When a massive star collapses, its core can be crushed into a neutron star: a ball about 20 kilometres across, packed so tightly that atoms are squeezed into neutrons. If the core is too heavy, it collapses further into a black hole. Some physicists have wondered whether there is a step in between: a star so dense that the neutrons themselves break apart into their building blocks, quarks. Such an object is called a quark star.
Strange matter
Neutrons and protons are each made of three quarks, held together so tightly that free quarks are never seen. Under enough pressure, the boundaries between neutrons might dissolve into one continuous soup of quarks. The idea of quark stars was first raised in 1965.
It became more intriguing with the strange matter hypothesis. In 1971 Arnold Bodmer, and in 1984 Edward Witten, pointed out that a mixture of up, down and strange quarks might be more stable than ordinary nuclei. If so, a quark star could be made of this “strange matter”, and it would be the true ground state of matter, more stable than the atoms around us. No experiment has found strange matter.
Candidates that faded
In April 2002 NASA’s Chandra X-ray Observatory reported two possible quark stars. RX J1856.5−3754 appeared to be only about 11 kilometres across, too small for a neutron star of its brightness, and the pulsar in the supernova remnant 3C 58 seemed colder than a neutron star should be. Better measurements later revised RX J1856’s radius to about 12 kilometres, about 24 across, comfortably within the range for an ordinary neutron star, and the claim was dropped.
Other candidates have been suggested since, including unusual supernovae and objects detected through gravitational waves. None has been confirmed.
How to find one
From outside, a quark star would look much like a neutron star. One clue could be size: a quark star might be smaller for its mass. Another would be speed of rotation: a pulsar spinning faster than once per millisecond would be strong evidence. No pulsar that fast has been found.