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More specifically the term gravitational collapse refers to the gravitational collapse of a star at the end of its life time, also called the death of the star. When all stellar energy sources are exhausted, the interior of a star will undergo a gravitational collapse. In this sense a star is a "temporary" equilibrium state between a gravitational collapse at stellar birth and a gravitational collapse at stellar death. The end states are called compact stars, either white dwarfs or neutron stars. Very massive stars cannot find a new dynamical equilibrium; they keep contracting. They are said to undergo a continued gravitational collapse or catastrophic gravitational collapse. With increasing speed the stellar density increases beyond any bound to infinite densities and the stars shrinks in a time much less than a second to a pointlike object. This is a physical singularity, a problem unsolved in present day physics. At the final stage the density of matter is so high that current gravitational theories do not apply. Before the singular state is reached, however, the condensed matter has the properties of a black hole and the ultimate fate cannot be observed, in principle.
The gravitational collapse of the interior of a star releases so much energy that the outer layers are blown away in an explosion. The remnants of explosions leading to the formation of white dwarfs are observed as planetary nebulae. Larger explosions, leading to the formation of a neutron star or black hole, are observed as supernovae, of which remnants can be observed. When the outer layers of a star are already removed (through a stellar wind for example), a catastrophic gravitational collapse can be seen as a gamma ray burst, a short flash of gamma rays lasting only seconds to minutes (see also gamma-ray astronomyGamma-ray astronomy is the astronomical study of gamma rays. Long before experiments could detect gamma rays emitted by cosmic sources, scientists had known that the universe should be producing these photons. Work by Feenberg and Primakoff in 1948, Hayak). Each gamma ray burst marks the birth of a black hole, usually in a very distant galaxy.
A general relativistic description of catastrophic gravitational collapse has two points of view: as seen by a comoving observer and as seen by a distant (stationary) observer.
An observer standing on a star in catastrophic gravitational collapse towards the black hole state undergoes a free fall (that is: in a comoving frame he does not feel gravity to first order). He only feels the tidal forceTidal force due to variations in gravity For a given gravitational field, the tidal acceleration at a point with respect to a body is obtained by vectorially subtracting the gravitational acceleration at the center of the body from the actual gravitationa (difference between the gravity on his head and his feet). This force increases beyond bounds as the star shrinks to a smaller radius. In the transverse direction the comoving observer during the catastrophic gravitational collapse will be squashed by the combination of the tidal force and the increasing curvature of space.
This free fall will end in a finite proper time, with an infinite length and with thickness zero, while in the limit volume zero is reached and the density is increased to infinity.
The comoving observer does not feel any particular force when he passes the Schwarzschild radiusThe Schwarzschild radius or gravitational radius is a characteristic radius associated with every mass. The term is used in physics and astronomy, especially in the theory of gravitation, general relativity. It was found in 1916 by Karl Schwarzschild and (the radius of a black hole, also called the event horizonFor the science fiction film, see Event Horizon (film An event horizon is a boundary in spacetime for a given observer beyond which no information, including light, can reach the observer. The most famous example is a black hole, which for a distant and s). In other words, this radius is not a physical singularity. If the black hole is large, perhaps a supermassive black holeA supermassive black hole is a black hole with a mass in the range of millions or billions solar masses. The Milky Way is believed to have one supermassive black hole, at the Galactic Center. Most if not all galaxies are thought to host a supermassive bla at the center of a galaxyThis article is about a celestial body. For alternate meanings see galaxy (disambiguation). Spiral Galaxy ESO 269-57 is about 150 million light-years away and 200,000 light-years across. Stars are almost always found in collections called galaxies togethe, the tidal forces may not even be strong at this radius. However, his observations of the outside world change dramatically. During the fall he will see the horizon on the surface rising upward through the gravitational deflection of light. Just below the horizon he will see more and more light coming from the back of the star until he can see the entire stellar surface. At the same time the part of the sky above him is becoming a smaller and smaller region around his zenith. When he passes the Schwarzschild radius, nothing is left of the outside world and he can't see any stars in the sky. Instead he sees the (shrinking) stellar surface in every direction. "Direction" becomes meaningless, or, rather, all directions become down.
Before the free falling observer passes the Schwarzschild radius, a scream for help can in principle reach the distant Earth or a spaceship. After passing this radius, all the signals he sends out will fall along with him in the gravitational collapse and never reach the outside world (hence the name event horizonFor the science fiction film, see Event Horizon (film An event horizon is a boundary in spacetime for a given observer beyond which no information, including light, can reach the observer. The most famous example is a black hole, which for a distant and s).