Enlarge / The new study did not make a breakthrough, but reduced the size of the question mark slightly.
How can we understand environments that cannot be reproduced on Earth? This is a challenge that astrophysicists face all the time. In some cases, it is largely a matter of understanding how well-understood physics is applied to extreme conditions and then comparing the results of these equations with observations. But a notable exception is the neutron star, where the corresponding equations become completely unsolvable and the observations do not provide much detail.
So while we are almost certain that there is a layer of near-pure neutrons near the surface of these bodies, we are very uncertain what might exist deeper inside them.
This week, Nature published a study that seeks to bring us closer to understanding. This does not give us an answer – there is still a lot of uncertainty. But this is a great opportunity to look at the process of how scientists can take data from a vast array of sources and begin to remove these uncertainties.
What’s after the neutrons?
The matter that forms neutron stars begins as ionized atoms near the core of a massive star. As the fusion reactions of the star cease to produce enough energy to counteract the attraction of gravity, this matter shrinks, experiencing increasing pressure. The crushing force is enough to remove the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. Eventually, even the electrons in the region force many of the protons, turning them into neutrons.
This finally provides a repulsive force against the crushing force of gravity. Quantum mechanics does not allow neutrons to occupy the same energy state in close proximity, and this prevents neutrons from approaching and thus blocks the collapse in a black hole. But it is possible that there is an intermediate state between a neutron spot and a black hole, in which the boundaries between neutrons begin to break down, leading to strange combinations of their constituent quarks.
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These types of interactions are controlled by the Strong Force, which binds quarks together in protons and neutrons and then binds these protons and neutrons in atomic nuclei. Unfortunately, calculations involving strong force are extremely expensive, computationally. As a result, it is simply not possible to make them work at the type of energies and densities present in the neutron star.
But that doesn’t mean we’re stuck. We have approximate values of the strong force that can be calculated at the respective energies. And while they leave us with considerable uncertainty, it is possible to use a variety of empirical evidence to limit these uncertainties.
How to watch a neutron star
Neutron stars are remarkable in that they are incredibly compact for their mass, squeezing more than the mass of the 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 farther away. So it seems impossible to do too much to depict these objects, right?
Not entirely. Many neutron stars are in systems with another object – in some cases a neutron star. The way in which these two objects affect each other’s orbits can tell us a lot about the mass of a neutron star. NASA also has a special observatory for neutron stars attached to the International Space Station. NICER (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 tracking the behavior of individual hotspots on the star’s surface.
More critical to this work, NICER can detect space-time distortion around large neutron stars and use it to generate a relatively accurate estimate of its size. If this is combined with a solid estimate of the neutron star’s mass, then it is possible to calculate the density and compare it to the type of density you would expect from something that is pure neutrons.
But we are not limited to photons when it comes to estimating the composition of neutron stars. In recent years, neutron star mergers have been detected by 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 patterns.
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