Life isn’t really like a box of chocolates, but it seems like something exists. Neutron stars – some of the densest objects in the Universe – can have structures very similar to chocolates, with gooey or hard centers.
The kinds of particle configurations that make up these centers are still unknown, but new theoretical work revealing this startling 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. Considering black holes as objects of immense (if not infinite) concentrations of matter, neutron stars take second place in the densest prize in the Universe. Once a star with a mass about 8 to 30 times that of the Sun runs out of material to fuse into its core, it is no longer supported by the external pressure of heat, allowing the core to collapse. under the effect of gravity while its shell of surroundings gases drift in space.
The resulting neutron star has a reduced mass of up to about 2.3 times the mass of the Sun, but is stuck in a sphere about 20 kilometers (12 miles) in diameter. These things are capitalized DENSE – and exactly what matters under such mind-boggling pressures is something scientists are dying to know.
Some studies propose that the nuclei stick together until they form shapes that resemble pasta. Others suggest that even deeper inside the star, 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 over a million “equations of state”. These 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, acoustic waves bouncing around stars can reveal what’s going on inside.
When the team used their equations of state to study the speed of sound in neutron stars, their structures were not uniform at all levels. In contrast, neutron stars at the lower end of the mass range, below 1.7 times the mass of the Sun, appeared to have a spongy mantle and a harder core, while those above 1 .7 solar mass had a hard mantle and a spongy core.
“This result is very interesting because it gives us a direct measure of the center compressibility of neutron stars,” says Rezzolla.
“Neutron stars apparently behave a bit like chocolate pralines: light stars look like those chocolates that have a hazelnut in their center surrounded by soft chocolate, while heavy stars can be thought of more like those chocolates where a layer hard contains a soft filling.”
This appears to fit both nuclear paste and quark soup interpretations of neutron star innards, but it also provides new information that could help model neutron stars over a range of masses in future work.
It could also explain how, regardless of mass, all neutron stars have roughly the same diameter of about 20 kilometers.
“Our extensive numerical study allows us not only to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, i.e. how much they distort each other through their gravitational fields.” says physicist Christian Ecker of Goethe University.
“This information will become particularly important for identifying the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”
Chocolate praline nuke nugget quark soup, anyone?
The research has been published in two articles in Letters from the Astrophysical Journal. They can be found here and here.
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