Supernovae 1987 A

Supernovae 1987 A[1]

Astronomers see numerous supernovae every year, but these are usually in very distant galaxies.  Supernovae are rare, occurring in any one galaxy once every fifty years or so.  This means that to see large numbers of supernovae, one must search numerous galaxies every day, which means looking far out into space.  On occasion, however, a supernova occurs in a nearby galaxy, providing astronomers with a close-up view of an exploding star.

The most important of the nearby supernovae is a supernova seen in February of 1987, named SN 1987A.[1]  It occurred in the Large Magellanic Cloud, which is 50 kpc away from Earth, so it occurred only (!) 163,000 years ago.  No other supernova observed since the start of the space age has occurred closer to Earth.  SN 1987A was observed with ground-based and space-based instruments, as well as with neutrino detectors buried deep under the Earth.  It occurred in a highly-visible region of the Southern Hemisphere sky, unobscured by dust.  Its source was a star that had been studied before the supernova occurred, with the final observation of the star occurring just hours before the explosion.  This supernova proved the theory that the core-collapse of a massive star produces a supernova, but it also showed that, contrary expectation, not all stars that explode in a supernova are red supergiants—sometimes blue supergiants also explode.

When we look at remnants of historical supernovae, such as Kepler’s of 1604 or Tycho’s of 1572 or somewhat older ones in the Milky Way, we see their completed transitions. Their conspicuous features are radio emission from particles that the shock wave accelerates to nearly the speed of light, and x-ray emission from gas heated to millions of degrees. For SN 1987A, we have much richer and more complete story that tells us what type of star exploded, what elements the explosion initially produced, and the detailed sequence of events that then unfolded. We can watch as the explosion destroys the evidence of the star’s pre-explosion behavior, as embodied in the three-ring circus of surrounding hydrogen. Nature made SN 1987A in a more intricate and interesting way than we imagined. It will be good to remember that lesson of humility.

Perhaps there’s an astronomer on Earth today whose name will go on the next one in the Milky Way itself. More likely, the first galactic supernova of the telescopic era will be caught by some large cooperative effort. Perhaps it will be an array of survey telescopes staring unblinkingly at the whole sky. Perhaps it will be specks of light produced by neutrinos as they slash through Antarctic ice, or a flare of ultraviolet light detected by a satellite, or the jiggle of the gravitational waves that a messy core collapse should produce. The next Milky Way supernova could outshine every star in the sky, or it might remain hidden.

[1] See an article, Robert P. Kirshner, “The Supernova of a Lifetime, Sky & Telescope (133, 2, February, 2017), pp. 36-40

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