- For the first time, astronomers have detected a neutron-star collision. Gravitational waves heard by two detectors pinpointed the source to a galaxy 130 million light-years away. The collision produced a radioactive “kilonova” that forged hundreds of Earths’ worth of platinum, gold, silver, and other atoms. The discovery solves a longstanding mystery about the origins of heavy elements.
Platinum and gold are among the most precious substances on Earth, each fetching roughly $1,000 an ounce.
However, their allure may grow stronger – and weirder – thanks to a groundbreaking new finding about their violent, radioactive, and cosmic origins.
The team alerted astronomers all over the world to the event right after it happened, helping them point telescopes directly at the scene of the crash and record unprecedented observations of the aftermath in visible light, radio waves, X-rays, and gamma rays.
These images revealed a radioactive soup giving birth to unfathomable amounts of platinum, gold, and silver – not to mention elements like the iodine found in our bodies, the uranium in nuclear weapons, and the bismuth in Pepto-Bismol – while shooting those materials deep into space.
The two neutron stars most likely merged to form a black hole, though the tiny bit of neutron star that escaped – and formed new elements – could get recycled into planets like Earth where aliens may eventually dig up the metals as we have.
“The calculations we did suggest most of the matter that came out of this event was in a swirling disk around a black hole. Half of that matter fell in, and half of it got ejected,” Brian Metzger, an astrophysicist at Columbia University who’s one of roughly 4,000 researchers involved in the discovery, told Business Insider. “The matter that ended up in your wedding band could have just as well fallen in.”
Astronomers detected the merger from 130 million light-years away, in the galaxy NGC 4993, on the morning of August 17.
“This is going to have a bigger impact on science and human understanding, in many ways, than the first discovery of gravitational waves,” Duncan Brown, an astronomer at Syracuse University who’s a member of the research collaboration, told Business Insider. “We’re going to be puzzling over the observations we’ve made with gravitational waves and with light for years to come.”
When two city-size atoms collide
Albert Einstein first predicted the existence of gravitational waves a century ago, but he didn’t believe they’d ever be detected because of their extraordinarily weak energies.
The Laser Interferometer Gravitational-Wave Observatory in the US defied Einstein in September 2015 when it “heard” the elusive phenomenon for the first time. Europe’s new Virgo gravitational-wave detector has also come online since then and worked with LIGO to make this fifth detection possible.
Unlike the four previous events, the latest one – which emanated from the constellation Hydra and was dubbed GW170817 – wasn’t created by colliding black holes. Its signal was weaker and closer to Earth by hundreds of millions of light-years, and it lasted 100 seconds as opposed to one second.
- 1M2H Collaboration/UC Santa Cruz/Carnegie Observatories
Brown and others think GW170817 is revolutionary in part because it provides clues about how the heaviest elements we find on Earth formed in space.
For example, giant stars that explode as supernovas – blasts that are brighter than billions of suns – are thought to form iron and lighter elements.
“Some of the heavy elements are made in supernova explosions, but it turns out this can’t explain the abundances,” Brown said of heavier elements. “They didn’t appear to be coming from supernova explosions, and so people have wondered for a long time where they came from.”
Researchers eventually hypothesized that pairs of colliding neutron stars could do the trick.
Most stars in the universe form in pairs, and the same is true of massive stars. Unlike the sun, however, big stars become supernovas when they die. At that point, their gravity crushes them into one of two forms: a black hole if they’re heavy or a neutron star if they’re light.
- LIGO-Virgo/Daniel Schwen/Northwestern
The latter is essentially one big atomic nucleus, since its gravity is powerful enough to squash all the particles together into an orb roughly the width of a metropolitan city – just one teaspoon of a neutron star weighs billions of tons.
“You smash these two things together at one-third the speed of light, and that’s how you make gold,” Brown said. “Turns out it’s not the philosopher’s stone – it’s not the things alchemists were looking at thousands of years ago.”
