What process would bring about a quark star? How would that differ from a neutron star?
A neutron star is the remnant core of a massive, normal star after it has undergone a supernova explosion. It consists mostly of neutrons and is held up against gravitational collapse by "neutron degeneracy pressure" - this is a quantum mechanical effect that resists two neutrons being in the same place and therefore tends to push them apart when they get too close together.
If the remant core of the original star is massive enough (greater than a few times the mass of the sun) then the force of gravity will be stronger than the force of neutron degeneracy pressure and so the star will continue to collapse right on past the neutron star stage. Currently, we think that the star will continue to collapse indefinitely past this point and eventually become a black hole. However, it is possible that there is another effect which comes into play - "quark degeneracy pressure". Quarks are the ultimate building blocks of protons and neutrons - each neutron is made up of three quarks - so it could be the case that as gravity overcomes neutron degeneracy pressure and causes the neutrons to "implode", the quarks put up some resistance of their own, and if the star isn't massive enough for its gravity to overcome that, then you would be left with a quark star instead of a black hole.
It is not yet known whether quark stars actually exist in nature because the details of what happens at such high densities are difficult to calculate. There has been some tentative evidence put forward for the existence of one quark star but it is by no means considered conclusive.
This page last checked on August 29th, 2015.