CHAPTER 15
THE FINAL SUPERNOVA
Our own Milky Way has not had a supernova in over four hundred years. Not a single brilliant new starlike point has lit up the heavens and cast shadows since just before the telescope was invented. Those wild halcyon supernova years in the eleventh, sixteenth, and early seventeenth centuries are for the history books alone.
Or so we thought. One strange night in February of 1987, our dwarf companion galaxy, the Large Magellanic Cloud, or LMC, had a star go ka-plooey with enough brilliance to wreck its poor solar system and send a flood of detritus through human bodies 166,000 light-years away. And enough brilliance to be seen by the naked eye.
Are you kidding? A naked-eye supernova after four centuries?
The author offered the chance for a peek to a bunch of friends, and we booked a flight to Costa Rica, which was far enough south for the exploding star to appear well above the horizon. Sure enough, there it was, neither challengingly faint nor eye-catchingly bright. It had been slowly brightening for two months and now matched the luminosity of the Little Dipper stars, meaning it was half the second-magnitude brightness of Orion’s belt. This supernova was magnitude 3. In keeping with long-standing tradition, it was called 1987A to signify the first observed supernova in that year, and until it faded a year later, it remained the sole naked-eye supernova since the lifetimes of Kepler and Galileo. It’s still the only supernova that anyone alive today can claim to have seen without optical equipment.
More excitement was in store. Each year your author leads an astronomy group to Chile’s marvelous Atacama Desert, and one afternoon each year we spend a few hours at the remote cutting-edge observatory run by the Carnegie Institution on a mountaintop called Las Campanas. The past and now present directors have been gracious in hosting an astronomy tour led by this astronomy journalist, even though the facility has no visitors’ center and no established procedure for putting up with nonworking astrophysicists. This is how we met Oscar Duhalde more than twenty years ago. He’s there nearly every night and runs the sophisticated equipment for astrophysicists who have been awarded time on one of their giant instruments, which include the famous twin 6.5-meter behemoths named the Magellan telescopes. And one night a few years ago, one of those astronomers casually said, “Did you know that Oscar was the one who discovered the 1987 supernova?”
All the guests in that forty-person group simultaneously swiveled their heads from him to Oscar, as if they were at a tennis match. Oscar? The nicest, most low-key, most unassuming man in the world? Our Oscar? He, of all the seven billion humans on the planet, was the first person in four hundred years to see a naked-eye supernova? And it had happened right here? Then why wasn’t his name in all the books?
He told us his story. On February 24, 1987, while doing his technical and instrument tasks, he stepped outside to walk to another observatory building. He looked up into the amazingly clear Chilean skies, where the Milky Way is typically bright enough to cast shadows. He glanced casually at the Large Magellanic Cloud, which was high up at that time of year in Chile, though it never clears the horizon in the mainland United States, and he instantly saw something so weird it was next to impossible.
There is no bright star or even dim star in front of that little nebulous patch that is the very nearest galaxy to ours. But that night there was. Oscar knew the stars intimately, the way your author and perhaps only 0.001 percent of the world does, and so he instantly recognized that this new star had to be a supernova. The entire process of spotting and identifying the first naked-eye supernova in four centuries had required no more than ten seconds.
Oscar returned to the big dome and told people. One of them, the Canadian astronomer Ian Shelton, was just finishing taking an image, by amazing coincidence, of the LMC, and he looked at the plate. Yes, a new star was there, and he was the first to capture it. So Shelton sent someone to make the two-hour drive down the eight-thousand-foot mountain to La Serena, where he could place a phone call and report it. Since Shelton was the first to report it, he is usually credited with being the discoverer of 1987A.
But everyone at the Carnegie Las Campanas Observatory knew who had seen it first. So, though it took a few years, you’ll now find most literature lists both men as co-discoverers.
But the fireworks were just beginning.