100 Earths’ worth of gold forged in one second
Metzger was among the first to seriously explore the physics of how this could happen.
He said neutron star mergers are a “messy process” that spill some of the stars’ guts into space, like “squeezing a tube of toothpaste” and having it shoot out of both ends. The collisions also accelerate thrown-off particles to a fraction of the speed of light while heating them to 10 million degrees.
“If you just ejected all of this stuff and it did nothing, it’d get extremely cold, and we’d never be able to see it,” Metzger said.
Though, of course, that’s not what happened on August 17.
“The heaviest elements, you can’t create them through nuclear fusion in a star,” Metzger added. “The way you form them is through neutron-capture.”
The process, known as the rapid process or r-process, goes like this: As the two neutron stars spiral toward each other – each roughly 1.4 times the mass of the sun – they spew out high-energy neutrons. Those neutrons smash into each other while moving outward, building giant atomic cores. But very big atoms are unstable, so they almost immediately break apart and decay into smaller atoms.
The same thing happens in special nuclear reactors that bombard uranium with neutrons to form heavier elements like plutonium. A neutron-star merger performs the r-process on a cosmic scale, though, bleeding off enough radioactive energy from decaying atoms to be visible from millions of light-years away – if astronomers know where to look at the right moment.
In 2010, Metzger coined the term “kilonova” for this flash of radioactive light because calculations showed it’d be dimmer than a supernova yet about 1,000 times as bright as a nova, which occurs when a star is born.
Scientists have seen what they suspected were kilonovas before but couldn’t confirm the masses of the two objects, as happened with GW170817.
- Skye Gould/Business Insider
Their observations of the recent kilonova revealed a striking amount of materials created just one second after the collision: roughly 50 Earth masses’ worth of silver, 100 Earth masses of gold, and 500 Earth masses of platinum.
The gold alone is worth about 100 octillion dollars at today’s market price, according to Metzger, or $100,000,000,000,000,000,000,000,000,000 – a 1 followed by 29 zeroes.
“You’d need Captain Kirk to go and get it for you, though, so we’re not in any danger of disrupting the market right now,” Brown said.
Fortunately, we don’t need a spaceship to find gold, platinum, and silver created by neutron stars – that’s where what we already have on Earth came from. Countless smash-ups over the millennia spread around enough of these exotic metals that when our planet formed, they were baked right into its crust.
A new era of astronomy is underway
In the worldwide call to arms on August 17, and in the days and months that followed, more than a third of all astronomers on the planet stepped up to help analyze and make sense of the event.
Vicky Kalogera, a member of the LIGO collaboration who’s an astrophysicist at Northwestern University, said she was one of nine people who wrote the main research study about the discovery. The writing process took the team two weeks of 12- to 16-hour international conference calls with hundreds of people from 910 institutions. The printed list of 4,000 or so authors is 28 pages.
“It was the hardest thing I’ve ever had to do in my life,” Kalogera told Business Insider, adding that more discoveries are on the way.
“These are rare events. For a galaxy like the one we’re observing, it’s somewhere between 30 and 470 neutron-star mergers per million years,” Kalogera said. “But LIGO is not sensitive only to this particular galaxy. We should see a few per year, because we’re listening to millions of galaxies.”
Brown said LIGO entered a planned yearlong upgrade shortly after the experiment detected GW170817. (LIGO was last booted up in November and ran through August.)
After the new work is finished in 2018, he said, LIGO should have a 50% boost in range, allowing it to gaze another 500 million light-years deeper into space and time. And in the early 2020s, a Japanese detector called Kagra and perhaps an Indian detector will join forces to listen to even more of the universe.
Researchers hope these improvements will reveal the secrets of a nearby supernova – perhaps Betelgeuse, which could explode at any moment.
“In some sense, this is the next big undiscovered country for gravitational waves,” Brown said. “But we’re only at the beginning of gravitational-wave astronomy, and we’ve been rewarded with these incredible discoveries.”