Ok, my interest is piqued. My limited knowledge of physics tells me that quarks cannot exist independently, and yet what is this I hear about quark stars? Anyone care to fill me in?
I would guess that the extreme pressures and temperatures in these stars rupture the neutrons into their constituent quarks. The physicists will figure it out eventually.
Here’s an article from the journal Nature on the subject.
Here’s a NASA answer. Most importantly they say “quark stars are probably a very unstable and fleeting phase in the life of a very massive neutron star.”
Sounds pretty simple.
I think I heard about these things (in a talk, not a paper), and I’m not sure I’m getting this exactly right, but here’s the basic idea as I understand it:
I assume you know what a neutron star is. There’s a law of quantum physics which states that you can only cram so much of a certain kind of particle into a given space. That’s why neutron stars can’t get any smaller, because the neutrons cannot be packed any tighter. However, neutrons are only comprised of up and down quarks. There are so many up quarks you can cram into a space, and so many down quarks you can cram into that same space.
But what if there were three types of quarks? Then you could convert one-third of the up quarks and one-third of the down quarks into this new kind of quark, this strange quark, and you could fit more of them in the same space. Of course, there are strange quarks. (There are, in fact, six types of quarks, but three of them are hermits.) So a strange star (or quark star) is one in which the pressure is so great that some of the stuff has turned into strange matter. It’s like a one-up on a neutron star.
Aha, rereading the OP, I understand that you probably know all of what I just said. Your question as to whether quarks can exist independently, though, I don’t know. The talk I saw didn’t specifically address this, so I don’t think it’s a big problem for the physicists. I guess that either existing in a big quark soup does not count as existing independently, or we see the formation of certain strange mesons or baryons, like the Λ[sup]0[/sup] (uds).
A couple of things:
It is my understanding that half of the down quarks become strange quarks, none of the up quarks do. u/d/d -> u/d/s
The NASA page is probably a bit outdated. Recent research indicates that quark stars may be stable. The jury is still out.
These things are confusing as heck because no one understands the details yet (if they exist at all).
“It is my understanding that half of the down quarks become strange quarks, none of the up quarks do. u/d/d -> u/d/s”
I believe that. It makes more sense than what I said, because it conserves charge.
Cool guys, thanks for what you’ve got there.
So they’re strange stars, and that’s not a value judgment but a physical description?
Strange…
It gets even more fun than that, Polycarp. Strange, along with Up, Down, Charm, Truth, and Beauty, is a flavor. 
Free quarks are still presumed impossible, but the quarks in a strange star still aren’t free, being in the presence of gazillions of other quarks. Presumably, it’s similar to the quark-gluon plasma which was produced at CERN a few years ago, except that CERN didn’t have the pressures available to keep it (semi)stable.
But what I don’t get is if they aren’t free quarks, then wouldn’t they have to be other types of particles? Isn’t that the essence of particle-ness, being comprised of joined quarks?
Think of it as a “soup” of quarks and gluons with the immense gravitational pressure keeping the soup liquified which would “freeze” into individual nucleons if the pressure wasn’t there. As a matter of fact, two researchers in Dallas claim to have found sesmic evidence for such strange quark matter “nuggets” having passed through the earth.