Sunday, April 3, 2016

SN 1987A, a supernova on the outskirts of the Tarantula Nebula in the Large Magellanic Cloud (a nearby dwarf galaxy). It is approximately 168,000 light-years away, and when its light reached Earth on February 23, 1987, it was visible to the naked eye.

The supernova occurred after a star known as Sanduleak -69° 202. This was a blue supergiant, a star about 20 times the size of the Sun. Blue supergiants have very short lifespans, a few million years at most. Their massive size creates such immense pressures that nuclear fusion occurs at a far faster rate than in a "main sequence" star like the Sun. As the star uses up its hydrogen (turning it into helium), fusion shuts down. The heat pushing outward stops, and the star collapses inward. The pressure mounts, and for a few brief seconds helium fusion occurs. The sudden burst of energy and heat pushes the outer layers of the star outward in a supernova.

It was long thought that blue giants could not create supernovas. That role was assigned to red giants, but SN 1987A proved astronomers wrong. We now know that blue supergiants can go nova. Afterward, they can spend time as a yellow supergiant (a cooling star), a yellow hypergiant (like a yellow supergiant, but one whose corona extends outward far more), and even a red supergiant (a very cool star). Many blue supergiants can become a red supergiant, then collapse back into a blue supergiant and repeat the cycle many times.

The material ejected from the surface of SN 1987A during the supernova was initially dark. A few months after the nova, ultraviolate light from Sanduleak -69° 202 hit this matter -- causing it to "turn on" and emit light itself.

We know now that there were two rings of matter ejected by Sanduleak -69° 202. One was the star's outer layer, and the other was matter from the core. In 2001, the core ejected hit the the outer layer ejected, causing the outer layer ring to heat up and emit massive amounts of X-rays. This also caused clumping in the combined rings.

You may as well enjoy it while you can. The shockwave has yet to hit the ejected matter. When it does, some time between 2020 and 2030, it will destroy the outer ring and these clumps.

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