Putting new limits on the interior of neutron stars

Extend / The new research didn’t make a big leap forward, but it did make the question mark smaller in size a bit.

How can we understand environments that cannot be replicated on Earth? This is a challenge astrophysicists face all the time. In some cases, it’s largely a matter of figuring out 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 intractable and observations don’t provide much detail.

So while we’re sure there’s a layer of nearly pure neutrons near the surface of these bodies, we’re not sure what might exist deeper within their interiors.

This week, Nature is publishing a study that tries to bring us closer to an understanding. It doesn’t give us an answer — there’s still a lot of uncertainty. But it’s a great opportunity to look at the process of how scientists can get data from a huge variety of sources and start to reduce those uncertainties.

What comes after neutrons?

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

This finally provides a force to push back against the overwhelming power of gravity. Quantum mechanics prevents neutrons from occupying the same energy state, in close proximity, and this prevents neutrons from getting closer and thus blocks collapsing into a black hole. But it is possible that there is an intermediate state between a neutron bubble and a black hole, 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 together 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 make them work with the kind of energy and density present in a neutron star.

But that doesn’t mean we’re stuck. We have approximations of the strong force that can be calculated at relevant energies. And while this leaves 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, compressing more than the mass of a Sun into an object just 20 km in diameter. The closest we know of is hundreds of light-years away, and most are much, much further away. So it seems like it’s impossible to do much in the way of imagining these objects, right?

Not entirely. Many neutron stars are in systems with another object – in some cases, a neutron star. The way these two objects influence each other’s orbits 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 a series of X-ray telescopes to take detailed images of neutron stars as they rotate. This made it possible to do things like track the behavior of individual hot spots on the surface of the star.

More critically for this work, NICER can detect space-time distortion around large neutron stars and use that to generate a reasonably accurate estimate of their size. If this is combined with a solid estimate of the neutron star’s mass, it is possible to figure out the density and compare it to the kind of density you would expect from something that is pure neutrons.

But we’re not limited to just photons when it comes to assessing the composition of neutron stars. In recent years, neutron star mergers have been detected using gravitational waves, and the exact details of this signal depend on the properties of the merging stars. Therefore, these mergers may also help rule out some potential models of neutron stars.

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