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On Aug. 6, 1967, Jocelyn Bell was looking at the squiggles drawn by a red pen on moving rolls of chart paper—the data from a radio telescope she was using to do her Ph.D. research on distant galaxies. She noticed one squiggle that looked odd. It was a “a bit of scruff,” she tells me from her office at Oxford University, where she’s now a visiting professor of astrophysics. The “scruff” was a series of sharp pulses that came every 1.3 seconds. Bell kept on observing it the following nights.
Over the next few months, Bell, her Ph.D. supervisor Antony Hewish, and a few colleagues kept the discovery tightly under wraps while they checked all the possible options, not least whether that was a signal from an extraterrestrial intelligence. Bell, half-jokingly, recalls being less than thrilled at the possibility that a bunch of aliens were contacting our planet and hijacking her Ph.D. project just half a year before her thesis defense.
Last week, astronomers were wowed by the symmetry of a neutron star collision.
On Dec. 21, she went to look at the data one more time before leaving for the Christmas holidays. She spotted another squiggle, similar to the first one, coming from a different part of our galaxy. It came as a relief to Bell: There was no way a second group of aliens would also be signaling Earth from another part of the sky at the same moment. The pulses had to be coming from a new, unknown type of astronomical object.
Not that this made much more sense than the alien option. Very short pulses implied a small body, about a tenth of a light-second across, which is not much bigger than Earth. The extreme regularity of the pulses, though, pointed at great reserves of energy, meaning the object had to be massive, which under any other circumstances would mean it was big. Once they published their findings, a science journalist describing the discovery, Anthony Michaelis from The Daily Telegraph, gave the new body a moniker that stuck: a pulsar.
The combination of tiny radius and large mass suggested to Bell, Hewish, and colleagues that it was a theorized object known as a neutron star. Today, all these decades later, astrophysicists still don’t know what goes on deep inside these objects. But last summer, in a dramatic demonstration, reported in The Astrophysical Journal Letters, they measured a neutron star 2.35 times as massive as our sun, the heaviest known. Although not everyone accepts the measurement just yet, it is not out of line. The heaviest precisely known neutron star is 2.08 solar masses, and a few more are above 2 solar masses—more massive than some theorists had thought possible. It has made them think afresh about what happens when matter is pushed to its utmost limits.
Take our sun, 1.4 million kilometers across, and shove its mass into a volume merely 20 kilometers in diameter. That is a neutron star. It is the densest object we know of made of ordinary matter, just a whisker away from a black hole. There may be several hundred million neutron stars in our galaxy alone.
Crunching a star into the size of a city is no easy task even for the fundamental forces of nature. Matter tends to resist compression, which is why planets and stars do not typically collapse under their own weight. A neutron star is born when an ordinary star is sufficiently massive, eight to 15 times as massive as the sun, has exhausted all its nuclear fuel and collapses to extreme densities. The outer layers of the star are blasted off into space as a supernova explosion, and the core remains as a neutron star.
Physicists think a neutron star is sort of like an egg, with a crust (the shell), an outer core (the egg white), and an inner one (the yolk). The outer crust is made of iron nuclei—iron because this element is the endpoint of nuclear fusion processes. Dig down, and the pressure increases relentlessly. The nuclei get pressed close together so that they morph into weird shapes. Physicists call this phase “nuclear pasta.”
In the outer core, the iron nuclei break down into their constituent protons and neutrons. The protons themselves don’t last long. They fuse with electrons to form more neutrons. This phase is home to a liquid consisting mainly of neutrons, a so-called neutron soup. It is no ordinary fluid, but a superfluid that violates many of our intuitions about fluid flow. If you had some in a beaker on Earth, it would climb up the walls.
Up to this point, neutron-star material is weird, but well within the range of conditions that physicists routinely study in their labs. Dig a bit deeper to the inner core, and that’s where it’s a total enigma. The core is denser than an atomic nucleus. Theorists don’t know whether neutrons are still intact there or if they break down further into even tinier particles, quarks. The ultra-low temperatures and enormous pressure could theoretically lead to a sort of a quark jelly.
It’s hard to imagine even how to study such an extreme material, which, by definition, is on the verge of imploding into a black hole. But you can get remarkably far by considering just two numbers: the size and mass of the neutron star. These reflect the squeezability of whatever form matter in the inner core takes. To describe this squeezability, physicists formulate a so-called equation of state, which relates density to pressure. There are many different models that propose different compositions, and each model—each equation of state—predicts a certain relationship between the neutron star’s size and mass. The heavier a neutron star is, the higher the pressure must be for a given density.
