30 years later, supernova 1987A is still sharing secrets

Ian Shelton was alone at a telescope in the remote Atacama Desert of Chile. After three hours getting a picture of the Large Magellanic Cloud, a wispy galaxy that orbits the Milky Way, he was plunged into darkness. High winds had taken hold of the rolltop door in the observatory’s roof, slamming it shut.

“This was maybe telling me I should just call it a night,” says Shelton, who was a telescope operator at Las Campanas Observatory on that evening of February 23, 1987.

He grabbed the photograph — an 8-by-10 inch glass plate — and headed off to the darkroom (yes, these were the days of developing images by hand). As a quick quality check, he compared the just-developed picture with an image he had taken the previous night.

Shelton noticed a star that hadn’t been there the night before. “I thought, this is too good to be true,” he says. He stepped outside and looked up. There it was — a faint point of light that wasn’t supposed to be there. He walked down the road to another telescope and asked astronomers there what they would say about an object that bright appearing in the Large Magellanic Cloud, just outside the Milky Way.

“Supernova” was the group’s response, Shelton says. He ran outside with the others — including Oscar Duhalde, who recalled seeing the same thing earlier in the evening — to double-check with their own eyes.

“The supernova has gotten dimmer by a factor of 10 million, but we can still study it.”

Robert Kirshner

They were witnessing the explosion of a star, quickly dubbed supernova 1987A. It was the closest supernova seen in nearly four centuries and so bright it was visible without a telescope. “People thought they’d never see this in their lifetime,” says George Sonneborn, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

With roughly 2 trillion galaxies in the observable universe, there’s almost always a star exploding somewhere. But a supernova close enough to be seen with the unaided eye is a rare event. In the Milky Way, astronomers estimate, one goes off every 30 to 50 years. But the most recent one seen was in 1604. At a distance of about 166,000 light-years, SN 1987A was the closest since the time of Galileo.

Supernovas are “important agents of change in the universe,” says Princeton astrophysicist Adam Burrows. They end the lives of stars and trigger the birth of new ones. They change the fate of entire galaxies by stirring up the gas needed to build more stars. Most, perhaps even all, of the chemical elements heavier than iron are forged in the chaos of the explosion. Lighter elements — “the calcium in your bones, the oxygen you breathe, the iron in your hemoglobin,” Burrows says — are created over the star’s lifetime and then spewed into space to seed a new generation of stars and planets — and life.

Thirty years after its discovery, supernova 1987A remains a celebrity. It was the first supernova for which the original star could be identified. It offered up the first neutrinos detected from beyond the solar system. Those subatomic particles confirmed decades-old theories about what happens in the heart of an explosion. And today, the supernova’s story continues to be written. New observatories draw out more details as shock waves from the explosion keep plowing through interstellar gas. “The supernova has gotten dimmer by a factor of 10 million, but we can still study it,” says astrophysicist Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics. “We can study it better and over a wider range of light than we could in 1987.”

A daily adventure

Communication was a bit slower when 1987A exploded. Shelton’s attempts to call the International Astronomical Union in Cambridge, Mass., failed. So a driver took off to La Serena, a town about 100 kilometers away, to alert the IAU by telegram.

Lots of researchers didn’t believe the news at first. “I thought, that’s got to be a joke,” says Stan Woosley, an astrophysicist at the University of California, Santa Cruz. But as word spread via telegram and telephone, it quickly became clear that it was not a prank. Amateur astronomer Albert Jones in New Zealand reported seeing the supernova the same night before clouds moved in. About 14 hours after the discovery, NASA’s International Ultraviolet Explorer satellite was already watching. Astronomers around the world scrambled to redirect telescopes both on the ground and in space.

“The whole world got excited,” Woosley says. “It was a daily adventure. There was always something coming in.” At first, astronomers suspected that 1987A was a class of supernova known as type 1a — the detonation of a stellar core left behind after a star like the sun quietly sheds gas at the end of its life. But it soon became clear that 1987A was a type 2 supernova, the explosion of a star many times heavier than the sun. Observations taken the next day in Chile and South Africa showed hydrogen gas hurtling away from the explosion at roughly 30,000 kilometers per second — about one-tenth the speed of light. After the initial flash, the supernova faded for about a week but then resumed brightening for about 100 days. It eventually maxed out with the light of roughly 250 million suns.

