A crash of stars reveals the origins of heavy elements

Last August 17, at 8:41 a.m. Eastern time, Earth received a message from deep space that solved — perhaps — a decades-old puzzle.

The message began as a subtle quiver in the fabric of space, a gravitational wave. It grew to a cosmic cacophony that included gamma rays, radio waves and visible light. It all emanated from a galaxy roughly 130 million light-years away, where the dense cores of two long-dead stars collided. In the debris from the crash, some of the heaviest atoms in the cosmos, such as gold, platinum and uranium, were born.

For over 60 years, scientists had debated where such elements came from. Some physicists favored supernovas, the violent explosions of massive stars. Others suspected that heavy elements might be generated in the explosive collisions of superdense neutron stars, remnants of supernovas. But no direct conclusive evidence had been available to settle the question. Thanks to the August 2017 gravitational wave signal, though, astronomers could train a full array of instruments on the collision site. Their data now confirm that precious heavy metals and heavier radioactive atoms emerged from the neutron star smashup.

Astrophysicist Friedrich-Karl Thielemann, of the University of Basel in Switzerland, and colleagues had just wrapped up a paper about the issue for the Annual Review of Nuclear and Particle Science.

“It was a relief,” he says. “All the things discussed in my review have been happening.”

Atom factories

Researchers had long ago figured out how lighter elements in the cosmos had formed. The unimaginably high temperatures of the Big Bang gifted the cosmos with hydrogen and helium (plus a dash of lithium) by fusing together primordial protons. Further fusion reactions in the cores of the first stars forged heavier elements, such as the carbon and oxygen needed for life. But stellar fusion can produce elements no heavier than iron, atomic number 26 on the periodic table.

“After that, it gets murky,” says Matthew Mumpower, a physicist at Los Alamos National Laboratory in New Mexico. Stars are in the business of producing energy to radiate into space, which fusion accomplishes nicely. But fusing nuclei heavier than iron consumes energy, rather than releasing it. Populating the rest of the periodic table — the dozens of elements with atomic numbers higher than iron — requires a different strategy.

Adding protons to make nuclei with higher atomic numbers doesn’t usually work. Protons carry a positive electrical charge; it’s tough to stuff more protons into heavy atoms because of the overwhelmingly repulsive positive charge from the protons that are already there.

Fortunately, there are neutrons. They are electrically neutral and so can slip into a nucleus easily. And they possess a secret weapon — the ability to transform into protons. A nucleus capturing a neutron can then emit an electron, turning the neutron into a proton, and thereby raise the atomic number — creating a new, heavier element.

Neutron by neutron

This neutron capture process can proceed slowly or rapidly. When slow, it’s called the s-process: Neutrons enter the nucleus much more slowly than they can create protons. The s-process takes place over thousands of years in the bloated interiors of aging stars. It’s responsible for about half of the elements heavier than iron.

The rest of the periodic table, including its heaviest members, relies on rapid neutron capture: the r-process. If the s-process resembles the gradual carving of a canyon by a trickle of water, then the r-process is like the rupture of a dam wiping out a village. In the r-process, a tsunami of neutrons overwhelms the atoms, penetrating them faster than the rate of changing into protons. Each nucleus becomes stuffed with neutrons until it can’t hold any more. The nucleus becomes dramatically unstable and either splits into two lighter elements, is transformed by high-energy light, or decays as one of its neutrons morphs into a proton, creating a heavier element that can now take on more neutrons.

It’s the r-process that has kept scientists busy for six decades. While physicists struggled to understand the dizzying variety of ways that new nuclei could be created, astronomers searched the cosmos for a site that could produce the necessary torrent of neutrons.

Evidence from existing stars suggested that elements born in the r-process must come from a single type of source. Every star that harbors r-process elements — including the sun — has them in the same relative amounts.

“The relative ratio of r-process elements in the sun is actually universal,” says Alexander Ji, an astronomer at the Carnegie Observatories in Pasadena, California. “The fact that it’s so consistent means that it has to be drawn from a single place.”

For decades, the prime suspects were supernovas, the cataclysmic deaths of stars much more massive than the sun. But the more theorists pursued that possibility, the less likely the supernova explanation seemed. “We were having trouble in supernova models getting enough neutrons fast enough,” says Jennifer Johnson, an astronomer at Ohio State University.

