Matter is the substance of which all physical objects are composed. The density of matter is the ratio of its mass to volume and is a measure of the composition of matter and the compactness of the constituent entities in it. In units of grams per cubic centimeter (g/cm3) the density of water is 1.0 g/cm3, and the densities of all macroscopic objects found on earth do not exceed roughly 20 g/cm3. However, some stellar objects are believed to be formed of matter with much higher densities. In the 1920s, a star called Sirius B, a binary companion of the star Sirius, was found to be a highly compact object having the mass of our sun but the size of a planet and thus must be composed of matter of very high density, estimated to reach millions of g/cm3. Sirius B is now known to belong to a class of stellar objects called the white dwarf stars. Dense matter physics began as an effort to understand the structure of the white dwarf stars. It matured into a branch of science devoted to the study of the physical properties of dense matter of all types that may be of interests to astrophysical and cosmological investigations.
Since the type of matter under study cannot be found terrestrially, it is impossible to subject it to direct laboratory examination. Hence the study of dense matter physics is mainly theoretical in nature. In the 1920s the emergence of quantum mechanics was making a strong impact on physics, and a theory of dense matter based on the quantum mechanical behavior of electrons at high density was constructed. It marked the dawn of dense matter physics, and this theory remains valid today for the study of white dwarf stars. The subsequent identification of other compact stellar objects such as the neutron stars and black holes greatly intensified the study of dense matter physics. We survey here what can be expected theoretically from dense matter and the implications of present theories on the structure of these compact stellar objects.
On the experimental side, the study is benefited by the fact that if the concept of matter density is extended to include microscopic bodies such as the atomic nuclei, then a substance called nuclear matter, which possesses extremely high density, may be identified. Through nuclear physics study, it is then possible to subject matter with such high density to laboratory examinations. Such experimental information provides an invaluable guide to the study of matter forming the neutron stars.
Compact stellar objects are mainly the remains of stars whose nuclear fuels have been exhausted and are drained of the nuclear energy needed to resist the pull of the gravitational force. As the gravitational force contracts the stellar body, it also grows in strength. This unstable situation is described as gravitational collapse, which continues until a new source of reaction strong enough to oppose the gravitational force becomes available. The search for the physical properties of dense matter responsible for resistances to gravitational collapse is an important aspect in dense matter physics since the results have important astrophysical implications.
The structure and stability of a compact stellar object depend on its composition and the equation of state of the form of matter that it is composed of. The equation of state expresses the pressure generated by the matter substance as a function of its density and temperature. The determination of the composition and the equation of state of dense matter is a prime objective in dense matter studies. These topics are discussed in Sections 3 and 4 after a brief introduction of the basic theoretical method involved is presented in Section 2.
Compact stellar objects perform rotations, pulsations and emissions, and to understand these processes we would need to know, in addition to the equation of state, the properties of dense matter under nonequilibrium condictions. These are called the transport properties which include the electrical and thermal conductivity and viscosity. These intrinsic properties of dense matter are discussed in Section 5. The effects of a strong magnetic field on the transport properties, however, are not included in this writing. Properties related to radiative transfer, such as emissivity and opacity, are discussed in Section 6. However, radiative transfer by photons in dense matter is completely superceded by conductive transfer, and since it does not play an important role, the photon emissivity and opacity in dense matter will not be discussed. Instead, Section 6 concentrates on the much more interesting topic of neutrino emissivity and opacity in dense matter.
The properties of dense matter will be discussed in several separate density domains, each of which is characterized by typical physical properties. In the first density domain,ranges from l02 to l07 gm/cm3, the physical properties are determined to a large extent by the electrons among the constituent atoms. The electrons obey an important quantum mechanical principle called Pauli's exclusion principle which forbids two electrons to occupy the same quantum state in a system. All electrons must take up quantum states that differ in energy, and as the electron density is increased, more and more of the electrons are forced to take on new high-energy quantum states. Consequently, the total energy of the electrons represents by far the largest share of energy in the matter system. It is also responsible for the generation of an internal pressure in the system. All white dwarf stars are believed to be composed of matter with densities falling in this domain, which is known to sustain stable stellar configurations. The physical mechanism mentioned here for electrons is central to establishing the physical properties of dense matter at all density domains, and for this reason it is first introduced in Section 2.
The second density domain ranges from 107 to 1012 g/cm3, where nuclear physics plays a key role. Above l07 g/cm3 the constituent atomic nuclei of the dense matter experiences nuclear transmutations. In general, an increase in density above this point leads to the appearance of nuclei that are richer in neutron content than those occurring before. This process, called neutronization, continues throughout the entire density domain. The process also suppresses the increase in electron number with increase in matter density and thus deprives the matter system of its major source of energy. Matter with densities belonging to this density domain experiences a gradual reduction in compressibility with increasing density and is no longer be able to sustain stable stellar configurations after its density exceeds 108 g/cm3.
As matter density approaches 1012 g/cm3, some nuclei become so rich in neutrons that they cease to bind the excess neutrons; nuclei now appear to be immersed in a sea of neutrons. The onset of such a phenomenon is called neutron drip, a term suggesting that neutrons are dripping out of the nuclei. This leads to the third density domain ranging from 1012to 1015 g/cm3. A rapid increase in neutron density accompanying an increasing matter density leads to the production of energetic neutrons, since neutrons (like electrons) obey Pauli's exclusion principle. Hence the same quantum mechanical mechanism characterizing the first density domain becomes operative here. As soon as neutrons were discovered experimentally in the 1930s, this mechanism was invoked to suggest the possible existence of stable neutron stars, long before neutron stars were actually identified in astronomical observations. Unlike the electrons, however, neutrons interact among themselves with nuclear forces that are comparatively strong and must be handled with great care. The average density of atomic nuclei, or nuclear matter density, is of the order of 1014 g/cm3. Much of the needed physics in understanding matter with density in this density domain must come from nuclear physics.
Our understanding of matter with densities above 1015 g/cm3 is very tentative; for this reason we shall assign matter with densities above 1015 g/cm3 into the fourth and last density domain. In this area we shall discuss the physical basis for topics like hyperonic matter, pion condensation, and quark matter.
Since the study of dense matter is highly theoretical in nature, we begin our discussion with an introduction to the basic theoretical method needed for such an investigation in establishing the composition of dense matter and its equation of state.