"Ordinary" neutron stars are small (with radius of about 10 km), fast rotating and heavy objects, created in explosions of massive (8M☺<M<M☺) supenova like that in 1054* in Crab Nebula.
Crab Nebula [European Southern Observatory]
They consist of neutrons** - rather than from u and d quarks in "proper" propotions. [see Proton & Co. ].
Proton [dr T. Wróblewski, IF PAP, Słupsk]
The neutron ball is similar to a giant atomic nucleus with Z=1057. With such a big compression, neutrons start to "crowd-up", following the Pauli's rule, which does not allow them to be in the same quantum state. It is energetically useful to replace some neutrons with protons, or even by isolated quarks.
It seems [ A.R. Bodmer, 1971 , E. Witten 1984 ] that the state of the matter with a lowest possible energy level is a mixture of the "normal" u and d quarks with strange quarks s. It is solely the big mass of the s quark (30 times more than that of u) which protects us from falling nucleus of 56 Fe, for example, to a quark state.
Diameter of a star consisting of quark matter would be 7-8 km. It could rotate very quickly, even quicker than 0.5 ms. The fast rotating quasar PSR1937+21 has a rotation period slightly too big - 1,56 ms. But the quasar J1808.4-3658, taking into account his mass and radius seems to be a strange star or even a quark one***.
One of the quarks [dr T. Wróblewski, IF PAP, Słupsk]
*That supernova explosion was noted
in China, and probably at rock paintings of Indians in New Mexico. Of course,
the explosion had place much earlier, but the signal reached Earth just in
1054. [see Supernowa]
** Properly, there must be some protons, electrons and mions created during
neutrons decay in weak interactions.
*** D. Li et al. Phys. Rev. Lett. 83 (1999) 3776
Source: I. Bombaci, A. Drago, INFN Notizie, Feb. 2003 (INFN,
Italy)