Exotic star

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 most well evidenced and understood is the quark star.

Exotic stars are largely theoretical, because it is difficult to test in detail how such forms of matter may behave, and (prior to the fledgling technology of gravitational-wave astronomy) we also lack satisfactory means of detection of cosmic objects that do not radiate electromagnetically or through known particles, or ways to verify and distinguish novel cosmic objects from other known objects. For preon stars (and similar) we also lack knowledge about whether quarks are indeed composed of some kind of sub-particle, and therefore whether preons physically exist.

However candidates for such objects are occasionally identified as such, based on indirect evidence gained from properties we can observe, when the object exhibits properties suggesting such a nature.

A quark star is a hypothesized object that results from the decomposition of neutrons into their constituent up and down quarks under gravitational pressure. It is expected to be smaller and denser than a neutron star, and may survive in this new state indefinitely if no extra mass is added. Effectively, it is a very large nucleon. Quark stars that contain strange matter are called strange stars.

...
Wikipedia