Stars that are at least about three times heavier than the sun suffer a spectacular end. They manage to use all elements up to iron as fuel in different shells in their interior. Their core, which is only 10,000 kilometers across, then usually consists of iron and heavier elements. What happens to the dying star now depends mainly on this core. When it exceeds the Chandrasekhar limit of 1.44 solar masses, its matter can no longer resist its own gravity – and the star collapses into a neutron star.

Its matter is compressed so strongly that the electrons of the atomic shell fuse with the protons of the atomic nucleus to neutrons (thereby electron neutrinos are released). The nucleus simultaneously ejects its shell in a gigantic explosion, a supernova. A neutron star is a very compact object – up to 3 solar masses are contained in a sphere 20 kilometers in diameter. One cubic centimeter of it weighs about as much as an iron cube with one kilometer edge length. At its surface, gravity is 200 billion times stronger than on Earth. If an object fell to the ground from a height of one meter, it would hit the ground after a microsecond at 7.2 million kilometers per hour.

Neutron stars thus have at least 1.44 times the mass of the Sun and a radius of only a few tens of kilometers, which makes them the densest objects in the universe. Mostly their mass is between 1.17 and 2.35 times the mass of the Sun. But in the middle of a supernova remnant known as HESS J1731-347, researchers have now found a stellar corpse that is far lighter. An article in Nature Astronomy reports on this small and extremely light neutron star, which has a radius of about 10 km and a mass of only 77% of that of the Sun.

Victor Doroshenko and colleagues calculated the mass of this neutron star, which thus challenges the current understanding of stellar physics. The authors argue that this object may not be a normal neutron star at all, but a more exotic – and previously undiscovered – object called a “strange star,” a hypothetical star made of quark material.

Theoretically, because such objects have not yet been detected, any neutron star could become a quark star if its mass approaches the so-called Tolman-Oppenheimer-Volkoff limit without exceeding it. This limit is between 1.5 and 3 solar masses. Physicists are interested in quark stars because they should be one of the few places where the hypothetical Strange Matter can also exist. It consists of the strange quarks belonging to the standard model of physics. Strange matter, if it is heavy enough with more than 1000 proton masses, should be stable.

How could Doroshenko & Co. weigh the neutron star in the first place? Normally, they show up only by the X-rays they emit. With their typical diameter, they are far too small to image in a telescope. However, a bright star recently found in the same location (HESS J1731-347) allowed Doroshenko and his co-authors to determine the distance to the pair of stars, and thus the mass of the neutron star and the density of matter within it.