Twenty-five years ago today, something marvelous happened. A star died. It is more exact to say the star died 165,000 years ago, but it was at 7:36 Universal Time on February 23, 1987, that evidence of the explosive death first reached Earth, and frankly it doesn’t suit my purpose to measure from the cosmic date.
This supernova, called in astronomical parlance SN 1987A — the “A” means it was the first one observed that year — was the first easily naked-eye supernova since 1604, before telescopes became astronomical tools. “We have, in a sense, been cheated,” Isaac Asimov wrote rather petulantly in his 1985 book The Exploding Suns. “If people have missed the chance of seeing a very bright, if temporary, pinpoint of light in the heavens, astronomers have missed considerably more than that. Were a bright supernova to burst into view, and were modern instruments focused upon it, we could find out in a few days more about supernovas, and about stellar evolution in general, than we have managed to learn during all the nearly four centuries since the last supernova was visible to the unaided eye.”
Asimov, like many, expected the next supernova to be the death throe of a red giant, a swollen, fluffy star that has fused all the hydrogen in its core. Yet the star that lit up the skies in 1987 wasn’t on anyone’s radar as a pre-supernova. In fact, we might have had better luck predicting a snail was pregnant than foretelling the imminent death of Sanduleak -69° 202. A hot, dense blue giant of about 20 solar masses in a gigantic complex of forming stars was not exactly a usual suspect: it looked too young.
No one, having seen the Southern Hemisphere sky, can fail to envy the inhabitants of that realm of the world. After the initial shock of discovering the Milky Way’s dusty arc of spectacular stars, the eye sees off to one side two fuzzy patches. The larger is the Large Magellanic Cloud, a dwarf galaxy and satellite of the Milky Way that lies about 165,000 light-years away. Inside this patch of stars is the Tarantula Nebula, the largest, most active region of star formation in the Local Group of galaxies to which our spiral belongs. And on the Tarantula’s edge Sk -69° 202 died.
Astronomers in Chile and New Zealand saw the bright spot in the LMC the night of February 23-24, 1987. From those observers, the word moved to the Central Bureau for Astronomical Telegrams in Cambridge, Massachusetts, and from Cambridge to the world.
Astronomers went berserk. They aimed the Earth-orbiting International Ultraviolet Explorer at the rapidly expanding supernova, hastily submitted papers to research journals, and scrutinized old photographic plates to find out which star had exploded. Former S&T technical editor Ronald A. Schorn, writing in the April 1987 issue, caught some of the excitement: “‘This is it, Ron!’ yelled supernova expert J. Craig Wheeler over the telephone, ‘This is it!’”
“I’ll never forget it, it changed my life,” says Richard McCray (JILA, University of Colorado Boulder), one of many astronomers to pounce on this chance of a lifetime. “Everybody knew how important it was.”
New "Star" Heralds Enlightenment
The explosion changed stellar astronomy. Theorists had long predicted that speedy, nearly massless particles called neutrinos would flood into space after the stellar core’s collapse, carrying with them most of the explosion’s energy. Lo and behold, three different ground-based experiments caught a handful of stealthy neutrinos at roughly the same time — 7:36 UT on February 23rd.
The neutrinos were the biggest discovery from SN 1987A, says Stanford Woosley (University of California, Santa Cruz). "They were predicted, but seeing them verified a 50-year-old theory" of how massive stars die, he explains. These neutrinos, the first detected from a source outside the solar system, turned neutrino astronomy into “a genuine observational science,” as Schorn put it in the May 1987 S&T.
These particles, like a rattling cough, presage stellar death even before there’s any visible sign. Neutrinos don’t interact much with matter, so they stream easily up through a star’s layers from the core after the collapse halts. The shock wave from the post-collapse rebound takes more time (on the order of several hours) to move up through the star’s innards, explaining why the neutrinos arrived at Earth before the supernova’s light.
SN 1987A truly surprised astronomers, first and foremost with its blue-giant progenitor. Because of the star's unique nature, the explosion reached only one-tenth the brightness expected of a star dying via core collapse. The common interpretation today is that Sk -69° 202 actually merged with another star before its death, causing it to throw off the puffy outer layers seen in red giants.
The merger also would explain the second biggest surprise of SN 1987A: three giant rings, one small one around the supernova, the other two above and below, all aligned along an imaginary line like wheels on an axle. Spectroscopic observations of these rings showed they have relatively large amounts of nitrogen, suggesting the material underwent thermonuclear processing (as you would expect inside a red giant). They are also about 20,000 years older than SN 1987A, so while they aren’t part of the supernova, whatever led to their expulsion may have ultimately catalyzed the star’s death.
The list of SN 1987A’s contributions is long indeed. It was the first explosion to show that a supernova would produce X-rays not just via impact — when its debris hit surrounding gas — but from radioactivity inside itself, particularly from the decay of a heavy form of cobalt called Co-56. Careful measurements of the supernova’s interaction with surrounding gas pinpointed the Large Magellanic Cloud’s distance, a distance used to calibrate all other cosmic size and age measurements. And last September astronomers reported in Science that SN 1987A appears to have produced a surprisingly large amount of dust — about 200,000 Earth masses. While it’s not clear that supernova-created dust lasts long enough to do anything worthwhile in space, in general dust is a good thing: it absorbs ultraviolet radiation that would otherwise prevent molecules and stars from forming.
Mysteries do remain. "In ways, we've exceeded Asimov's expectations, but in others we failed," Woosley says. "Part of the problem with learning about stellar evolution is that '87A seems to have been weird. Somehow that always seems to be the case for things nearby that we can study well."
The most puzzling question is what happened to the star's collapsed core. Despite a neutrino pulse that matched just what astronomers expected to see from the creation of a superdense neutron star, to date (and despite an early false alarm) no compact object has been found. The dust may be somewhat to blame. McCray hopes that upcoming observations with the Atacama Large Millimeter/submillimeter Array in Chile will reveal the elusive remnant.
But truth be told, SN 1987A isn’t the most marvelous thing about February 23rd to me. The most marvelous thing is that it’s my birthday.