The right track

Despite several surprises along the way, SN 1987A didn’t lead to a fundamental shift in how astronomers thought about supernovas. “It rubbed our nose in the fact that we were on the right track,” says astrophysicist David Arnett of the University of Arizona in Tucson. The general idea — suspected for decades and largely confirmed by 1987A — is that a type 2 supernova goes off when a heavyweight star runs out of fuel and can no longer support its own weight.

Stars live in a delicate balance between gravity and gas pressure. Gravity wants to crush a star. High temperatures and extreme densities in the center of a star allow hydrogen nuclei to slam together and create helium, liberating copious amounts of energy. That energy pumps up the pressure and keeps gravity in check. Once a star’s core runs out of hydrogen, it fuses helium into carbon, oxygen and nitrogen. For stars like the sun, that’s about as far as they get. But if the star is more than about eight times as massive as the sun, it can keep going, forging heavier elements. All that weight on the core keeps the pressure and temperature extremely high. The star forges progressively heavier elements until iron is created. But iron is not a stellar fuel. Fusing it with other atoms doesn’t release energy; it saps energy from its surroundings.

Without an energy source to fight against gravity, the bulk of the star comes crashing down on its core. The core collapses on itself until it becomes a ball of neutrons, which can survive as a neutron star — a hot orb about the size of a city with a density greater than that of an atomic nucleus. If enough gas from the dying star rains down on the core, the neutron star loses its own battle with gravity and forms a black hole. But before that happens, the initial onrush of gas from the rest of the star hits the core and bounces, sending a shock wave back toward the surface, tearing apart the star. In the ensuing explosion, elements heavier than iron are forged; more than half of the periodic table may originate in a supernova.

Newly formed elements aren’t the only things a supernova spits out. Theorists had predicted that neutrinos, nearly massless subatomic particles that barely interact with matter, should be released during the core collapse, and in no small quantity. Despite their ghostly nature, neutrinos are suspected to be the main driving force behind the supernova, injecting energy into the developing shock wave and accounting for about 99 percent of the energy released in the explosion. And because they pass through the bulk of the star unimpeded, neutrinos can get a head start out of the star, arriving at Earth before the blast of light.

Confirmation of this prediction was one of the big successes from 1987A. Three neutrino detectors on different continents registered a nearly simultaneous uptick in neutrinos roughly three hours before Shelton recorded the flash of light. The Kamiokande II detector in Japan counted 12 neutrinos, the IMB facility in Ohio detected eight and the Baksan Neutrino Observatory in Russia detected five more. In total, 25 neutrinos were recorded — a deluge in neutrino science.

“That was huge,” says astrophysicist Sean Couch of Michigan State University in East Lansing. “That told us beyond a shadow of a doubt that a neutron star formed and radiated neutrinos.”

While the neutrinos were expected, the type of star that went supernova was not. Before 1987A, astronomers thought that only puffy red stars known as red supergiants could end their lives in a supernova. These are gargantuan stars. One nearby example, the bright star Betelgeuse in the constellation Orion, is at least as wide as the orbit of Mars. But the progenitor of 1987A, known as Sanduleak -69° 202 (SK -69 202 for short), was a blue supergiant, hotter and more compact than the red supergiant that was widely expected. 1987A didn’t fit the mold.

“SN 1987A taught us that we did not know everything,” Kirshner says. More surprises came after the launch of the Hubble Space Telescope.

A necklace of pearls

When Hubble was launched in 1990, 1987A was one of its first targets. Early images were fuzzy because of a now infamous defect in the telescope’s main mirror (SN: 4/18/15, p. 18). Corrective optics installed in 1993 revealed some unexpected details of the fading explosion.

“Those first pictures from Hubble were jaw-dropping,” says Shelton, now a teacher in the Toronto area. A thin ring of glowing gas — faintly seen in earlier images from the ground — encircled the site like a Hula-Hoop. Above and below that ring were two fainter rings, the trio forming an hourglass shape.

 “No other supernova had shown that kind of phenomenon,” says Richard McCray, an astrophysicist at the University of California, Berkeley. Not because it doesn’t happen, he says, but because other supernovas were too far away.

