Gravitational compression

{{short description|Compression of a massive object by its own gravity}}

In astrophysics, gravitational compression is a phenomenon in which gravity, acting on the mass of an object, compresses it, reducing its size and increasing the object's density.

File:Stellar core.gif of a star such as the Sun, gravitational pressure is balanced by the outward thermal pressure from fusion reactions, temporarily halting gravitational compression.]]

At the center of a planet or star, gravitational compression produces heat by the Kelvin–Helmholtz mechanism. This is the mechanism that explains how Jupiter continues to radiate heat produced by its gravitational compression.{{cite web | url = http://www.iki.rssi.ru/nineplanets/jupiter.html | title = Jupiter | publisher = Space Research Institute,Russian Academy of Sciences |access-date = 2009-11-05}}

The most common reference to gravitational compression is stellar evolution. The Sun and other main-sequence stars are produced by the initial gravitational collapse of a molecular cloud. Assuming the mass of the material is large enough, gravitational compression reduces the size of the core, increasing its temperature until hydrogen fusion can begin. This hydrogen-to-helium fusion reaction releases energy that balances the inward gravitational pressure and the star becomes stable for millions of years. No further gravitational compression occurs until the hydrogen is nearly used up, reducing the thermal pressure of the fusion reaction.{{cite web |url = http://www.space.com/scienceastronomy/astronomy/star_formation_010116.html | title = How a Star is Born: Clouds Lift on Missing Link | date = 16 January 2001 | author = R.R. Britt | website = Space.com | access-date = 2009-11-05}} At the end of the Sun's life, gravitational compression will turn it into a white dwarf.

{{Cite web

| title = White Dwarf Stars

| publisher = Astrophysics Science Division, NASA Goddard Space Flight Center.

| date = November 2006

| url = http://imagine.gsfc.nasa.gov/docs/science/know_l2/dwarfs.html

| access-date = 2009-11-05

}}

At the other end of the scale are massive stars. These stars burn their fuel very quickly, ending their lives as supernovae, after which further gravitational compression will produce either a neutron star

{{Cite web

| author = M. Coleman Miller

| title = Introduction to neutron stars

| publisher = University of Maryland

| url = http://www.astro.umd.edu/~miller/nstar.html

| access-date = 2009-11-05

}} or a black hole

{{Cite web

| author = N. Strobel

| title = Black Holes

| publisher = Nick Strobel's Astronomy Notes

| date = June 2, 2007

| url = http://www.astronomynotes.com/evolutn/s13.htm

| access-date = 2009-11-05

}} from the remnants.

For planets and moons, equilibrium is reached when the gravitational compression is balanced by a pressure gradient. This pressure gradient is in the opposite direction due to the strength of the material, at which point gravitational compression ceases.