Shove the mass of our sun into a volume 20 kilometers in diameter—that is a neutron star.
Astronomers have a battery of techniques to measure the mass of neutron stars. One of the best methods is via pulsar timing: measuring the regularity of pulses over years and decades. Radius is much more difficult to measure precisely.
Scientists approach the problem from several sides. They combine nuclear theory and experiments with observations of gravitational waves, radio pulses, and X-rays. The X-ray data is an especially important new development, coming from the NICER (Neutron Star Interior Composition Explorer) instrument that NASA installed on the International Space Station in 2017. “If there is an inner core with matter different to neutrons and protons, the best chance of seeing its signatures is by observing heavy neutron stars,” says Achim Schwenk, a researcher at the Technical University of Darmstadt who has been analyzing NICER data.
When a neutron star is in a binary system, the motion of the neutron star and its companion are sensitive to the masses of both objects. One of the objects serves as the weight scale of the other, and vice versa. Another method is to study how deformable neutron stars are when they collide. The deformability tells us just how difficult it is for gravitational tidal forces to squish a neutron star as the other one comes closer. In 2017, two gravitational wave detectors—LIGO in the U.S. and Virgo in Italy—made history when they detected tiny ripples in spacetime. The ripples were triggered when two neutron stars smashed into each other, disturbing the fabric of the cosmos. Just last week, astronomers studying the aftermath of this event found that the emerging debris, “a heavy metal enriched fireball,” was more remarkably symmetrical than expected.
Through various techniques, theorists have been ruling out candidate equations of state. The discoveries of neutron stars heavier than two solar masses indicate that the matter inside the inner core can’t be very jelly-like—it must be extremely stiff to support such a mass. But the deformability measured by LIGO and Virgo showed that the equation of state is not too stiff.
Astronomical observations alone, though, are not enough. The range of densities in a neutron-star core is enormous, from about half to some five to six times as dense as an atomic nucleus—creating a sort of a “density ladder” within the star, as Jorge Piekarewicz, a researcher at Florida State University, calls it. He and others must apply different theoretical methods to describe all the different layers of a neutron star: the crust, inner core, and so on. No single technique can determine the entire equation of state. So the research is interdisciplinary by necessity. “This provides a unique synergy between many fields, all aimed at understanding the structure of matter under conditions that cannot be reproduced in terrestrial laboratories,” Piekarewicz says.
Nuclear experiments can still come close to reproducing these conditions. One strategy is to collide heavy nuclei such as gold using particle accelerators—for example, the Schwerionen synchrotron 18 accelerator at the GSI Helmholtz Centre for Heavy Ion Research in Germany. The collisions are an analogue of a neutron star merger, but on the femtometer scale. They squish matter to several times the densities in atomic nuclei, mimicking conditions in the outer and inner cores. Schwenk says the information on the equation of state from these collisions is remarkably consistent with the constraints from astrophysics.
At these densities, the fine details of subatomic particles can make a huge difference. Neutrons and protons, which are normally thought to be the same size, in fact differ slightly in atomic nuclei that contain more neutrons than protons—the neutrons gain an additional layer, or “skin,” in the jargon. Piekarewicz and his collaborators have argued that the thicker this skin is, the more pressure the neutrons will produce, and the larger neutron stars will be for a given mass. A team of experimentalists led by Kent Paschke from the University of Virginia measured the neutron skin at Jefferson Lab in Newport News, Virginia, to verify the theory.
The results have thrown out a new surprise, though. The Jefferson Lab experiment indicated that neutron-star material is extremely stiff, more than the gravitational-wave observations imply. Assuming both are right, that presents a paradox. It may mean that something new is going on inside neutron stars—perhaps an unexpected change of state that turns quark jelly into something even stranger. “If this stiff-to-soft-to-stiff result can be confirmed, this may suggest a possible phase transition in the neutron star interior,” says Piekarewicz. “To what—be it quarks, hyperons, or something else—is too soon to tell.”
Jocelyn Bell’s discovery of the weird “bit of scruff” that summer night of 1967 forever changed astronomy. It opened a window on the most extreme matter known in the universe. Neutron stars may not be aliens, but the search for what they’re made of may be just as compelling. 
Katia Moskvitch is a theoretical physicist and the author of the book Neutron Stars: The Quest to Understand the Zombies of the Cosmos.
Lead image: The spectacular merger of two neutron stars. NASA / CXC / Trinity University / D. Pooley et al. Illustration: NASA / CXC / M.Weiss
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