Placing new boundaries inside neutron stars

Enlarge / The new research didn’t make a breakthrough, but it did reduce the size of the question mark a bit.

How can we understand environments that cannot be reproduced on Earth? This is a challenge that astrophysicists constantly face. In some cases, this is largely a matter of determining how well understood physics applies to extreme conditions, and then comparing the output of these equations to observations. But a notable exception to this is a neutron star, where the relevant equations become completely unsolvable, and the observations don’t provide much detail.

So while we’re pretty sure there’s a layer of nearly pure neutrons near the surface of these bodies, we’re very uncertain as to what might exist deeper within their interiors.

This week, Nature publishes a study that tries to bring us closer to an understanding. That doesn’t give us an answer — there’s still a lot of uncertainty. But it’s a great opportunity to examine the process by which scientists can extract data from a wide range of sources and begin to reduce these uncertainties.

What comes after neutrons?

The matter that forms neutron stars begins with ionized atoms near the core of a massive star. Once the star’s fusion reactions stop producing enough energy to counter the pull of gravity, this material contracts, coming under increasing pressure. The crushing force is enough to eliminate the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the region’s electrons are forced into many protons, converting them into neutrons.

This finally provides a force to repel the overwhelming power of gravity. Quantum mechanics prevents neutrons from occupying the same energy state, in close proximity, which prevents neutrons from getting closer and thus blocks the collapse into a black hole. But it is possible that there is an intermediate state between a blob of neutrons and a black hole, a state where the boundaries between neutrons begin to break down, resulting in strange combinations of their constituent quarks.

These types of interactions are governed by the strong force, which binds quarks into protons and neutrons, and then binds those protons and neutrons into atomic nuclei. Unfortunately, calculations involving the strong force are extremely computationally expensive. As a result, it is simply not possible to operate them with the kind of energies and densities present in a neutron star.

But that doesn’t mean we’re stuck. We have approximations of the strong force which can be calculated at the relevant energies. And, although these leave us with substantial uncertainties, it is possible to use a variety of empirical evidence to limit these uncertainties.

How to look at a neutron star

Neutron stars are notable for being incredibly compact for their mass, squeezing more than the mass of a Sun inside an object that is only about 20 km in diameter. The closest we know is hundreds of light years away, and most are much, much further. So it would seem impossible to overdo it in imaging these objects, would it?

Not entirely. Many neutron stars are found in systems with another object, in some cases a neutron star. How these two objects influence each other can tell us a lot about the mass of a neutron star. NASA also has a dedicated neutron star observatory attached to the International Space Station. NICER (the Neutron star Interior Composition Explorer) uses an array of X-ray telescopes to obtain detailed images of neutron stars as they rotate. This allowed him to do things like track the behavior of individual hotspots on the star’s surface.

More critical to this work, NICER can detect the distortion of spacetime around large neutron stars and use it to generate a reasonably accurate estimate of its size. If this is combined with a solid estimate of the mass of the neutron star, then it is possible to determine the density and compare it with the kind of density you would expect from something that is made up of pure neutrons.

But we are not limited to just photons when it comes to evaluating the composition of neutron stars. In recent years, neutron star mergers have been detected via gravitational waves, and the exact details of this signal depend on the properties of the stars performing the merger. So these mergers may also help rule out some potential neutron star models.

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