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.
At press conferences held across continents, scientists today announced the first detection of gravitational waves created by two neutron stars smashing into each other.
First theorised by Albert Einstein in 1916, gravitational waves are kinks or distortions in the fabric of spacetime 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.
Image: A Hubble Space Telescope image (left) shows the oval galaxy NGC 4993 as it looked four months before the new gravitational wave detection, while a picture from the Swope Telescope in Chile (right) shows where a bright spot appeared in the galaxy in August 2017. Photographs by Hubble/STScI and 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley
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.
"This was our holy grail but it eluded us even when gravity waves from black holes had been detected. Forty four years later we have found the holy grail!" Explained Winthrop Professor David Blair is a Node Director at the ARC Centre of Excellence for Gravitational Wave Discovery, and from the School of Physics, University of Western Australia.
Unlike colliding black holes, broken neutron stars expel metallic, radioactive debris that can be seen by telescopes—if you know when and where to look.
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, Nature, 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.
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 spacetime on Earth is minuscule—on the order of a fraction of the width of an atomic nucleus.
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.
Image: Using the Swope and Magellan telescopes in Chile, astronomers recorded the neutron star explosion as it suddenly appeared as a bright spot in visible light (left) and then faded (right). After about seven days, the spot was no longer detected in visible wavelengths. Photograph by 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley
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 a few 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 tantalising sign that something dramatic had happened. The team sent out a telegram alerting others to the discovery. Within 42 minutes, five other groups, 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 4889, 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 cosmic fabric of spacetime, generating the gravitational waves that 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.
Image: The fabric of spacetime distorts as two neutron stars spiral in toward their demise in an illustration. Illustration by NSF/LIGO/Sonoma State University/A. Simonnet
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.
"We had not expected to detect such an event so close; as a result it was very loud. To be accompanied by a gamma-ray burst was incredible." says Dr Eric Howell is an ARC DECRA Fellow in the School of Physics and Astrophysics, University of Western Australia.
Also, the explosion and its aftermath didn’t play out exactly as predicted. The gamma-ray burst was relatively wimpy, with much fainter rays 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.
Image: An all-sky map shows the confirmed detections of gravitational waves to date, and one candidate detection. The bands show where spacetime was wrinkled by each event, while the numbers signify the date of detection; the latest event, GW170817, was recorded on August 17, 2017. Illustration by LIGO/Virgo/NASA/Leo Singer/Axel Mellinger
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.
But further observations will have to wait: The galaxy’s position in the sky is so close to the sun right now 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.
"On August 17, the LIGO and Virgo gravitational wave detectors recorded two neutron stars, for the first time, merging about 130 million light years away (one light year is about 10 trillion kilometres). Explains Associate Professor Jeff Cooke is an ARC Future Fellow at the Centre for Astrophysics & Supercomputing, Swinburne University of Technology.
"However, this distance is 'very close' in astronomical terms and is essentially in our ‘back yard’. As such, it gave us a great view of the event. Although an event like this was predicted to be detected (eventually) by the very sensitive LIGO/Virgo detectors, no one expected it to occur so soon..”
Lead image: Two neutron stars crash into each other in an explosive event called a kilonova in this illustration. On October 16, 2017, astronomers announced the first confirmed detection of ripples in spacetime called gravitational waves created by this kind of violent—and visible—event.
Credit: Illustration by Robin Dienel; Courtesy the Carnegie Institution for Science