identities and merging into the general interstellar medium of the Large Magellanic Cloud. This is not a complete loss, however, because by studying this destructive process, astronomers of the 21st century will be able to probe the ring material with a detail and accuracy never before possible.

via NASA APOD

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