Gravitational collapse
From Wikipedia, the free encyclopedia
Gravitational collapse in astronomy is the inward fall of a massive body under the influence of the force of gravity. It occurs when all other forces fail to supply a sufficiently high pressure to counterbalance gravity and keep the massive body in (dynamical) equilibrium. Gravitational collapse is at the heart of the structure formation in the universe. 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 end states are called compact stars.
The types of compact stars are:
- White dwarfs, whose electron degeneracy pressure opposes gravity.
- Neutron stars, whose neutron degeneracy pressure opposes gravity.
- Black holes, whose physics at the center are unknown.
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. They contract past the point where all of their matter is inside the event horizon of a newly forming black hole, where we can no longer observe it. It is clear that the collapse continues at least until several solar masses of matter are all in a space the size of an atomic nucleus. After that, what happens is completely unknown.
The equations of general relativity by themselves, without considering any other known physics, predict the formation of a singularity. Quantum mechanics usually forbids any form of matter to be compressed into a space smaller than its wavelength, but at this scale, quantum mechanics breaks down, predicting that spacetime dissolves into quantum foam. Furthermore, quantum mechanics and general relativity are mathematically incompatible, and there is no adequate theory of quantum gravity to replace them and tell us what might really happen. String theory does not allow matter to be compressed to less than the smallest dimension of spacetime, but we cannot yet test string theory.
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). Each gamma ray burst marks the birth of a black hole, usually in a very distant galaxy.
Contents |
[edit] 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.
[edit] 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). He only feels the tidal force, the 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 co-moving observer during the catastrophic gravitational collapse will be squashed by the tidal force, that is by the increasing curvature of space.
General relativity predicts that his 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. However, this does not take account of any other known or conjectured physics, which would prevent any of these infinities by a variety of mechanisms, or of entirely unknown physics yet to be discovered.
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). In other words, this radius is not a physical singularity. If the black hole is large, perhaps a supermassive black hole at the center of a galaxy, 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. However, light will bend around the event horizon so that stars in all directions will be visible within that horizon. Light that goes around the black hole more than once will produce bands near the horizon with increasingly distorted images of the entire sky.
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. At no point is the stellar surface below visible, because light cannot move upward from anywhere inside the event horizon. Near the center, the outside world is reduced to a point surrounded by a series of ever thinner rings. There is nothing to see in any direction except straight up. All other directions become down, as measured by trajectories of light.
Of course all of this observation is purely hypothetical, based on current mathematical presentations of black holes. As black holes are believed to be physical maximum entropy objects, no observing body could be sustained within them. Any human or machine observer would be torn to subatomic particles by tidal forces.
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).
[edit] 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). Also radiation is seen to tick slower and thus is observed at a longer wavelength (gravitational redshift). 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.
See also Scientific wager.
