Neutron Star Interior
It is a rare opportunity when we can use astronomical observations to push the frontiers of physics in a way that is not possible in any laboratory on Earth. Yet neutron stars, the dead, dense remnants of massive stars, provide us with just that opportunity.
Crushed under the inward pull of gravity once a massive star’s fuel is exhausted, the stellar core that becomes a neutron star reaches matter densities that are not naturally encountered anywhere else in the universe. (Black holes don’t count: Although they may infinite energy densities, they hide behind horizons and are inaccessible). In fact, the matter that makes up neutron stars has fundamentally changed character, taking us into regimes that are still poorly understood in physics. As a result, a lot of recent efforts have focused on probing these stars’ interiors with astronomical instruments. To be sure, there are some aspects of that unusual matter that we can confidently predict, based on theoretical calculations and laboratory experiments. For example, during the implosion of the star, the electrons that normally surround he nucleus of an atom in the core get pushed into the atomic nuclei. There, they combine with the protons through weak interactions, one of the four types of interactions that take place between particles in the universe. (The other three are strong, gravitational, and electromagnetic.) That combination produces a neutron—thus giving this new star its name—as well as nearly massless, ghostly particles called neutrinos that rapidly escape from the star, carrying with them a large amount of energy the first observational confirmation of this process happened with Supernova 1987 observed in 1987 in the Large Magellanic Cloud (see S&T Feb 2017, p. 36), when two neutrino observatories detected a burst of particles around the same time as the supernova’s visible light appeared in the sky.
Scientists can’t create such neutron-rich matter. Simply smashing together and squeezing them to high densities in heavy ion colliders, such as the RHIC experiment in Brookhaven National Laboratory, doesn’t work. First, collisions in accelerators create very hot matter, which behaves differently than cold matter inside a neutron star. (It’s “cold” not because the temperature is low, but because the thermal energy is so small compared to the energies of other internal interactions that it’s unimportant.) Second, the weak interaction acts over a relatively long time-scale, much longer than the time that particles have to interact with one another when crushed against each other in a collider. Imagine that a plane were flying nearby, in the opposite direction to yours. You might have time to catch a glimpse of a passenger on that flight, but you certainly wouldn’t have time for a hearty handshake—even the if it were physically possible.
Still, if the creation of neutrons during the implosion were all that could happen to the core’s atomic nuclei, astronomers would by now consider the question of what lies inside a neutron star a solved problem. But transforming run-of-the-mill atoms into a super-dense soup of (almost) entirely neutrons turns out to be only the beginning of the particles’ journey.
The Fate of Collapsing Matter
As neutrons become squeezed further together in neutron star cores, they reach densities that are difficult to fathom While the star’s crust may look more or less like normal mater, the core can reach densities that are a hundred trillion (! that is 1014) times higher than the densest natural elements on Earth.
Under those conditions, neutrons not only start vehemently repelling one another but also interacting in new ways. This is because neutrons are examples of particles called fermions, which require increasingly higher energy to be confined closer and closer together. To counter this rising energy, neutrons may find it “energetically favorable”–basically less hassle—to dissolve into their even smaller constituents, called quarks, creating a quark soup. Alternatively, they may form different combinations of quarks than what normally make up a neutron or proton. Such hyper-nuclei, called hyperons, can be created in laboratories but survive only or a short time. In neutron stars, they might be stable.
Yet another possibility is that fermions pair up with one another to form a type of particle called a boson. These particles, which behave differently from fermions, can transition into an unusual superfluid state of matter (the same state that is observed in low-temperature helium fluids, superconductors, and some other metals, called a Bose-Einstein condensate). This sate will have strange properties, such as flowing without friction. If this transition indeed occurs inside the core—and we have good reason to think that it does—it will relieve some of the pressure built up by the high densities that mater experiences there.
But which of these possibilities actually take place in a neutron star’s interior? One of the things that complicate this puzzle is that, while we’ve observed hyperons and many types of bosons as standalone particles in laboratories, a quark has never been observed by itself, in what is called an “unconfined” state. Therefore, predicting and testing the behavior of quark matter becomes very difficult.
But if one could see into the cores of neutron stars and prove that they contain quark matter, it would constitute a major advance in our understanding of these smallest constituents of matter.
Probing the Invisible
How would astronomers go about probing the deep interiors of not only the densest but also the smallest stellar objects in the universe? At roughly 20 km across, neutron stars are smaller than some of the Solar System’s asteroids. Yet they pack into that tiny volume up to two times the mass of the Sun. And unlike asteroids, they are hundreds to many thousands of light-years away. It turns out that measuring the exact sizes of neutron stars, which can be done from a distance, provides the best possible tool for getting a complete picture of their interiors. Our calculations tell us that, if only neutrons remain in the interior, the pressure building up from the repulsive interactions will support a star of a particular size. If any constituents other than neutrons form, their interactions would cause a different amount of repulsion, creating a star of a different size. Thus, astronomers can discover the possibilities in the realm of physics simply by measuring exact diameters of these stars.
Astronomers are used to measuring the sizes of far-away objects by collecting and analyzing the light they emit. Indeed, nearly all our knowledge about the sizes of normal stars comes from measuring both the total light emitted by the star, referred to as its luminosity, as well as the breakdown of that emission into different wavelengths of light, known as its spectrum. The spectrum of the star allows us to determine its temperature; the hotter the stat, the higher the energy of the light that it emits. It’s this temperature that regulates exactly the amount of radiation that emerges from each patch of the surface, allowing us to determine the star’s intrinsic brightness. Comparing that to the total observed luminosity give us an exact measurement of the star’s surface area.
