When Neutron Stars Collide

Monday, November 12, 2018

Berkeley astrophysicists predict – with astonishing accuracy – what the merger of two neutron stars looks like, and answer a longstanding question about how the cosmos creates heavy elements 

Pictured above at The National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley Lab, Theoretical astrophysicist Daniel Kasen studies how cosmic explosions serve as probes of cosmology and fundamental physics. He is associate professor of physics and astrophysics at UC Berkeley, and a faculty scientist at Lawrence Berkeley National Laboratory. He earned his MS and PhD in physics from Berkeley and joined the faculty in 2010.


Cosmic Thunder and Lightning

A neutron star forms when a massive star runs out of fuel. The star’s outer layers explode in a supernova. Its core collapses into a sphere so dense that protons and electrons are forced together to form neutrons. “Neutron stars are among the densest objects in the universe,” Kasen explains, “with a mass something like our sun compressed into the size of a small city.”

“Occasionally two neutron stars spiral together and collide violently,” he continues. “That process generates gravitational waves and expels matter that radiates across the electromagnetic spectrum. You can learn about the cosmic origin of elements and the physics of neutron stars by studying the material that gets ejected as the two stars come together.”

Gravitational waves are generated by disturbances in space-time caused by violent cosmic collisions. They are often described in terms of sound – a ‘chirp’ produced when the detector receives a gravitational wave (GW) signal. Kasen compares the signals coming from GW170817 to thunder and lightning. “Electromagnetic radiation is the lightning,” he says. “Gravitational waves are the thunder.”


The Birth of Multi-Messenger Physics

“This is the first time we’ve seen both gravitational waves and electromagnetic radiation from a single cosmic event,” he adds. “And that’s been a holy grail for astrophysics.” The availability of different kinds of signals – messengers – to study the same event is called multi-messenger physics.

“Multi-messenger physics has important scientific implications,” Kasen points out. “One of the main results from this event was confirmation that gravity waves travel at essentially the speed of light. Observers saw both light and gravitational waves arrive almost simultaneously, just as Einstein’s General Theory of Relativity predicts. As a result, other theories of gravity that propose different speeds can now be ruled out.”

“Not only could astronomers pinpoint the location and distance of this collision,” he continues. “It became possible, for the first time, to directly study how ultra-dense matter is stretched and torn and fused into heavy elements in a violent collision.”

Kasen and collaborators had been working for almost a decade to develop theoretical models detailing what could be happening when neutron stars collide and what the light coming from such an event would look like. Telescope observations of GW170817 gave them their first chance to see how their models compare with astronomical observations. “Luckily, at only 130 million light years away, this event was remarkably close to us,” Kasen notes.

Eliot Quataert, Berkeley professor of astronomy and physics and one of Kasen’s collaborators, said in a media release, “We were anticipating LIGO finding a neutron star merger in the coming years but to see it so nearby – for astronomers – and so bright in normal light has exceeded all of our wildest expectations. And, even more amazingly, it turns out that most of our predictions of what neutron star mergers would look like as seen by normal telescopes were right!”


Heavy Elements and Kilonova Models

Astronomers have long wondered where and how the cosmos creates heavy elements. Fusion reactions in the superheated cores of stars generate elements up to iron, with 26 protons in its nucleus. Building heavier elements requires increasing the number of protons in atomic nuclei, but electrical forces make it extremely difficult to push positively charged protons into an already positively charged nucleus.

“The forces of repulsion get more extreme as the atoms get heavier,” Kasen explains. “The trick to going beyond iron is to bombard atoms with neutrons. Neutrons have no charge, so they can more easily enter the nucleus. Once captured, they decay into protons.”

For decades, theorists had proposed that colliding neutron stars could provide the dynamic, neutron-rich conditions required to generate elements heavier than iron. And there was speculation that such an event would produce a radioactive glow in observable wavelengths of light, enabling astronomers to confirm heavy elements were forming. But no such event had been seen, and no one had delved deeply into the details.

In 2010, Kasen and Quataert co-authored a paper led by Brian Metzger, then a Berkeley graduate student who is now professor of astronomy and physics at Columbia. They were the first to estimate the brightness of glowing debris from a neutron star merger. Their calculations predicted an explosion that would burn a thousand times brighter than a stellar nova, prompting them to coin the term ‘kilonova’ to describe it.

Since then, Kasen and collaborators have been developing increasingly detailed kilonova models, using supercomputers to simulate not only the radioactive glow, but also the physics of the collision itself and the material ejected in the explosion.

“The Berkeley group did a lot of the pioneering work on what the light coming from a kilonova would look like.” Kasen reports, “and how we can use it to infer the chemical makeup and speed of the debris cloud.”

Theory usually follows observation – after a discovery, theorists work to explain what happened, why, and how. “Astronomy is primarily an observational science,” Kasen points out. “This was one of those rare cases where theorists developed very detailed predictions of this phenomenon, then observers went out and found it.”

With no observational data to start from, Kasen and colleagues had to go back to first principles, creating kilonova simulations by integrating known fundamentals of everything from atomic and nuclear physics to electromagnetism, general relativity, and hydrodynamics. They also pushed into new territory, taking a deep dive into the quantum mechanics needed for their simulations. “In a sense,” Kasen notes, “we had to build our atoms from the ground up, calculating how they absorb and emit light.”

The depth and complexity of kilonova models require intensive computer power. Kasen and collaborators have thus far depended on the supercomputers at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Lab. They are now embarking on a new project called Exastar, part of a broad effort supported by Department of Energy to build the next generation of supercomputers.


Models Match Observations

Details predicted by the team’s kilonova models were confirmed by observations of the event itself. The overall brightness, the speed with which the glow faded, and even changes in color over time were well within expected ranges.

Jennifer Barnes, now an Einstein postdoctoral fellow at Columbia, worked with Kasen while she was a graduate student at Berkeley. “One of the interesting things we found,” Kasen recalls, “was that some of these elements, in particular the lanthanides and actinides that sit at the bottom of the periodic table, are incredibly opaque. They block out visible light and reradiate it in red or infrared.”

During the first hours after the merger, visible light glowed in the blue region of the spectrum, indicating formation of comparatively light elements like silver. Over the next several days the glow moved increasingly into the red, signaling formation of heavier elements like gold, platinum, and the lanthanides. The colors finally peaked at the predicted infrared wavelengths.

“Our group had special expertise in understanding the physics needed to visualize what we would see.” Kasen said.  “By comparing the data to our models, we also were able to determine the total mass of elements produced. It was the equivalent of six percent of the mass of the sun; about 2/3 of it composed of the heaviest elements and 1/3 of lighter elements. Within that mixture would be about 100 earth masses of gold and platinum, along with uranium, plutonium, and even some rare super-heavy elements like Berkelium and Californium.”

“Now that we have data,” he says, “we can probe physics in a new regime of extreme densities and temperatures. We’re now looking at what we can rule out in terms of recently proposed theories, including some theories of dark matter.”


A New Field of Study

Kasen and collaborators are fine-tuning their models in preparation for the chance to analyze more kilonovae. Just a few days after GW170817 was detected, LIGO was shut down and will come back on line in early 2019 with improved sensitivity. “They expect the rate of gravity wave detections to go up by almost a factor of ten,” Kasen reports. “So my guess is they’ll find many more kilonova events.”

“What’s so exciting,” he concludes, “is that we’re seeing the creation of a whole new field of study in astrophysics. What started out as theorists’ imaginings has now been observed, and we have much to learn.”


This article appears in the 2018 Physics@Berkeley magazine

Devi Mathieu