Enlarge / New research has not made any progress, but has slightly reduced the size of the question mark.
How can we understand environments that cannot be replicated on Earth? This is a challenge that astrophysicists face all the time. In some cases, it is largely a matter of figuring out how well-understood physics is applied to extreme conditions and then comparing the output of these equations with observations. But a notable exception to this is a neutron star, where the relevant equations become completely intractable and the observations do not provide much detail.
Thus, while we are fairly certain that there is an almost pure neutron layer near the surface of these bodies, we are not sure what could exist deeper inside them.
This week, Nature publishes a study that tries to bring us closer to an understanding. He gives us no answer, there is still a lot of uncertainty. But it is a great opportunity to look at the process of how scientists can take data from a wide variety of sources and begin to reduce these uncertainties.
What happens after neutrons?
The matter that forms neutron stars begins as ionized atoms near the nucleus of a massive star. Once the star’s fusion reactions stop producing enough energy to counteract the attraction of gravity, this matter contracts and experiences increasing pressures. The crushing force is enough to remove the boundaries between the atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the region’s electrons are forced into many of the protons, turning them into neutrons.
This ultimately provides a force to be reckoned with against the crushing power of gravity. Quantum mechanics prevents neutrons from occupying the same energy state, very close together, and this prevents neutrons from getting closer and thus blocks the collapse of a black hole. But there may be an intermediate state between a neutron drop and a black hole, one in which the boundaries between neutrons begin to break, giving rise to strange combinations of their constituent quarks.
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Such interactions are governed by the Strong Force, which binds quarks into protons and neutrons and then binds these protons and neutrons into atomic nuclei. Unfortunately, calculations involving strong force are extremely expensive, computationally. As a result, it is simply not possible to get them to work with the type of energies and densities present in a neutron star.
But that doesn’t mean we’re trapped. We have approximations of the strong force that can be calculated at the relevant energies. And while they 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 stand out for being incredibly compact in their mass, compressing more than one mass of the Sun into an object that is only about 20 km in diameter. The closest we know is hundreds of light-years away, and most are far, far away. So it seems impossible to do too much in the way of imagining these objects, right?
Not at all. Many neutron stars are found 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 neutron star observatory dedicated to the International Space Station. NICER (the neutron star’s inner composition explorer) uses a series of X-ray telescopes to get detailed images of neutron stars as they rotate. This has allowed him to do things like track the behavior of individual hot spots on the star’s surface.
More critically for 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 this 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 to the type of density you would expect from something that is pure neutrons.
But we do not limit ourselves only to photons when it comes to evaluating the composition of neutron stars. In recent years, neutron star fusions have been detected by gravitational waves, and the exact details of this signal depend on the properties of the merging stars. Therefore, these fusions may also help rule out some potential neutron star models.