For neutron stars, the approach we take isn’t too different. So far, much of the information we’ve obtained about their sizes has come from nearly the same methods we apply to normal stars, but with a few complications. First off, it is difficult to see down to the surface of many neutron stars. This is because the majority of neutron stars that have ever been observed are enshrouded by strong magnetic fields (called magnetospheres), as well as energetic charged particles that swarm around in these fields like a cloud. These types of neutron stars are exciting in their own right, observable as the beautiful radio and gamma-ray pulsars that spin like lighthouses with exquisite regularity. However, it is difficult to see down to the star’s surface without being overwhelmed by the light emitted from the surface of the neutron star itself, we focus instead on those stars that have very weak magnetic fields, which allows the surface’s x-ray glow to shine through. Some of these stars are not pulsars at all, while others have very weak pulsar emission.
The second difficulty astronomers encounter in measuring sizes comes from the extremely strong gravitational fields neutron stars possess. Thanks to the vast amount of matter packed into such a small volume, a neutron star strongly bends space-time around itself and the path of the light that the star emits becomes significantly distorted. Indeed, the gravitational field is so strong that a neutron star hovers just this side of catastrophe—any denser, and it would collapse into a black hole.
What this means for measuring the size of a neutron star is that an extra step is required to map the observed light through its distorted path back to its origin. This procedure allows us to accurately account for every piece of the surface that contributes to the observed light and obtain a measurement of the entire surface area.
The results we have obtained to date with this method have been pretty striking: Neutron stars turn out than what we would predict if they were made up only of neutrons. More precisely, if none of the possible interactions that give rise to new particles or unconfined quarks take place in their cores, we would expect neutron stars to be 25-26 km across. The measurements point to 20-22 km instead. That may seem like a small difference, but it is in fact large: The central density of two such stars differs by a factor of two. This is enough to have a profound effect on the amount of repulsion the particles experience.
As with all scientific experiments, these results need to be confirmed with independent methods. So far, though, the measurements indicate that neutrons are taking at least one of the possibilities available to them to partially release the pressure valve. Which one, we’re still unsure. We need both additional, independent observations and theoretical investigations to find out.
Gravity to the Rescue
One of the powerful techniques that can reveal the sizes of a neutron star relies on the general relativistic effects that arise due to the star’s extreme gravity—the exact same effects that cause complications in surface area measurements. This time though, they come to our help We apply a technique known as pulse profile modeling to a special class of pulsars. In these sources, the magnetic fields that are anchored on the surface are weak enough that the swarm of particles around the star doesn’t overwhelm the light from its surface. Nevertheless, these magnetic fields are strong enough to guide charged particles toward the star’s magnetic poles, producing a hotspot where the poles meet the crust. As the star spins on its axis, the hotspots come in and out of sight, generating a characteristic pulse in the x rays.
Measuring the pulses’ shapes allows us to determine the size of the star that emitted them. This is because the amount that the light path bends as it leaves the surface of a neutron star depends on how large the star is. In other words, two neutron stars of the same mass but with different sizes, say 20 and 25 km, would create a different pattern of modulation in the light they emit. These patterns can be calculated very precisely and compared to the observed pulses, revealing the sizes of these pulsars.
We are poised to conduct this experiment with an instrument called the Neutron Star Interior Composition Explorer (NICER), which is scheduled for launch to the International Space Station (ISS) this year (2017). NICER is approximately a meter across and consists of carefully designated optical elements that focus the incoming x rays onto 56 silicon detectors. After its journey on a SpaceX resupply mission, it will be unpacked and mounted onto its home on the ISS platform. A star-tracker-based pointing system will then allow the high-precision x-ray timing instrument to point to and track pulsar targets over nearly half of the sky.
What makes NICER unique is its unprecedented capability to record the arrival times of incoming photons with 100-nanosecond precision. This capability will enable the highly faithful reconstruction of the pulse waveforms for a number of pulsars. The detectors will also capture the pulsars’ spectra. Coupled with the precisely determined pulse shape, these measurements will provide all the information necessary for a precise size measurement within a year after its launch.
Another exciting avenue into the neutron star interior will become possible through the detection of gravitational waves with LIGO. Even though the first two events detected by LIGO were coalescing black hole binaries, LIGO is also sensitive to signals from merging neutron stars (S&T Dec, 2015, p. 26). Shortly before the expected coalescence, the pair of inspiraling neutron stars start distorting and pulling each other apart through tidal interactions, obeying the same principles as the Moon’s effect on Earth’s oceans, but far more severe. How severe it is depends on how deformable the stars are, which in turn depends on their size, density, and interior composition. Remarkably, the distortions caused by these tidal interactions are then encoded into the gravitational wave signals that are emitted throughout the inspiral, offering one more penetrating glimpse into the neutron star interior.
If NICER and LIGO experiments confirm the existing measurements of small sizes, the results would point to new physics that emerges when matter becomes ultra-dense. Or the experiments may offer other surprises—that remains to be seen.
But no matter how small and impenetrable they may seem, neutron stars will not be able to hold onto their innermost secrets for much longer
 See Feryal Őzel, “The Inside Story of Neutron Stars,” Sky and Telescope (134, 1, 2017, pp. 16-21). Feryal Őzel is a professor of astronomy and physics at the University of Arizona and a current Guggenheim Fellow. She studies neutron stars and black holes and is a member of the NICER team.