SCIENCE
What is a neutron star? End-shutdown
EITHERn August 6, 1967, Jocelyn Bell was looking at scribbles drawn in red pen on moving scrolls of graph paper: data from a radio telescope she was using for her Ph.D. research on distant galaxies. She noticed a strange looking doodle. It was “a bit scruffy,” she tells me from her office at Oxford University, where she is now a visiting professor of astrophysics. The “scruff” was a series of high-pitched pulses that came every 1.3 seconds. Bell continued to watch him the following nights.
Over the next several months, Bell, his Ph.D. Supervisor Antony Hewish and a few colleagues kept the discovery under wraps while reviewing all possible options, especially whether it was a signal from extraterrestrial intelligence. Bell half jokingly remembers being less than thrilled at the prospect of a group of aliens making contact with our planet and abducting his Ph.D. project barely half a year before the defense of his thesis.
Last week, astronomers were amazed by the symmetry of a neutron star collision.
On Dec. 21, he went to look at the data one more time before leaving for Christmas vacation. He saw another scribble, similar to the first, coming from a different part of our galaxy. This was a relief to Bell: there was no way that a second group of aliens was also sending signals to Earth from another part of the sky at the same moment. The pulses had to come from a new and unknown type of astronomical object.
Not that this made much more sense than the alien option. The very short pulses implied a small body, about a tenth of a light-second across, not much bigger than Earth. However, the extreme regularity of the pulses pointed to large reserves of energy, which meant that the object had to be massive, which under any other circumstances would mean it was large. Once they published their findings, a science journalist who described the discovery, Anthony Michaelis of the daily telegraphit gave the new body a nickname that stuck: a pulsar.
The combination of a minute radius and a large mass suggested to Bell, Hewish, and their colleagues that it was a theoretical object known as a neutron star. Today, all these decades later, astrophysicists still don’t know what happens inside these objects. But last summer, in a dramatic demonstration, reported in The letters of the astrophysical journal, they measured a neutron star 2.35 times more massive than our sun, the heaviest known star. Although not everyone accepts the measure yet, it is not out of line. The heaviest precisely known neutron star is 2.08 solar masses, and some more are over 2 solar masses, more massive than some theorists thought possible. It has made them think again about what happens when matter is pushed to its extreme limits.
youImagine our sun, 1.4 million kilometers in diameter, pushing its mass into a volume just 20 kilometers in diameter. That is a neutron star. It is the densest object we know of made of ordinary matter, just a thread away from a black hole. In our galaxy alone there may be several hundred million neutron stars.
Turning a star into the size of a city is no easy task even for the fundamental forces of End-shutdown. Matter tends to resist compression, which is why planets and stars rarely collapse under their own weight. A neutron star is born when an ordinary star is massive enough, eight to 15 times more massive than the sun, has used up all its nuclear fuel, and collapses to extreme densities. The outer layers of the star are thrown into space as a supernova explosion, and the core remains as a neutron star.
Physicists believe that a neutron star is like an egg, with a shell (the shell), an outer core (the egg white), and an inner core (the yolk). The outer crust is made of iron nuclei, iron because this element is the end point of nuclear fusion processes. You dig down and the pressure builds relentlessly. The nuclei are pressed very close together so that they morph into strange shapes. Physicists call this phase “nuclear paste.”
In the outer core, the iron nuclei break down into their constituent protons and neutrons. The protons themselves don’t last very long. They fuse with electrons to form more neutrons. This phase harbors a liquid consisting mainly of neutrons, the so-called neutron soup. It is not an ordinary fluid, but a superfluid that violates many of our intuitions about fluid flow. If you had something in a beaker on Earth, it would climb up the walls.
Up to this point, neutron star material is rare, but it falls within the range of conditions that physicists routinely study in their laboratories. Dig a little deeper to the inner core, and that’s where it’s a total conundrum. The nucleus is denser than an atomic nucleus. Theorists don’t know if the neutrons are still intact there or if they break down further into even smaller particles, quarks. The ultra-low temperatures and enormous pressure could theoretically lead to a kind of quark jelly.