Using various orbiting detectors, the Hubble Space Telescope, and sophisticated new terrestrial telescopes like the newly built ALMA array in the Atacama Desert just a few hundred miles north of the discovery mountain, astronomers feasted on a wealth of data from this nearest supernova since the early seventeenth century. They saw X-rays streaming off to collide with surrounding material that had been previously shed by the unfortunate ex-star, making it glow like holiday decorations. They detected a mass of neutrinos that actually arrived ahead of the visible supernova. They scoured pre-nova photographs and identified the star that had exploded—the first time an exploding star could be pinpointed and examined in an image taken before the cataclysmic detonation. Turned out, it was an enormous blue supergiant weighing eighteen times more than our sun with the catchy name of Sanduleak -69°202.
Supernova 1987A as it looked a half a year later when a lethal shock wave of ultraviolet light smashed into a ring of material in its vicinity. These gassy knots had been shed into space twenty thousand years earlier by the now-destroyed star during a prior outburst of lesser violence. The type 2 supernova was the first naked-eye supernova in more than four hundred years. (Hubble Space Telescope / NASA)
Those neutrinos were a really cool thing. Theory had insisted that this type of supernova should produce a flood of those strange fundamental weightless (or perhaps near-massless) particles, and by 1987 the world had two separate neutrino-detecting experiments in operation. The fact that the neutrinos were detected before the visible light was observed supplied key information that helped settle the old issue of whether neutrinos had mass. If they possessed even a little heft, they might solve the longtime riddle about the universe’s missing mass and whether our cosmos will someday stop blowing itself up and instead start collapsing simply because there are so many neutrinos and each is contributing to the collective gravitational pull. But with any mass at all, they couldn’t travel at light speed. So now, finally, supernova 1987A released neutrinos and light at the same time (although theorists said that the neutrinos actually had a few hours’ head start). Bottom line: If they had mass, they’d arrive at Earth hours, days, months, years, or even centuries after the visible light of the explosion, depending on their individual weight. But it so happened that they beat the light here by a few hours, after both had zoomed through space for 166,000 years.
So, puzzle solved: Neutrinos have no significant mass. This was the first belated Valentine’s Day present SN 1987A delivered to astrophysicists, and the good-news section of the good news/bad news story of that solar-system-destroying cataclysm.
Because neutrinos normally don’t interact with the baryonic matter that makes up our planet’s elements and their constituent subatomic particles, a signal that a neutrino has arrived involves indirect detections. Even then, trillions of neutrinos must arrive before a single one can change one element to a different one, which is how all that elaborate underground neutrino-finding equipment operates.
But even before the extreme light of a hundred million suns had started to leave the fragmenting Sanduleak star, and thus hours before Oscar saw that new little star in the LMC, neutrinos from SN 1987A were penetrating the Earth. Four trillion neutrinos per second passed unfelt through each eyeball of the planet’s nearly six and a half billion humans. But as they penetrated the ground, a few were finally detected.
Lying deep in the Kamioka zinc mine in Japan and in the Morton salt mine under Lake Erie are two massive pools of black water designed to detect flashes of light from a rare neutrino effect. At 7:35:35 universal time on February 23, two hours before Oscar noticed the optical light arriving on earth, the neutrinos reached Japan’s Kamioka Observatory’s Institute for Cosmic Ray Research, which can detect underground neutrinos by looking at Cherenkov radiation, a blue glow caused by anything traveling faster than the speed of light.1
The Kamiokande II detector observed eleven neutrinos from the supernova. (Some sources say that Kamiokande detected not eleven but twelve neutrinos from SN 1987A. The author is tempted to split the difference and say around eleven and a half neutrinos.)
These near-dozen neutrinos detected in that Japanese underground water pool and the eight more found by detectors in that old Morton salt mine underneath Lake Erie were historic. The chance of a neutrino interacting with an atom is so incredibly minuscule that detecting those nineteen or twenty meant that at that moment, ten billion neutrinos from the supernova must have been simultaneously zooming through every fingernail-size area of every earthly human and animal, marking the instant 166,000 years ago when the star’s core bounced back from its collapsing layers. This happened just before a blinding brilliance left the star to begin a 1,600-century journey to Earth.
Even if all this unfolded outside the awareness of nearly the entire human race, these astounding detection capabilities deserve a salute. These new methods in which science successfully pinned down the mayhem happening in our companion galaxy were an astounding change from Kepler’s Star of 1604—the previous time humans tried to make sense of the naked-eye death of a massive sun.