Neutron stars represent nature close to the edge of what is possible. Their own gravity is so powerful it crushes protons and electrons together until they fuse, leaving material so dense more mass than the Sun is squeezed into space the size of a small city. Now evidence has emerged that for some neutron stars, even this extraordinary compaction is insufficient, with their centers made up of quark matter, where quarks break out of their usual triads to exist independently.

When a supernova explodes, it throws off most of what was once the star and leaves behind an almost unimaginably dense core. If what remains has a mass more than double the Sun's, it will collapse to a black hole. Somewhat less massive cores have sufficient gravity to squeeze matter into a state even beyond the extremes of white dwarfs, letting nothing but neutrons survive.

Physicists once considered this the densest and most exotic matter possible, black holes aside, but some have pondered the possibility of something yet more extreme; quark matter where the subatomic particles that normally comprise electrons, protons, and neutrons behave like an exotic liquid. Dr Aleksi Vuorinen of the University of Helsinki argues that two recent developments suggest quark matter is rare but real.

"Confirming the existence of quark cores inside neutron stars has been one of the most important goals of neutron star physics ever since this possibility was first entertained roughly 40 years ago," Vuorinen said in a statement.

Just two weeks ago, a team of physicists proposed gravitational waves could help us understand neutron stars' cores, arguing the star's composition should influence oscillations that alter the waves we can detect. Vuorinen claims to have taken a big step down that road. Others have used the wave shape from the first detected neutron star merger to establish an upper limit of 13 kilometers (8 miles) on those stars' radii.

In Nature Physics, Vuorinen and co-authors combine this newfound precision with recent discoveries of particularly massive neutron stars to model the interiors of neutron stars of different sizes. Most of the neutron stars whose mass we have been able to measure fall between 1.4 and 1.7 solar masses. The authors conclude there is no reason to think these go beyond neutron composition to incorporate quark matter.

However, three objects have been found recently that edge towards the point of collapse to black holes, each with approximately twice the Sun's mass. Extrapolating from what we have learned about smaller neutron stars, the team calculate that these would be neutron/quark star hybrids, whose quark matter cores might even exceed half their size.

"There is still a small but nonzero chance that all neutron stars are composed of nuclear matter alone,” Vuorinen acknowledged, owing to some uncertainty about the way sound operates in such extreme conditions.