gravitational collapse - Catastrophic gravitational collapse toward a black hole

A phenomenon which occurs when the supply of nuclear energy in the core of a star runs out, and the star cools and contracts; a prediction of general relativity. This disturbs the precise balance inside the star between the inward pull of gravity and the star's gas pressure. Once the star radius is less than a critical value (Schwarzschild radius, R = 2MG/c², value 3 km/2 mi for the Sun), the collapse cannot be reversed by any force now known to physics. Implosion to a black hole seems inevitable.

Gravitational collapse in astronomy is the inward fall of a massive body under the influence of the force of gravity. An initial smooth distribution of matter will eventually collapse and cause the hierarchy of structures, such as clusters of galaxies, stellar groups, stars and planets. For example, a star is born through the gradual gravitational collapse of a cloud of interstellar matter. The compression caused by the collapse raises the temperature until nuclear fuel ignites in the center of the star and the collapse comes to a halt. The thermal pressure gradient (leading to expansion) compensates the gravity (leading to compression) and a star is in dynamical equilibrium between these two forces.

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 types of compact stars are:

White dwarfs, whose electron degeneracy pressure opposes gravity. Neutron stars, whose neutron degeneracy pressure opposes gravity.

The collapse to a white dwarf takes place over tens of thousands of years, while the star blows off its outer envelope to form a planetary nebula. In theory, a white dwarf-sized object could collapse to a neutron star by accreting matter from a companion star, but in fact a white dwarf accreting that much matter would undergo catastrophic fusion, blowing the star apart completely in a Type 1a supernova. Neutron stars are actually formed by gravitational collapse of larger stars, in the other types of supernova.

Very massive stars, above the Tolman-Oppenheimer-Volkoff limit cannot find a new dynamical equilibrium with any known force opposing gravity.

The gravitational collapse of the interior of a star releases so much binding energy that the outer layers are blown away in an explosion. 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 astronomy).

Catastrophic gravitational collapse toward a black hole

A general relativistic description of catastrophic gravitational collapse has two points of view: as seen by a co-moving observer and as seen by a distant (stationary) observer.

Viewed by a co-moving observer

An observer standing on a star in catastrophic gravitational collapse towards the black hole state undergoes a free fall (that is, in a co-moving frame he does not feel gravity to first order). This force increases beyond bounds as the star shrinks to a smaller radius. In the transverse direction the co-moving observer during the catastrophic gravitational collapse will be squashed by the tidal force, that is by the increasing curvature of space.

The co-moving observer does not feel any particular force when he passes the Schwarzschild radius (the radius of a black hole, also called the event horizon). However, light will bend around the event horizon so that stars in all directions will be visible within that horizon.

When the observer passes through the event horizon and continues falling, the part of the sky above him becomes a smaller and smaller region around his zenith.

Before the free-falling observer passes the Schwarzschild radius, a call 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 horizon).

Viewed by a distant (stationary) observer

A stationary observer at Earth or in a distant orbit will have an entirely different view on the catastrophic gravitational collapse.

A clock of the free falling observer is in a stronger part of the gravitational field and when viewed from a distance appears to tick slower (gravitational time dilation). As the free falling observer (in his time) falls faster and faster toward the Schwarzschild radius, the stationary observer sees him progressing slower and slower towards the Schwarzschild radius and will never see him passing that stage. Instead the stationary observer will see collapse progressively dimmer and redder, until the entire star plus comoving observer disappears in much less than a second. The last photon the stationary observer will receive, comes from a stage of the collapsing star just outside the Schwarzschild radius.