Why do neutron stars only have neutrons?

A neutron star is basically a supernova remnant, that is, one of the possibilities of the outcome of a supernova event. They are the most dense and smallest stars in the universe. Typically about 12-20 kilometers in diameter, they can have mass upto 3 times that if sun, however the largest was weighed at 2. read more

There remains only neutrons, and the star is as dense as the neucleus of an atom, that is 10^15 g/cm^3. This is called a neutron star. The core of the star collapsed has to be between 1.1 and 3 times the mass of sun in order to be a neutron star. Smaller, a white dwarf; larger, a black hole. read more

In general, compact stars of less than 1.39 M☉ (the Chandrasekhar limit) are white dwarfs, whereas compact stars with a mass between 1.4 M☉ and 3 M☉ (the Tolman–Oppenheimer–Volkoff limit) should be neutron stars. read more

According to a new analysis, exotic states of matter such as free quarks or BECs do not arise inside neutron stars. "Neutron star clocked at mind-boggling velocity". New Scientist. A neutron star has been clocked traveling at more than 1500 kilometers per second. read more