A superdense celestial object that is formed when the remnants of old stars collapse on themselves, denser than a neutron star but not dense enough to become a black hole. Quark stars were first hypothesized in the 1980s, but the first was not discovered until early 2002. Like neutron stars, quark stars are composed of neutrons that have undergone enough pressure by the collapse of the star to have lost their differentiation and dissolved into a mass of quarks and gluons. The up and down quarks of which neutrons are composed then change into strange quarks, with the resulting strange matter compacting into an even denser mass than a neutron star. Also called strange star.