Life isn't really like a box of chocolates, but it seems that something out there is. Neutron stars – some of the densest objects in the Universe – can have structures very similar to chocolates, with either gooey or hard centers.

What kinds of particle configurations those centers consist of is still unknown, but new theoretical work revealing this surprising result could put us a step closer to understanding the strange guts of these dead stars, and the wild extremes possible in our Universe.

Neutron stars are pretty incredible. If we consider black holes to be objects of immense (if not infinite) concentrations of matter, neutron stars win second place in the Universe's Most Dense Award. Once a star with a mass of around 8 to 30 times that of the Sun's runs out of matter to fuse in its core, it's no longer supported by heat's outward pressure, allowing the core to collapse under gravity as its shell of surrounding gases drift off into space.

The resulting neutron star has a reduced mass of up to around 2.3 times the mass of the Sun, but it's squeezed into a sphere around just 20 kilometers (12 miles) across. These things are capital-letters DENSE – and what exactly happens to matter under such mind-blowing pressures is something scientists are dying to know.

Some studies propose that nuclei crowd together until they form shapes that resemble pasta. Others suggest 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 might be akin to 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'. 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 that bounce around in 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 across the board. Rather, the neutron stars on the lower end of the mass range, below 1.7 times the mass of the Sun, seemed to have a squishy mantle and harder core, while those above 1.7 solar masses had a hard mantle and a squishy core.

"This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be," Rezzolla says.

"Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their center surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling."

This seems to fit with both the nuclear pasta and quark soup interpretations of neutron star innards, but it 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 around 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 through their gravitational fields," says physicist Christian Ecker of the University of Goethe.

"These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars."

Chocolate praline nuclear pasta quark soup, anyone?

The research has been published in two papers in The Astrophysical Journal Letters. They can be found here and here.