When an incredibly massive star supernovas, it leaves behind one of two things: a neutron star or a black hole. Neutron stars are the leftover cores of massive stars after they have ejected all their other material. They are incredibly small, only about 20 kilometers in diameter, or 175 times smaller than the moon. But these objects have about 1.4 times the mass of our sun, making them incredibly dense (just one teaspoon of material from a neutron star would weigh a billion tons in Earth gravity).
If a neutron star is dense enough, it will warp the very fabric of space-time and become a black hole. But our physical models of the universe suggest that something else might happen to astronomically dense neutron stars that are not quite dense enough to collapse into black holes—they might turn into what are known as "strange stars," or strange quark stars. These theoretical and mysterious objects are the subject of a wonderful new episode from the aptly named YouTube channel Space Time.
When a massive star runs out of fusion fuel and collapses, the individual neutrons of separate atoms smash together. All the electrons and protons are also crushed together to form additional neutrons, creating a substance that is just a flowing mass of almost pure neutrons. When this neutron substance, called neutronium, grows to about 10 kilometers in diameter, the rest of the collapsing material from the star ricochets off it and causes a massive explosion—a supernova.
What remains is a neutron star, and at the very core of that star, it is possible that conditions are so dense that the neutrons begin to overlap and occupy the same quantum state. In these conditions, it is possible that the neutrons break down into their component elementary particlescalled quarks.
Physicists have successfully created minuscule amounts of quark-gluon plasma for small fractions of a second in the Large Hadron Collider, and they have been able to deduce information about quarks from the particles that decay from this quark-gluon plasma. (Gluon is a subatomic particle believed to bind quarks together.)
In the center of a neutron star, however, the immense pressure would force the quarks into a superfluid rather than a plasma. (A superfluid is a fluid with no viscosity, and therefore can flow without losing any energy.) This quark superfluid would have a greater density than even neutronium, if indeed it exists, and we call hypothetical neutron stars with quark superfluid cores quark stars.
There are six types, or "flavors," of quarks: up, down, strange, charm, top, and bottom. Neutrons are composed of one up quark and two down quarks, and if neutrons were to decay into quark matter inside a neutron star, this ratio of up and down quarks would not be stable except under immense pressures. However, it is possible that about half of the down quarks convert into strange quarks during the breakdown process of the neutrons. Quark matter with a third up, down, and strange quarks is known as strange matter.
Strange matter would allow the three types of quarks to exist at a very low energy state, making it possibly the most stable matter in the universe. It is therefore possible that entire stars are composed of strange matter—stars that were big enough to supernova, but just under the limit that would result in a black hole while over the limit that would produce a neutron star. On paper, a strange star, as these theoretical stars are called, could exist in our universe forever.
It's pretty mind-bending stuff, so kick back and watch the Space Time video above to try to get a better sense of all the strange things happening in our universe—including electroweak stars, where quarks themselves are consumed by the massive heat and pressure at the center of an ultra-dense neutron star.
Source: Space Time