A rare source for rare elements

In 1982 astrophysicists Eugene Symbalisty and David Schramm suggested that collisions between neutron stars might work. Suspected to exist in the 1930s and first detected in the 1960s, neutron stars betrayed their presence by emitting regular pulses of radiation, earning the designation of pulsar. The first binary pulsar — a pair of neutron stars orbiting each other — had been discovered just eight years before Schramm and Symbalisty’s suggestion. Observations of the binary pulsar revealed that over the next few hundred million years, the two neutron stars would spiral closer together and eventually merge. Such a collision would probably eject gobs of neutron-rich material, which could then be folded into the next generation of stars and planets.

Other data also implicated neutron stars as r-process sources. Radioactive elements on Earth, for example, can reveal how much of these elements were created long ago based on how much remains around now. The r-process element plutonium-244, for instance, has a half-life of 81 million years. Its abundance today indicates initial yields much lower than expected from something as relatively common as a supernova. It seems that r-process synthesis must be a relatively rare event.

Dwarf galaxies that orbit the Milky Way have also helped trace heavy element origins. These wispy collections of stars are considered probable remnants of the early universe, untouched since their formation. Because they are simple and pristine, they preserve a clear record of how their elements were synthesized.

In 2015, Ji (then at MIT) and colleagues examined the dwarf galaxy Reticulum II. They found that most of its brightest stars were loaded with r-process elements. Nine other dwarf galaxies showed comparatively few of these elements, indicating that at some point Reticulum II hosted a single, rare event that polluted its stars with r-process debris. Ji and collaborators concluded that a collision between two neutron stars could explain all the observations.

“You would need to have 1,000 supernovas to bring [the r-process abundances] up to the level observed,” says astrophysicist Anna Frebel of MIT, a coauthor of the Reticulum II study. But the dwarf galaxy is so tiny that it would never survive that sort of onslaught. “Just a few supernovas would blow this thing apart,” she says.

While the case for neutron stars was growing, the evidence remained circumstantial. No one had yet witnessed a definitive collision — until last August.

Gravitational tip-off

The initial signal came from LIGO, the Laser Interferometer Gravitational-Wave Observatory, which operates instruments in Louisiana and Washington state. LIGO is designed to sense ripples in space-time from a variety of cosmic calamities. Virgo, a gravitational wave detector in Italy, helped triangulate where the gravitational wave signal was coming from.

Just 1.7 seconds after LIGO spotted the gravitational waves, NASA’s Fermi space telescope recorded a flash of gamma rays coming from the same direction. An automated system alerted the astronomical community. “It was a worldwide effort where basically every telescope pointed at this thing,” Ji says.

Details of the gravitational wave signal revealed that two neutron stars had spiraled together and merged. Colors of light from the crash scene revealed telltale signatures of newly formed heavy elements. The timescale, energy, colors and amount of ejected material all closely matched theoretical predictions for a neutron star merger.

“People are still sorting out which parts of it are not exactly the same,” says Ji. “But the overall picture is almost on the dot.”

Extreme event

The numbers are staggering. The neutron stars, whipping around each other hundreds of times per second, merged in just a few milliseconds. The ejected debris weighed up to as much as 4 percent of the mass of the sun. That material sped away at 20 percent of the speed of light, crossing an expanse equal to Pluto’s distance from the sun in under 30 hours. The entire synthesis of new elements occurred in less than one second. Researchers estimate that the synthesized gold alone was as massive as a few dozen Earths.

“This event was a double slam dunk,” says Brian Metzger, an astrophysicist at Columbia University. The observations revealed how much r-process material was produced and provided an estimate of how often such collisions occur. With just one event, the uncertainties in those estimates are pretty large. But given how long LIGO was searching and how deep into space it could peer, astronomers estimate that every year there are roughly 1,500 such collisions in one cubic gigaparsec, a sphere about 4 billion light-years across.

At that rate there is one collision in the Milky Way every 100,000 years. Supernovas are 100 to 1,000 times more frequent. But given the amount of material that neutron stars' mergers produce, their infrequent collisions are probably enough to account for the heavy elements found in our galaxy, says Mumpower of the Los Alamos lab.

The quest to understand the origin of the elements is far from over, though. “People who tell you that the site of the r-process has been solved are oversimplifying,” Mumpower says. “There are so many uncertainties, just on the nuclear physics alone.”

Last August’s neutron star collision told researchers that they are on the right track. But they don’t yet know if this event was typical or not. LIGO’s detectors are now undergoing an upgrade to improve their sensitivity. Once back online, LIGO could detect dozens of neutron star collisions per year, providing a much clearer picture of how the heavy elements are created and dispersed. “We’re in for a fun ride over the next decade,” Metzger says.