Gravitational Collapse (redirected from Gravitationally collapsed)

gravitational collapse [‚grav·ə′tā·shən·əl kə′laps]

(astronomy)

The implosion of a star or other astronomical body from an initial size to a size hundreds or thousands of times smaller.

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Gravitational Collapse 

(in astronomy), the catastrophically rapid compression of a star under the action of gravitational attraction.

According to existing astronomical conceptions, gravitational collapse plays a decisive role in the late stages of the evolution of massive stars. During the billions of years of its prior existence, a star is in equilibrium: the forces of gravitational attraction, which tend to compress the star’s material, are balanced by the forces of hot gas pressure, which counteract compression. Thermonuclear reactions proceeding in the star’s central regions at temperatures of tens of millions of degrees are the sources of the star’s radiant energy. After several billion years, the star’s nuclear sources of energy are exhausted. Meanwhile, the star continues to lose energy, radiating light into space from its surface and neutrinos from its interior. This leads to a very slow contraction of the star’s central regions. If the star’s mass is not less than 1.2 solar masses, then the density and pressure in the star’s central regions increase so much that nuclear reactions begin to occur involving the breakdown of complex nuclei, during which an enormous amount of heat is absorbed. This leads to the following: with the increase in the density of the gas the forces of hot gas pressure do not rise as fast as the gravitational forces, the equilibrium between these forces is upset, and under the influence of gravity, now not balanced by the force of gas pressure, the star tends to contract—gravitational collapse occurs.

The process takes a fraction of a second, but in this time the density of the central parts of the star increases to that of the atomic nucleus, about 10₁₄ g/cm₃. Now the already powerful repulsive forces of the nuclear particles pressing on each other slow or even halt the compression of matter in the star’s central regions. The falling outer layers of the star encounter the layers that have come to rest, and an outward-traveling shock wave is generated, which is reinforced by neutrinos emanating from the interior and by the detonation of the remnants of the nuclear “fuel” in the star’s envelope. The star’s outer layers are ejected into space. This ejection process is observed as the explosions of supernovas. The core remaining after the ejection of the envelope of a star with a mass not exceeding 2 solar masses is a neutron star. Astronomers observe such stars as sources of pulsating radio emission—pulsars.

If the mass of the star’s core is large (greater than 2 solar masses), then the repulsion of the nuclear particles is not able to withstand the gravity, and the star’s core, after rapid cooling, will continue to contract. In this case, its gravitational field increases so much that the effects of the general theory of relativity begin to play a role, and no force can any longer halt the contraction. This stage of a star’s evolution is called relativistic gravitational collapse. When the star’s radius becomes equal to a critical value (determined by the star’s mass and equal to 3 M₀ km, where M₀ is the star’s mass expressed in solar masses), the gravitational field no longer releases radiation or particles. Such a celestial object is called a black hole or frozen star.

REFERENCE

Zel’dovich, Ia. B., and I. D. Novikov. Teoríia tiagoteniia i evoliutsiia zvezd. Moscow, 1971.

I. D. NOVIKOV