An exotic star is a hypothetical compact star composed of something other than electrons, protons, neutrons, and muons; and balanced against gravitational collapse by degeneracy pressure or other quantum properties. These include quark stars (composed of quarks) and perhaps strange stars (based upon strange quark matter, a condensate of up, down and strange quarks), as well as speculative preon stars (composed of preons, a hypothetical particle and "building block" of quarks, if quarks prove to be decomposable into component sub-particles). Of the various types of exotic star proposed, the kind we have best understanding of, and evidence for, is the quark star.