Recent observations made by the Chandra X-ray telescope in conjuntion with the Hubble Space Telescope of the neutron stars RX J1856.5-3754 and 3C58 have given anomalous results. One explanation suggested for these results are that the stars may be comprised of free quarks or possibly other sub-nuclear matter in a crystalline form.
RX J1856.5-3754 appears to be a solid body about 7 miles in diameter, with a surface temperature of 700,000 celsius. Given its mass and size, calculations show it is too small to be a neutron star; the neutron degeneracy pressure is not sufficient to prevent gravity compressing it to a black hole. However it has been hypothesised that at very high densities, it's possible for a 'soup' of free quarks to be stable. Normally quarks are 'bound' and as you try to move them further apart the force between them actually increases, preventing them from ever being observed singly like other particles. The gravitional energy is such in these stars however to possibly provide enough energy to liberate the quarks from their bondage, allowing them to roam free over the volume of the star.
A paper due to be published on June 20, 2002 in The Astrophysical Journal by Jeremy Drake (and co-authors) of the Harvard-Smithsonian Center for Astrophysics will hopefully go into greater depth and answer some of the questions arising from this hypothesis.
Another way to think of the composition of a 'quark star' is that in some ways it is like a vast nuclear particle; like for instance a proton or neutron, which are 'nuggets' of three bound quarks. The star is similar, it is a huge nugget of quark material, where the quarks are bound by gravity! As you move up the gravity well, the potential energy gets less until the free quarks in the nugget behave normally and become bound again.
The observations of 3C58, which was formed in the supernova explosion reported by Chinese and Japanese astronomers in 1181 show the star to be much cooler than predicted. Patrick Slane and Steven Murray, of the Harvard-Smithsonian Center for Astrophysics (a.k.a. CfA), and David Helfand of Columbia University, New York used the Chandra X-ray telescope to observe the supernova remnant. The amount of x-rays given off by the star is far lower than predicted by existing models used to explain the cooling of a neutron star; and the star has an estimated temperature of 1,000,000 celsius, which is too low... Again the gravitional energy is such that new forms of stable matter may be present, comprised of sub-nuclear particles that we can only create on earth in particle accelerators, for fractions of a second. The matter may even be stable enough for it to crystalise!
Of course other explanations may account for these anomalous results, but taken at pure face-value new forms of matter are needed for these objects to exist. Regardless, these stars are so extreme they can provide valuable tests of quantum mechanics on a macroscopic scale.
Primary source :-
http://chandra.harvard.edu
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