kb/data/en.wikipedia.org/wiki/Compact_object-0.md

title	chunk	source	category	tags	date_saved	instance
Compact object	1/3	https://en.wikipedia.org/wiki/Compact_object	reference	science, encyclopedia	2026-05-05T13:31:55.959127+00:00	kb-cron

In astronomy, the term compact object (or compact star) refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density compared to ordinary atomic matter. The term is used as a generalization for cases where the exact nature of a significant gravitational effect isolated to a small radius is not known. Since most compact object types represent endpoints of stellar evolution, they are also called stellar remnants, and accordingly may be called dead stars in popular media reports. The state and type of a stellar remnant depends primarily on the mass of its progenitor star. A compact object that is not a black hole may be called a degenerate star. In June 2020, astronomers reported narrowing down the source of fast radio bursts (FRBs), which may now plausibly include "compact-object mergers and magnetars arising from normal core collapse supernovae".

== Formation == The usual endpoint of stellar evolution is the formation of a compact star. Every active star will eventually evolve to a point where the outward radiation pressure from nuclear fusion in its interior can no longer counteract its own gravity. When this happens, the star collapses under its own weight and undergoes the process of stellar death. For most stars, this will result in the formation of a very dense and compact stellar remnant, also known as a compact star. Compact objects have no internal energy production, but will—with the exception of black holes—usually radiate for millions of years with excess heat left from the collapse itself. According to the most recent understanding, compact stars could also form during the phase separations of the early Universe following the Big Bang. Primordial origins of known compact objects have not been determined with certainty.

== Lifetime == Although compact objects may radiate, and thus cool off and lose energy, they do not depend on high temperatures to maintain their structure, as ordinary stars do. Barring external disturbances and proton decay, they can persist virtually forever. Black holes are however generally believed to finally evaporate from Hawking radiation after trillions of years. According to our current standard models of physical cosmology, all stars will eventually evolve into cool and dark compact stars, by the time the Universe enters the so-called degenerate era in a very distant future. A somewhat wider definition of compact objects may include smaller solid objects such as planets, asteroids, and comets, but such usage is less common. There are a remarkable variety of stars and other clumps of hot matter, but all matter in the Universe must eventually end as dispersed cold particles or some form of compact stellar or substellar object, according to thermodynamics.

== White dwarfs ==

The stars called white or degenerate dwarfs are made up mainly of degenerate matter; typically carbon and oxygen nuclei in a sea of degenerate electrons. White dwarfs arise from the cores of main-sequence stars and are therefore very hot when they are formed. As they cool they will redden and dim until they eventually become dark black dwarfs. White dwarfs were observed in the 19th century, but the extremely high densities and pressures they contain were not explained until the 1920s. The equation of state for degenerate matter is "soft", meaning that adding more mass will result in a smaller object. Continuing to add mass to what begins as a white dwarf, the object shrinks and the central density becomes even greater, with higher degenerate-electron energies. After the degenerate star's mass has grown sufficiently that its radius has shrunk to only a few thousand kilometers, the mass will be approaching the Chandrasekhar limit – the theoretical upper limit of the mass of a white dwarf, about 1.4 times the mass of the Sun (M☉). If matter were removed from the center of a white dwarf and slowly compressed, electrons would first be forced to combine with nuclei, changing their protons to neutrons by inverse beta decay. The equilibrium would shift towards heavier, neutron-richer nuclei that are not stable at everyday densities. As the density increases, these nuclei become still larger and less well-bound. At a critical density of about 4×1014 kg/m3 – called the neutron drip line – the atomic nucleus would tend to dissolve into unbound protons and neutrons. If further compressed, eventually it would reach a point where the matter is on the order of the density of an atomic nucleus – about 2×1017 kg/m3. At that density the matter would be chiefly free neutrons, with a light scattering of protons and electrons.

== Neutron stars ==