Life isn’t really like a box of chocolates, but it feels like something is. Neutron stars, some of the densest objects in the Universe, can have structures very similar to chocolates, with sticky or hard centers.
What kinds of particle configurations these centers consist of is still unknown, but new theoretical work that reveals this surprising result could bring us closer to understanding the strange innards of these dead stars and the wild extremes possible in our Universe.
Neutron stars are pretty amazing. If we consider black holes to be objects of immense (if not infinite) concentrations of matter, neutron stars win second place for the Densest in the Universe Award. Once a star with a mass between 8 and 30 times that of the Sun runs out of matter to fuse into its core, it is no longer supported by the outward heat pressure, allowing the core to collapse ·lapse under gravity as its surrounding layer of gases drifts away. in space
The resulting neutron star has a reduced mass of up to about 2.3 times the mass of the Sun, but is contained within a sphere about 20 kilometers (12 mi) in diameter. These things are capital DENSE, and what exactly happens to matter under these mind-boggling pressures is something scientists are dying to know.
Some studies suggest that the nuclei pile up into shapes that resemble pasta. Others suggest even further inside the star, the pressures become so extreme that atomic nuclei cease to exist altogether, condensing into a “soup” of quark matter.
Now, theoretical physicists led by Luciano Rezzolla of Goethe University in Germany have discovered how neutron stars could be like chocolates with different fillings.
The team combined theoretical nuclear physics and astrophysical observations to develop a set of more than a million “equations of state.” They are equations that relate the pressure, temperature and volume of a given system, in this case a neutron star.
Using these, the team developed a scale-dependent description of the speed of sound in neutron stars. And this is where it gets interesting. The speed of sound in a given object, be it a star or a planet, can reveal the structure of its interior.
Just as seismic waves on Earth and Mars propagate differently through materials of different density, revealing structures and layers, sound waves bouncing off stars can reveal what’s going on inside them.
When the team used their equations of state to study the speed of sound in neutron stars, their structures were not uniform in every sense. Rather, neutron stars at the lower end of the mass range, below 1.7 times the mass of the Sun, appeared to have a soft mantle and harder core, while those above 1.7 solar masses had a hard mantle and a soft core.
“This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be,” says Rezzolla.
“Apparently, neutron stars behave a bit like chocolates: light stars look like those chocolates that have a hazelnut in the center surrounded by soft chocolate, while heavy stars can be thought of more like those chocolates where a hard layer contains a soft filling.”
This seems to fit both the nuclear pasta and quark soup interpretations of neutron star interiors, but also provides new information that could help model neutron stars across a range of masses in future work.
This could also explain how, regardless of their masses, all neutron stars have roughly the same diameter of about 20 kilometers.
“Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other at through their gravitational fields”. says physicist Christian Ecker of Goethe University.
“These insights will be particularly important for identifying the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”
Chocolate Praline Nuclear Quark Soup, anyone?
The research has been published in two papers in The Astrophysical Journal Letters. They can be found here and here.