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 very much a matter of figuring out how to apply well-understood physics to extreme conditions and then comparing the output of these equations with observations. But the notable exception to this is the neutron star, where the relevant equations become quite intractable, and the observations don’t provide much detail.
Therefore, while we are sure that there is a layer of nearly pure neutrons near the surface of these objects, we are not entirely sure what might be present in their innermost depth.
This week, Nature is publishing a study that attempts to bring us closer to understanding. It does not give us an answer – there is still a lot of uncertainty. But it’s a great opportunity to look at the process of how scientists can take data from a wide range of sources and begin to reduce these uncertainties.
What about neutrons?
The matter that makes up neutron stars begins as ionized atoms near the core of a massive star. Once a star’s fusion reactions stop producing enough energy to counteract the gravitational pull, this material contracts, and experiences increasing pressures. The crushing force is enough to eliminate the boundaries between atomic nuclei, creating a giant soup of protons and neutrons. In the end, even the electrons in the region are forced to form many protons, converting them into neutrons.
This finally provides a force to compress the crushing force of gravity. Quantum mechanics prevents neutrons from occupying the same energy state, in close proximity, and this prevents neutrons from getting close, and thus prevents collapsing into a black hole. But it is possible that there is an intermediate state between a mass of neutrons and a black hole, where the boundaries between neutrons begin to collapse, resulting in strange clusters of their constituent quarks.
These types of interactions are subject to the strong force, which binds quarks together into protons and neutrons and then binds those protons and neutrons into the nuclei of an atom. Unfortunately, calculations involving extreme force are computationally very expensive. As a result, it is not possible to get them to work with the kind of energies and densities found in a neutron star.
But that doesn’t mean we’re stuck. We have rough estimates of the strong force that can be calculated at the relevant energies. And while these leave us with great uncertainty, it is possible to use a variety of empirical evidence to reduce these uncertainties.
How do you look at a neutron star
Neutron stars are incredibly compact for their mass, squeezing a mass more than the mass of the Sun inside an object only about 20 km wide. The closest we know of is hundreds of light-years away, and most of it is much farther away. So, it seems like it’s impossible to do much with the way these things are portrayed, right?
Not completely. Many neutron stars are in other-body systems – 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 (Nutron Star Interior Composition Explorer) uses an array of X-ray telescopes to obtain detailed images of neutron stars as they rotate. This allowed her to do things like track a file Single hotspot behavior on the surface of the star.
And most importantly for this work, NICER . can Spacetime distortion detection around large neutron stars and use this to generate a reasonably accurate estimate of their size. If combined with a solid estimate of a neutron star’s mass, it’s possible to find out the density and compare it to the kind of density you’d expect from something that is pure neutrons.
But we’re not just limited to photons when it comes to evaluating the formation of neutron stars. In the last years , neutron star merger Detected via gravitational waves, the exact details of this signal depend on the properties of the stars doing the merging. Therefore, these mergers could also help rule out some potential neutron star models.