Compact object

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".

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.

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.