They give birth astride a grave, the light gleams an instant, then it's night once more.
The supernova that was spotted in the Large Magellanic Cloud in 1987 reached 3rd magnitude and was the brightest to grace our skies in 383 years.
Courtesy David Malin, Ray Sharples, and the Anglo-Australian Observatory.
Samuel Beckett, Waiting for Godot
The last person to see and chronicle a supernova outburst in our galaxy was Johannes Kepler. That was in 1604, when the star now named after him rivaled Venus in brightness. By some measures we're overdue for another brilliant supernova, yet the next star to explode in our galaxy is more likely to be a visual pipsqueak compared to Kepler's Star. Yet even a dim supernova is unlikely to be overlooked; its birth will be trumpeted by physicists' subterranean particle detectors rather than by astronomers' telescopes.
What's seen of Supernova 1987A today are the faded star and surrounding rings of gas that it has lit up. This three-color Hubble Space Telescope composite is from several images taken in 1994, 1996, and 1997.
Courtesy the Hubble Heritage Team (NASA/STScI/AURA).
Supernovae are stars that brighten by a dozen magnitudes or so and at their peak are some 10,000 times more luminous than ordinary novae. (The physical processes operating during these two explosions are completely different: supernovae blow themselves to smithereens; ordinary novae don't.) The enormous luminosity of supernovae at their brightest makes it possible to readily spot them in distant galaxies for weeks they can match the light output from all the other stars in a hefty system like our own Milky Way. Indeed, the identification of supernovae as a unique phenomenon had to wait until the 1920s, when galaxies themselves were recognized as independent star systems.
"Behold, directly overhead, a certain strange star was suddenly seen, flashing its light with a radiant gleam.... Astonished and stupefied, I stood still.... When I had satisfied myself that no star of that kind had ever shone forth before...I began to doubt the faith of my own eyes.... Having confirmed that my vision was not deceiving me...and marveling that the sky had brought forth a certain new phenomenon to be compared with the other stars, I immediately got ready my instrument."
Tycho Brahe reflecting on the supernova of 1572
In his book De nova stella Tycho included this sketch of Cassiopeia with the supernova of 1572 at the top, near a star now called Kappa.
S&T photo by Craig Michael Utter.
No supernova outburst has been studied in our Milky Way for nearly 400 years. Therefore, everything we know about the workings of these stars has come from observing them in other galaxies. At such immense distances even these celestial powerhouses are dim, and their secrets have to be teased from the paltry number of photons that strike our detectors. Furthermore, catching these critters in the act has been a matter of chance, even after systematic searches began in the 1930s. The idea is simple: look at enough galaxies and, sooner or later, you'll find a supernova.
A recent theoretical model (curves) tracks the light variations of SN 1969L. Note that this 'plateau' Type II supernova's ultraviolet (U) light rises and fades faster than its blue ((B)) and yellow (V) light. Such a star halfway to the center of the galaxy could shine as brightly as Venus but would probably be dimmed by interstellar dust. The theoretical curves are for a star having 15 times the Sun's mass and 240 times the Sun's diameter.
Courtesy Sergei Blinnikov.
Even so, these distant supernovae are detected days or weeks after their outbursts begin. What hasn't been observed are the earliest stages of a supernova's development. Particularly interesting, for example, will be observations to assess the chemical composition and physical state of the fastest-moving pieces of the star's blown-off atmosphere. Such information should provide insight about the end point in the evolution of a very massive star as well as clues about how heavy elements are injected into the interstellar medium.
Wonderfully detailed mathematical models of supernova explosions have been built on theorists' computers (see the diagram above, right). They tell us what to expect, but wouldn't it be nice to have a sanity check? This may now be possible thanks to a group of neutrino (symbol ν, the lowercase Greek letter "nu") observatories that can warn us when a star blows up in our cosmic backyard the Milky Way and its nearby attendants in the Local Group even before its light begins to turn on! Although this new tool is wielded by high-energy physicists, its impact will likely rest with amateur astronomers and other small-telescope users.