Around 130 million years ago, two dead stars violently collided and set off a sequence of events that, over the last two months, have whipped astronomers on Earth into an absolute frenzy.
First theorized by Albert Einstein in 1916, gravitational waves are kinks or distortions in the fabric of space-time caused by extremely violent cosmic events. Until now, all confirmed detections involved a deadly dance between two black holes, which leave no visible signature on the sky.
But with this latest event, teams using about a hundred instruments at roughly 70 observatories were able to track down and watch the cataclysm in multiple wavelengths of light, allowing astronomers to scrutinize the source of these cosmic ripples for the first time.
“We saw a totally new phenomenon that has never before been seen by humans,” says Andy Howell of the University of California, Santa Barbara. “It’s an amazing thing that may not be duplicated in our lifetimes.”
Unlike colliding black holes, shredded neutron stars expel metallic, radioactive debris that can be seen by telescopes—if you know when and where to look.
“We felt the universe shaking from two neutron stars merging together, and that told us where to go and point our telescopes,” says Howell, whose team was among several that chased down the stars tied to the gravitational wave signal.
Ultimately, about 3,500 people were involved in the gravitational wave detection and ensuing astrophysical forensics; the results of the massive project are reported today in 40 papers appearing in several scientific journals, including Science and Physical Review Letters.
Together, the observations are helping astronomers verify some long-standing theories in physics and resolve a debate about the origin of gold and other heavy elements in the cosmos—discoveries made possible by the nascent field of gravitational wave astronomy.
"This is the first time that we hear the death spiral of two neutron stars, and we also see the fireworks that came from the final merger," Vicky Kalogera of Northwestern University said today during a press event held in Washington, D.C.
The first, though indirect, evidence for the existence of gravitational waves emerged in 1974. But actually snagging the waves proved elusive for decades, because the amount by which they distort space-time on Earth is minuscule—on the order of a fraction of the width of an atomic nucleus. (Find out more about gravitational waves.)
To try and sense these ridiculously small shifts in the cosmos, researchers created the Laser Interferometer Gravitational-wave Observatory, or LIGO. The observatory’s twin detectors each use lasers to measure minute changes in the distance between pairs of mirrors created when gravitational waves wash over Earth; a third detector, run by the European Virgo team, now does the same.
In early 2016, scientists at LIGO announced a breakthrough: Their highly sensitive instruments had at last captured their quarry. Since then, LIGO has confirmed three more events, each created by black holes merging, and the team’s leaders have been awarded the 2017 Nobel Prize in physics.
But early in the morning of August 17, the LIGO detectors recorded something new. Gravitational waves toggling the distance between those pairs of mirrors contained telltale clues suggesting their source was not black holes, but merging dead stars.
Two seconds after those signals shook the LIGO detectors, NASA’s orbiting Fermi Gamma-ray Space Telescope caught a flash of gamma-rays coming from the same general region of sky as the LIGO signal. Lasting just under two seconds, the flash looked like a short gamma-ray burst—the type of cosmic explosion thought to be produced by colliding neutron stars.
Coincidence? The LIGO-Virgo team didn’t think so. The team sent up the equivalent of an astronomical bat signal, telling observers that if they acted quickly, they could survey the debris left over by the stars’ mutual annihilation and, for the first time, watch the aftermath of gravitational waves being born.
That signal triggered follow-up observations by teams around the globe, all of which were clamoring to help put together the pieces of this cosmic puzzle. But first, crucially, the teams needed to know where to point all their fancy hardware.
Dancing With the Stars
Enter Charlie Kilpatrick, a postdoc at the University of California, Santa Cruz. After the gravitational wave and gamma-ray triggers had come in, Kilpatrick and his colleagues had quickly gotten to work sifting through a pile of galaxies in roughly the same region as the source of the new signals.
They had under their command a small and unpretentious telescope on the ground in Chile, and as soon as the Chilean sky darkened, they planned to target each of those galaxies and look for signs of activity. But they had to be quick: That portion of the sky would only be visible for an hour or two before slipping below the horizon.
About 10 hours after the LIGO-Virgo alert went out, the fifth galaxy Kilpatrick looked at glittered with a bright spot that hadn’t been there before—a very tantalizing sign that something dramatic had happened. The team sent out a telegram alerting others to the discovery. Within 42 minutes, five other groups, including Howell’s, had the galaxy in their crosshairs.