It’s hard to even imagine how to study material so extreme that, by definition, it’s about to implode into a black hole. But you can get very far by considering just two numbers: the size and mass of the neutron star. These reflect the squeezability of whatever form the matter in the inner core takes. To describe this compressibility, physicists formulate the so-called equation of state, which relates density to pressure. There are many different models proposing different compositions, and each model, each equation of state, predicts a certain relationship between the size and mass of the neutron star. The heavier a neutron star is, the higher the pressure must be for a given density.
Push the mass of our sun into a volume 20 kilometers in diameter, that’s a neutron star.
Astronomers have a battery of techniques to measure the mass of neutron stars. One of the best methods is through pulsar timing: measuring the regularity of the pulses over years and decades. The radius is much more difficult to measure accurately.
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 other than neutrons and protons, the best chance to see their signatures is by looking at heavy neutron stars,” says Achim Schwenk, a researcher at the Technical University of Darmstadt who has been analyzing the 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 a scale for the other and vice versa. Another method is to study how deformable neutron stars are when they collide. Deformability tells us how hard it is for gravitational tidal forces to crush one neutron star as the other gets closer. In 2017, two gravitational wave detectors, LIGO in the US and Virgo in Italy, made history when they detected tiny ripples in space-time. The ripples were triggered when two neutron stars collided with each other, disrupting the fabric of the cosmos. Just last week, astronomers studying the consequences of this event discovered that the emerging debris, “a ball of fire enriched with heavy metals”, it was more remarkably symmetrical than expected.
Through various techniques, theorists have been ruling out candidate equations of state. Discoveries of neutron stars heavier than two solar masses indicate that the matter within the inner core cannot be very gelatinous, it must be extremely rigid to support such a mass. But the deformability measured by LIGO and Virgo showed that the equation of state is not too rigid.
However, astronomical observations alone are not enough. The range of densities in the core of a neutron star is enormous, from about half to about five or six times as dense as an atomic nucleus, creating a kind of “density ladder” within the star, as Jorge Piekarewicz, Florida Investigator. State University, he calls it. He and others must apply different theoretical methods to describe all the different layers of a neutron star: the crust, the inner core, etc. No single technique can determine the complete 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 may still come close to reproducing these conditions. One strategy is to collide heavy nuclei such as gold using particle accelerators, for example the heavy-ion synchrotron accelerator 18 at the GSI Helmholtz Center for Heavy Ion Research in Germany. The collisions are analogous to a neutron star merger, but on the femtometer scale. They crush matter to several times the density of atomic nuclei, mimicking the conditions of the outer and inner nuclei. Schwenk says that the information about the equation of state of these collisions is remarkably consistent with the constraints of astrophysics.
At these densities, the fine details of subatomic particles can make a big difference. Normally thought to be the same size, neutrons and protons actually differ slightly in atomic nuclei that contain more neutrons than protons: neutrons get an extra 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 experimenters led by Kent Paschke of the University of Virginia measured the neutron skin at the Jefferson Laboratory in Newport News, Virginia, to verify the theory.
However, the results threw a new surprise. The Jefferson Lab experiment indicated that the neutron star material is extremely stiff, more so than implied by gravitational wave observations. Assuming both are right, that presents a paradox. It may mean that something new is happening inside neutron stars, perhaps an unexpected change of state that turns the quark jelly into something even stranger. “If this hard-to-soft-to-hard result can be confirmed, this may suggest a possible phase transition in the interior of the neutron star,” Piekarewicz says. “To what, whether it’s quarks, hyperons or anything else, it’s too early to say.”
Jocelyn Bell’s discovery of the strange “nape piece” that summer night in 1967 forever changed astronomy. She opened a window into the most extreme matter known in the universe. Neutron stars may not be aliens, but the search for what they’re made of can be just as compelling.
Katia Moskvitch is a theoretical physicist and author of the book Neutron stars: the quest to understand the zombies of the cosmos.
Main 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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