I was aware of strange quarks, but unaware of whether any kind of ordinary matter collapse can actually leave you with only strange quarks. I forget what a neutron is made of, but I believe all the particles made of quarks require some mix of different ones.
Looking for help with same, I found This Rather Prescient Slide-Show Display.
Hadrons (particles made up of quarks and gluons) are of two types: mesons and baryons. Mesons (such as pions) are made up of a quark and an antiquark; the color charge of the quark cancels the anticolor of the antiquark. Baryons (such as protons and neutrons) are made up of three quarks, and the three color charges (red, green, blue) add up to a colorless particle.
(Geek alert: these quarks are merely the "valence quarks" of the particle; a real hadron is filled with a "sea" of virtual quark-antiquark pairs of all quark flavors.)
Neutrons are made up of two down quarks and an up quark. Protons are made up of two up quarks and a down quark. Other types of baryons also include one or more of the heavier quarks (strange, charm, bottom, top).
There are other states of matter that are permitted by quantum chromodynamics. The most famous is the quark-gluon plasma. In this state, the hadrons lose all integrity, and the quarks and gluons are free to travel throughout the medium, rather than being confined. This requires a fantastic amount of energy, however, and once it cools enough the plasma crystalizes into hadrons.
However.
Most calculations of quark-gluon plasmas include only up and down quarks. This is sensible, because we try to create such plasmas using heavy ions, which are made up of protons and neutrons. But this may not be a good approximation in all cases. Some calculations indicate that if there are enough strange quarks in the plasma, the plasma may be stable. That is, it may represent a lower energy state than the equivalent baryon-number worth of neutrons. If this is the case, then it is possible that a neutron star might spontaneously transition into this state of matter, becoming one giant subatomic particle (and releasing energy in the process). The properties of such a "strange star" would be very different from a standard neutron star.
Neutron stars are the collapsed cores of some massive stars. They pack roughly the mass of our Sun into a region the size of a cityAt these incredibly high densities, you could cram all of humanity into a volume the size of a sugar cube. Naturally, the people thus crammed wouldn't survive in their current form, and neither does the matter that forms the neutron star.
This matter, which starts out in the original star as a normal, well-adjusted combination of electrons, protons, and neutrons, finds its peace (aka a lower energy state) as almost all neutrons in the neutron star. These stars also have the strongest magnetic fields in the known universe. The strongest inferred neutron star fields are nearly a hundred trillion times stronger than Earth's fields, and even the feeblest neutron star magnetic fields are a hundred million times Earth's, which is a hundred times stronger that any steady field we can generate in a laboratory. Neutron stars are extreme in many other ways, too. For example, maybe you get a warm feeling when you contemplate high-temperature superconductors, with critical temperatures around 100 K? Hah! The protons in the center of neutron stars are believed to become superconducting at 100 million K, so these are the real high-T_c champs of the universe.
All in all, these extremes mean that the study of neutron stars affords us some unique glimpses into areas of physics that we couldn't study otherwise.
So, like, how do we get neutron stars? Neutron stars are believed to form in supernovae such as the one that formed the Crab Nebula, and the stars that eventually become neutron stars are thought to start out with about 15 to 30 times the mass of our sun. These numbers are probably going to change as supernova simulations become more precise, but it appears that for initial masses much less than 15 solar masses the star becomes a white dwarf, whereas for initial masses a lot higher than 30 solar masses you get a black hole instead. In any case, the basic idea is that when the central part of the star fuses its way to iron, it can't go any farther because at low pressures iron 56 has the highest binding energy per nucleon of any element, so fusion or fission of iron 56 requires an energy input. Thus, the iron core just accumulates until it gets to about 1.4 solar masses (the "Chandrasekhar mass"), at which point the electron degeneracy pressure that had been supporting it against gravity gives up the ghost and collapses inward.
At the very high pressures involved in this collapse, it is energetically favorable to combine protons and electrons to form neutrons plus neutrinos. The neutrinos escape after scattering a bit and helping the supernova happen, and the neutrons settle down to become a neutron star, with neutron degeneracy managing to oppose gravity. Since the supernova rate is around 1 per 30 years, and because most supernovae probably make neutron stars instead of black holes, in the 10 billion year lifetime of the galaxy there have probably been 10^8 to 10^9 neutron stars formed. One other way, maybe, of forming neutron stars is to have a white dwarf accrete enough mass to push over the Chandrasekhar mass, causing a collapse. This is speculative, though, so I won't talk about it further.
The guts of a neutron star We'll talk about neutron star evolution in a bit, but let's say you take your run of the mill mature neutron star, which has recovered from its birth trauma. What is its structure like? First, the typical mass of a neutron star is about 1.4 solar masses, and the radius is probably about 10 km. By the way, the "mass" here is the gravitational mass (i.e., what you'd put into Kepler's laws for a satellite orbiting far away). This is distinct from the baryonic mass, which is what you'd get if you took every particle from a neutron star and weighed it on a distant scale. Because the gravitational redshift of a neutron star is so great, the gravitational mass is about 20% lower than the baryonic mass.
Anyway, imagine starting at the surface of a neutron star and burrowing your way down. The surface gravity is about 10^11 times Earth's, and the magnetic field is about 10^12 Gauss, which is enough to completely mess up atomic structure: for example, the ground state binding energy of hydrogen rises to 160 eV in a 10^12 Gauss field, versus 13.6 eV in no field. In the atmosphere and upper crust, you have lots of nuclei, so it isn't primarily neutrons yet. At the top of the crust, the nuclei are mostly iron 56 and lighter elements, but deeper down the pressure is high enough that the equilibrium atomic weights rise, so you might find Z=40, A=120 elements eventually. At densities of 10^6 g/cm^3 the electrons become degenerate, meaning that electrical and thermal conductivities are huge because the electrons can travel great distances before interacting.
Deeper yet, at a density around 4x10^11 g/cm^3, you reach the "neutron drip" layer. At this layer, it becomes energetically favorable for neutrons to float out of the nuclei and move freely around, so the neutrons "drip" out. Even further down, you mainly have free neutrons, with a 5%-10% sprinkling of protons and electrons. As the density increases, you find what has been dubbed the "pasta-antipasta" sequence. At relatively low (about 10^12 g/cm^3) densities, the nucleons are spread out like meatballs that are relatively far from each other. At higher densities, the nucleons merge to form spaghetti-like strands, and at even higher densities the nucleons look like sheets (such as lasagna). Increasing the density further brings a reversal of the above sequence, where you mainly have nucleons but the holes form (in order of increasing density) anti-lasagna, anti-spaghetti, and anti-meatballs (also called Swiss cheese).
When the density exceeds the nuclear density 2.8x10^14 g/cm^3 by a factor of 2 or 3, really exotic stuff might be able to form, like pion condensates, lambda hyperons, delta isobars, and quark-gluon plasmas. Yes, you say, that's all very well for keeping nuclear theorists employed, but how can we possibly tell if it works out in reality? Well, believe it or not, these things may actually have an effect on the cooling history of the star and their spin behavior!