“It’s been sort of slow to dawn on me what a big deal this is,” Kilpatrick says.
Over the next several days, a fleet of observatories joined the party. For weeks, the gravitational wave source, near the fringe of an oval-shaped galaxy called NGC 4993, was the most stared-at spot on the sky.
In that region of space, two neutron stars had been spiraling around one another for ages, moving through a breathless dance destined to end in a second, even more violent death. Millions of years in the making, their lethal coda was so furious that it warped and distorted the fabric of space-time, generating the gravitational waves that rippled across the cosmos at the speed of light and eventually alerted us to their demise.
Big Bang Theory
Thanks to the quick detective work, scientists were able to study the explosion across the electromagnetic spectrum, in everything from radio waves to gamma-rays.
The merger now resolves a long-standing debate about the origin of heavy elements in the periodic table: precious metals, including gold and platinum, and things like the neodymium scientists use when building lasers like LIGO’s.
For a long time, scientists thought these metals were forged mainly in the bellies of large stars that die explosive deaths. But more recent work suggested that such supernovae didn’t eject enough of this stuff into the cosmos to account for what we see.
Building these elements requires an excess of neutrons, one of the particles that make up atomic nuclei; as one might suspect, these are set free in enormous quantities when neutron stars are ripped apart.
By studying the explosion in infrared light, teams determined that the debris contained at least ten thousand Earths worth of precious metals—more than enough to seed the cosmos with the observed amounts.
“These events can actually account for all the gold and all the heavy elements in the universe today,” says Enrico Ramirez-Ruiz of the University of California, Santa Cruz. The observations, he says, are “just breathtaking—the level and quality of the data, it’s just beautiful.”
However, other parts of the story told by these events are still shrouded in mystery. For starters, it’s not exactly clear what was left behind after the two neutron stars collided. All we know is that the remnant of the collision is about 2.6 times as massive as the sun.
Given that mass, and the starting neutron stars, it’s almost certainly a black hole, says the University of Arizona’s Feryal Ozel. Other less likely possibilities include an anomalously hypermassive neutron star; but that kind of object could break what scientists know about the physics of neutron stars.
Regardless of its identity, the collision’s remnant raises a host of questions about the densest known objects in the universe.
“No one has observed either a neutron star or a black hole with well-measured mass between 2 and about 5 solar masses,” says Caltech’s Alan Weinstein, a member of the LIGO team.
Also, the explosion and its aftermath didn’t play out exactly as predicted. The gamma-ray burst was relatively wimpy and much fainter than similar events seen before, says Caltech’s Mansi Kasliwal. Plus, it took longer than expected for x-rays and radio waves to hit detectors following the blast.
That could mean the jets of high-speed radiation sent out by the explosion were not aimed directly at Earth, and were instead slightly off axis, says Daryl Haggard of McGill University, whose team used the Chandra X-ray Observatory to spy on the merger.
Or, it could mean something more complex is going on. Perhaps, Kasliwal suggests, a cocoon of energetic debris thrown off by the explosion has choked whatever jet was initially produced. Scientists are hoping that continued observations in radio waves, which should be visible for quite a while longer, will help resolve the issue.
“Even though the radio emission arrived late to the party, it will be the last to leave—and comes bearing gifts!” says Caltech’s Gregg Hallinan.
Observations reported in late December, which include more than a hundred days of radio data, support the idea of a cocoon-choked jet. That raises the question of whether such mergers are indeed responsible for short gamma-ray bursts. The team reporting those results, published December 20 in the journal Nature, suggest that perhaps colliding neutron stars are responsible for a previously unrecognized type of fleeting astrophysical burst.
For now, further observations will have to wait: The galaxy’s position in the sky is so close to the sun that it’s dangerous for some telescopes to observe it. When it moves slightly farther from our star’s glare, telescopes will again swivel to gaze at the last lingering remnants of the blast.
In the meantime, astronomers will no doubt be celebrating their good fortune at seeing the blast in such detail in the first place.
“This thing exploded 130 million years ago,” says Maria Drout, of the Carnegie Observatories. “But if it had happened a month later, we wouldn’t have been able to see it at all. The detectors would have been turned off, and it would have been behind the sun.”