Compact Star

Compact stars are compact objects having a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.
Compact stars are often the end point of Stellar evolution i.e. death of stars. They are also called Stellar Remnants.
The state and type of a compact star depend primarily on the mass of the star that it formed from.
White dwarfs, Neutron stars and Black holes are different types of Compact stars.

Death of Star

Ordinary stars generate thermal energy by nuclear fusion of high elements.
Stellar structure determined by the balance of thermal pressure (due to nuclear burning) and gravity.
As the hydrogen fuel in the core is exhausted, thermal pressure support decreases and the core begins to contract under gravity, raising the temperature to initiate He-burning to produce carbon.
In low mass stars (like the Sun), nuclear burning does not produce beyond carbon.

More massive stars reach sufficient temperature and pressure to cause carbon fusion.
Nuclear burning continues from fusion of carbon to oxygen, neon, magnesium, silicon and iron forming onion like layered internal structure.
The fusion process continues until iron group of nucleus is produced. ( 56 Fe is the most stable nucleus.)

As Iron has the maximum binding energy per nucleon, fusion of iron does not produce energy and nucleus burning steps.

As fusion ceases, thermal pressure support against the core vanishes resulting in a core collapse.

Core Collapse

As the outer layer collapse towards the center, the rapidly shrinking core heats up, causing photo-integration of iron nuclei into the helium nuclei and neutrons. The helium
nuclei also split into protons and neutrons through photo-disintegration.
As the core-density increases, it becomes energetically favorable for electrons and protons to combine (“electron capture”), producing neutrons and neutrinos.

(Crab Nebula exploded in 1054)

Neutrinos rarely interact with normal matter and freely escape the core carrying away energy and further accelerating the collapse.
The collapse may be halted by “degeneracy pressure” or strong nuclear force.
Once the collapse halts, the infalling matter rebounds, producing a shock wave propagating outward.
The shock loses energy by dissociation of heavy elements which can stop the explosion.
The neutrinos from the core which are carrying thermal energy, resuscitate the shock which cause the supernova to explode.
When the progenitor star is below 20 solar masses, the degenerate remnant is a Neutron Star.
The hot newly born “proto-neutron star” cools by emitting neutrinos.
For heavier stars the remnant collapse to Black Hole.

Brief Description of Different Types of Compact Stars

White Dwarfs

White dwarfs are the stellar remnants that most of the main sequence stars end up getting into at the end of their stellar evolution.
White dwarfs are thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star or black hole.
The first white dwarf discovered was in the triple star system of 40 Eridani, which contains the relatively bright main sequence star 40 Eridani A, orbited at a distance by the closer binary system of the white dwarf 40 Eridani B and the main sequence red dwarf 40 Eridani C.
The nearest known white dwarf is Sirius B, at 8.6 light years, the smaller component of the Sirius binary star where its companion Sirius A is a bright star.

(The small white spot is Sirius B which is a white dwarf and the bright star is Sirius A)

White dwarfs can’t generate energy through nuclear fusion. The energy that they emit comes from their stored thermal energy.
Hence white dwarfs cool over a period of time into a cold Black dwarf. Because the length of time it takes for a white dwarf to reach this state is calculated to be longer than the current age of the known universe (approximately 13.8 billion years), it is thought that no black dwarfs yet exist. The oldest known white dwarfs still radiate at temperatures of a few thousand kelvins, which establishes an observational limit on the maximum possible age of the universe.
In a normal star, energy is transported by radiation or by convection. Since white dwarfs are degenerate, electrons can travel long distance before undergoing collisions. The energy transfer is therefore due to electron as in conduction.
The conduction is very efficient, so the interior of the white dwarf is nearly isothermal.
The thin non-degenerate envelope transfers heat less efficiently, causing heat to leak slowly. The mass of an isolated, nonrotating white dwarf cannot exceed the Chandrasekhar limit of ~1.4 M☉. This limit may increase if the white dwarf is rotating rapidly and nonuniformly.
White dwarfs in binary systems can accrete material from a companion star, increasing both their mass and their density. As their mass approaches the Chandrasekhar limit, this could theoretically lead to either the explosive ignition of fusion in the white dwarf or its collapse into a neutron star.

Neutron Star

(Crab Pulsar)

Black Holes

(Sagittarius A*)

Conclusion