Astronomy: Neutron star shows the lighter side of dense matter

A small and extremely light neutron star — with a radius of approximately 10 km and mass of only 77% that of the Sun — is described in a paper published in Nature Astronomy. This star, which is lighter than theoretically expected, may expand our knowledge of the state in which cold dense matter exists in the Universe.

Neutron stars typically have a mass 1.4 times that of the Sun, and have a radius of just tens of kilometres, making them some of the densest objects in the Universe. However, they have been known to range in mass from 1.17 to 2.35 times the mass of the Sun.

Victor Doroshenko and colleagues calculated the mass of a neutron star found within the supernova remnant known as HESS J1731-347. At 0.77 solar masses it is lower than expected and challenges current understanding of stellar physics. The authors argue that this object might not be a normal neutron star at all, but a more exotic — and as yet undiscovered — object known as a ‘strange star’, a hypothetical star made of quark material.

Neutron stars have been heavily studied in the past, since they can be relatively easy to identify: they sometimes emit copious X-rays and often lie at the centre of supernova remnants. However, an optically bright star recently found at the same location (HESS J1731-347), allowed Doroshenko and co-authors to determine the distance to the pair of stars, and thus calculate the mass of the neutron star and the density of matter within it.