Sets new boundaries for the interior of neutron stars

Enlarge / The new research has not had a breakthrough, but it has shrunk the size of the question mark 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 is largely a matter of finding out how well-understood physics applies to extreme conditions and then comparing the output data from these equations with observations. But a notable exception to this is a neutron star, where the relevant equations become completely insurmountable, and observations do not give many details.

So while we are pretty sure that there is a layer of almost pure neutrons near the surface of these bodies, we are very uncertain about what may exist deeper inside them.

This week, Nature publishes a study that tries to move us closer to an understanding. It does not give us an answer – there is still a lot of uncertainty. But it is a great opportunity to look at the process of how researchers can take data from a wide range of sources and start pondering these uncertainties.

What is after neutrons?

The substance that forms neutron stars starts as ionized atoms near the nucleus of a massive star. When the star’s fusion reactions stop producing enough energy to counteract gravity, this matter contracts, and experiences increasing pressure. The crushing power is enough to eliminate the boundaries between atomic nuclei, and create 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 eventually provides a force to push back against the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same state of energy, in the immediate vicinity, and this prevents the neutrons from getting closer, thus blocking the collapse of a black hole. But it is possible that there is an intermediate state between a lump of neutrons and a black hole, one where the boundaries between the neutrons begin to break down, resulting in strange combinations of their quarks.

This type of interaction is controlled by the strong force, which binds quarks together into protons and neutrons and then binds these protons and neutrons to atomic nuclei. Unfortunately, calculations involving the strong force are extremely costly, computationally. As a result, it is simply not possible to get them to work with the kind of energies and densities found in a neutron star.

But this does not mean that we are stuck. We have approaches to the strong force that can be calculated by relevant energies. And while they leave us with significant uncertainties, it is possible to use a wealth of empirical evidence to limit these uncertainties.

How to look at a neutron star

Neutron stars are remarkable for being incredibly compact for mass, squeezing more than one solar mass inside an object that is only about 20 km in diameter. The closest we know of is hundreds of light years away, and most are much, much longer. So it seems impossible to do too much in the way of depicting these objects, right?

Not completely. Many neutron stars are in systems with a different object – in some cases a neutron star. The way these two objects affect 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 obtain detailed images of neutron stars as they rotate. This has allowed it to do things like track the behavior of individual hot spots on the surface of the star.

More critical to this work, NICER can detect space-time distortion around large neutron stars and use it to generate a reasonably accurate estimate of its size. If it is combined with a solid estimate of the mass of the neutron star, then it is possible to find out the density and compare it with the type of density you expect from something that is pure neutrons.

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