Compact Stars | Clockwork of the Universe #1

​In this series I’d like to explore all kinds of topics from various branches of the science tree. The first subject of our dissections will be compact stars (also known as compact objects). Compact stars are very dense bodies — often remains of dead stars. The type of compact object depends mainly on the mass of the star (stars) it's formed from.

Formation

As I already said compact stars are usually formed when an old star dies. It happens when the radiation pressure (pressure exerted by electromagnetic radiation) from the nuclear fusion can no longer balance out its gravitational force. Due to this the star collapses under its own weight and undergoes the process of stellar death.

Compact stars have no means of producing energy but will — with the exception of black holes — radiate with excess heat long after their collapse.

White dwarfs

Also called degenerate dwarfs are the least compressed of stellar remnants. They form from the cores of main-sequence stars, such as our Sun. As they cool down they darken until they eventually become hypothetical black dwarfs.

White dwarfs are made up of what's called electron degenerate matter. Degenerate matter is a fancy name for matter so dense that atoms start to degrade into individual fermions (electrons, protons, neutrons and others). The main force keeping white dwarfs from further collapse is the Pauli exclusion principle which states that identical fermions cannot occupy the same quantum state — that means that two electrons with the same energy levels cannot occupy the same spot in spacetime. Electron degenerate matter is a collection of positively charged nuclei of atoms floating in a sea of electrons.

There is a limit to the mass of an electron-degenerate object and thus white dwarfs. It is the Chandrasekhar limit, beyond which electron degeneracy pressure cannot support the object from further collapse. The theoretical limit is approximately 1.44 M☉ (M☉ — solar mass) for objects with a composition typical for white dwarfs, however the real limit considering the general theory of relativity and with realistic Coulomb corrections is around 1.38 M☉. This limit can be further influenced by the chemical composition of the object and the object's rotation.

Neutron stars

Like white dwarfs they also form from the cores of stars — this time massive supergiant ones with total mass between 10 and 25 M☉.

Under the pressure present in neutron stars most protons and electrons combine creating neutrons (this process is called inverse beta decay) thus becoming neutron degenerate matter (sometimes referred to as neutronium).

The equivalent of Chandrasekhar limit for neutron degenerate matter is Tolman-Oppenheimer-Volkoff limit of probably around 2.2-2.9 M☉. Above this limit a neutron star collapses into a black hole or possibly into a more dense form of degenerate matter.

A very interesting occurrence are so-called glitches — sudden small increases of the rotational speed of a neutron star. It is theorized that glitches are caused by starquakes which in turn happen because as the neutron star slowly slows down it becomes more spherical, but because of the stiffness of their crust starquakes occur to release the tension created. Anti-glitches — sudden decreases of speed — have also been observed, but we currently have no explanation for them.

I’m afraid I will have to leave black holes for another article.

You can expect the next issue — most likely about the Boltzmann brain paradox — in October. See you then!