The central ring spanned 1.3 light-years across and was expanding at about 37,000 km/h. The ring’s size and how quickly it was growing indicated that the star dumped a lot of gas into space about 20,000 years before it exploded. That could explain why SK -69 202 was a blue supergiant when it exploded. Some type of earlier outburst might have whittled the star down to expose hotter, and therefore bluer, layers.

One leading idea for how the rings formed is that SK -69 202 might be the offspring of two stars that were once locked in orbit around one another and then spiraled together. As the stars merged, some excess gas might have been expelled in a ring aligned with the original orbit while other gas was funneled in the perpendicular direction. Rapid rotation of a single star or powerful magnetic fields also could have directed gas from an eruption into a loop around the star.

The primary ring has only gotten more intriguing with age. In 1994, a bright spot appeared on the ring. A few years later, three more spots developed. By January 2003, the entire ring had lit up with 30 hot spots, all drifting away from the center of the explosion. “It was like a necklace of pearls,” Kirshner says, “a really beautiful thing.” A shock wave from the supernova had caught up with the ring and started to heat up clumps of gas.

By now, the hot spots are fading and new ones are appearing outside the ring. Given how quickly the spots are waning, the ring will probably be destroyed sometime in the next decade, Claes Fransson, an astrophysicist at Stockholm University, and colleagues predicted in 2015 in Astrophysical Journal Letters. “In a way, this is the end of the beginning,” Kirshner says.

The elusive neutron star

One of the enduring mysteries of 1987A is what became of the neutron star that formed at the heart of the explosion. “It’s a cliffhanger,” Kirshner says. “Everybody thinks that the neutrino signal means that a neutron star formed.” But despite three decades of searching with many different types of telescopes, there’s no sign of it.

“It’s a bit embarrassing,” Burrows says. Astronomers haven’t been able to find the pinprick of light from a glowing orb in the middle of the debris. There is no steady pulse from a pulsar, formed by a rapidly spinning neutron star sweeping out beams of radiation like a cosmic lighthouse. Nor is there any hint of heat radiated by dust clouds exposed to the harsh light of a hidden neutron star. “That is one of the things most crucial to closing the chapter on 87A,” Burrows says. “We need to know what was left.”

The neutron star is probably there, researchers say, but it might be too feeble to see. Or perhaps it was short-lived. If more material rained down in the aftermath of the explosion, the neutron star could have gained too much weight and collapsed under its own gravity to form a black hole. For now, there’s no way to tell.

Answers to this mystery and others will depend on new and future observatories. As technology advances, new facilities keep providing fresh looks at the remains of the supernova. The Atacama Large Millimeter/submillimeter Array in Chile, which today combines the power of 66 radio dishes, peered into the heart of the debris with 20 antennas in 2012. ALMA is sensitive to electromagnetic waves that can penetrate clouds of detritus surrounding the supernova site. “That gives us a look at the guts of the explosion,” McCray says.

Within those guts lurk solid grains of carbon- and silicon-based compounds that formed in the wake of the supernova, researchers reported in 2014 in Astrophysical Journal Letters. These dust grains are thought to be important ingredients for making planets. Supernova 1987A appears to be creating a lot of this dust, suggesting that stellar explosions play a crucial role in seeding the cosmos with planet-building material. Whether that dust survives shock waves that are still ricocheting around the leftovers of the supernova remains to be seen.

The fate of that dust, the whereabouts of the alleged neutron star, the effects from the shock wave that continues to plow through space — these and other unknowns keep bringing astronomers back to 1987A. From Earth, the universe can seem unchanging. But over the last 30 years, 1987A has shown us cosmic change on a human timescale. A star was destroyed, new elements were created and a tiny corner of the cosmos was forever altered. As the closest supernova seen in 383 years, 1987A gave humankind an intimate peek at one of the most fundamental and powerful drivers of evolution in the universe.

“It was a long time coming,” Shelton says. “This particular supernova … deserves all the accolades it gets.” But even though 1987A was close, he adds, it was still outside the Milky Way. He and others are waiting for one to go off within this galaxy. “We’re overdue for a bright one here.”