A neutron star is the dense, collapsed core of a massive star that exploded as a supernova. The neutron star contains about a Sun's worth of mass packed in a sphere the size of a large city. NASA/Dana Berry

The life-cycle of stars is directly related to their mass. A star with a mass similar to the sun (about half to eight solar masses), when it runs out of fuel (hydrogen, which undergoes nuclear fusion to become helium), will first turn into a red giant star and start burning helium, and then, will shrink to become a white dwarf.

Stars with over 20 solar masses, on the other hand, will likely turn into black holes when they collapse, and those that are much more massive will become supermassive black holes. But there are also stars whose mass is between seven and 20 times that of the sun, and this variety ends up as neutron stars.

After black holes, neutron stars are the second-most dense object known in the universe. For a sense of perspective, a teaspoon of neutron star matter, if brought to Earth, would weigh a billion tons. There is a lot we don’t know about these objects, or how elementary particles behave in conditions of extreme density and pressure that exist inside them.

Even on the outside, theoretical models suggested neutron stars had a radius of between 10 and 16 kilometers (6 to 10 miles), but a new method of measurement proposes a far more specific radius — about 12 kilometers.

Led by a group from the University of Turku, Finland, researchers developed their method by “modeling how thermonuclear explosions taking place in the uppermost layers of the star emit X-rays to us. By comparing the observed X-ray radiation from neutron stars to the state-of-the-art theoretical radiation models, researchers were able to put constraints on the size of the emitting source.”

“We constrained it [neutron star radius] to be around 12 kilometers with about 400 meters accuracy, or maybe 1000 meters if one wants to be really sure. Therefore, the new measurement is a clear improvement compared to that before,” Joonas Nättilä, a doctoral candidate at the university who developed the method, said in a statement Wednesday.

Knowing accurately the radius of neutron stars helps scientists better understand how matter behaves inside them, by better estimating the “nuclear-physical conditions” inside these ultra-dense objects. In particular, it can aid scientists by allowing them to better determine the state of neutron matter, or how much it can be compressed at extremely high densities.

“The density of neutron star matter is circa 100 million tons per cubic centimeter. At the moment, neutron stars are the only objects appearing in nature, with which these types of extreme states of matter can be studied,” Juri Poutanen, the leader of the research group, said in the statement.

The LIGO-Virgo collaboration which recently detected the first gravitational waves caused by a collision between two neutron stars also compared its observations with these new constraints, according to the university’s statement.

The study, titled “Neutron star mass and radius measurements from atmospheric model fits X-ray burst cooling tail spectra,” appeared online in the journal Astronomy & Astrophysics.