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.
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.
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.
Source: The Daily Galaxy via Harvard Smithsonian Astrophysics Center and heritage.stsci.edu
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