The existence of neutron stars was first proposed by Baade and Zwicky in 1934, right after the discovery of the neutron by Chadwick in 1932. But what are neutron stars really made out of? Only neutrons? In fact, the answer to this is no. Although neutrons are the most abundant constituents of neutrons stars, other types of particles may exist in their interiors as well. Chiefly among them are protons and electrons, which together with neutrons form atoms, the building blocks of terrestrial matter. The amount of electrons in a neutron star is such that the star’s interior is electrically neutralized. Other particles like muons, which ought to exist in the cores of neutron stars too, are well known from terrestrial cosmic ray observations, which show that muons arrive very abundantly at the surface of the Earth. Presently it is being believed that numerous additional types of particles and possibly novel states of matter, some of which can be produced and studied in energetic collider experiments, may exist in the interiors of neutron stars. These range from hyperons (particles like the nucleon but containing a strange quark), to Bose-Einstein condensates of pions or kaons (a special phase of bosonic matter), to quark matter. Cold quark matter is not accessible in the laboratory, but may exist in different phases, each one with its unique property, in the central cores of neutron stars. Determining observational signatures that could indicate that these exotic forms of matter really exist inside of neutron stars is one of the present topics of research.
Author: Constança Providência