Quark Stars are what happens when matter is squeezed as tightly together as it can be before it implodes into a black hole. Matter in this state has some very unusual properties. For example, its pressure is largely independent of temperature, and because its atoms are so tightly squeezed, only the strong force has any significant influence on its behaviour.
Quark stars, or strange stars as they are sometimes called, are a theoretical type of neutron star. Some researchers suspect that about 0.1% of all neutron stars are quark stars. And, like neutron stars, these stars pack between 1.5 and 2.0 solar masses into a volume about the size of a small city. They, in theory, consist of matter composed of up, down and strange quarks. The idea here is that under extreme pressure, ordinary nuclear matter (organized into protons and neutrons, both consisting of arrangements of up and down quarks) dissociates into quarks and some of these up and down quarks then transform into strange quarks. This arrangement of three kinds of quarks may allow the matter to be packed together more efficiently. The critical pressure required for the transformation from nuclear matter to strange matter is currently unknown. Not enough is known yet about the strong force that governs the behaviour of quarks. What is known, however, is that under ordinary densities and temperatures, the strong force confines quarks into hadrons (protons and neutrons for example). The scale across which this force can act is very short, about 10-15 m and that's about the size of a hadron. However, when density increases to the point where quarks are squeezed together closer than 10-15 m, the hadrons "melt" into quarks and the strong force becomes the dominant force of the entire star. This defines the theoretical quark matter state. This is thought to be the same physical state of matter as the quark-gluon plasma state that dominated the several-microsecond-old universe.
These stars might be almost entirely composed of strange matter surrounded by a thin envelope of nuclear matter, or perhaps all neutron stars, especially those near the mass limit of about 2 solar masses, have within them a strange matter core. Physicists are currently looking for a possible strange matter signature as they observe neutron stars.
If two neutron stars with strange matter cores or two quark stars collide, one would expect pieces of strange matter to fly off into space. What might happen when these bits of strange matter collide with the ordinary nuclear matter of another star or a planet such as Earth? This very remote possibility is explored in the last section of my Neutron Star article.
In 2008, researchers at the University of Calgary in Canada proposed that three recently observed powerful supernova, each about 100 times brighter than a typical supernova, might have been neutron stars exploding into quark stars. They call this the quark-nova hypothesis.
Even though quark stars are only hypothetical at this point, I include them because of the possibilities they offer for getting us closer to a fuller understanding of the mysterious workings of the universe. As researchers continue to explore the possibility of quark stars, we will understand more about the physics of strange matter and quark plasma as well. You can even think of going back in time to the beginnings of the universe as you dig deeper into a quark star. Observations of these stars and the intense supernovas that may produce them might help us understand the incredibly intense first microseconds of the universe's existence.
Next up: Magnetars.
Next up: Magnetars.