1995–2015: What is left: Compact Objects

Abstract

A white dwarf, a neutron star or a black hole are left behind after a star dies. These objects are called ‘compact’ because they are so small compared to the Sun and all other stars. Let us leave the black holes out of the discussion for the moment. White dwarfs and neutron stars have about as much mass as the Sun: a white dwarf is a hundred times smaller, and a neutron star a thousand times smaller still. Thus the density is enormously higher and that has an important consequence: it affects strongly how the gas reacts to the compression caused by the force of gravity inside the compact star. The law of gravity is the same in all normal stars, in white dwarfs and in neutron stars, but the laws of compressibility are totally different. The latter law is called the ‘Equation of State’ or EoS. In normal stars the EoS is the well-known law of Boyle/Gay-Luzac. In white dwarfs the EoS is known from quantum physics and from the properties of electrons and protons. As a consequence, one can calculate reliably how large a white dwarf will be once one knows its mass. In neutron stars the Equation of State is uncertain, mainly because little is known about the behaviour of neutrons in a very dense gas. Models of neutron stars are therefore very uncertain, and consequently the size of a neutron star cannot be predicted reliably. Measuring this size is therefore an important research goal in the study of neutron stars.