Saturday, August 6, 2011

Black Hole

A black hole is a region of space from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics. Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.
Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity containing a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.
Black holes of stellar mass are expected to form when heavy stars collapse in a supernova at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may be formed.
Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter. Astronomers have identified numerous stellar black hole candidates in binary systems, by studying their interaction with their companion stars. There is growing consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our Milky Way.

Stellar black holes

In theory, any mass if sufficiently compressed would become a black hole. The Sun would suffer this fate if it were shrunk down to a ball about 2.5 km in diameter. In practice, a stellar black hole is only likely to result from a heavyweight star whose remnant core exceeds the Oppenheimer-Volkoff limit following a supernova explosion.

More than two dozen stellar black holes have been tentatively identified in the Milky Way, all of them part of binary systems in which the other component is a visible star. A handful of stellar black holes have also been discovered in neighboring galaxies. Observations of highly variable X-ray emission from the accretion disk surrounding the dark companion together with a mass determined from observations of the visible star, enable a black hole characterization to be made.

Among the best stellar black hole candidates are Cygnus X-1, V404 Cygni, and several microquasars. The two heaviest known stellar black holes lie in galaxies outside our own. One of these black hole heavyweights, called M33 X-7, is in the Triangulum Galaxy (M33), 3 million light-years from the Milky Way, and has a mass of 15.7 times that of the Sun. Another, whose discovery was announced in October 2007, just a few weeks after that of M33 X-7, is called IC 10 X-1, and lies in the nearby dwarf galaxy, IC 10, 1.8 million light-years away. IC 10 X-1 shattered the record for a stellar black hole with its mass of 24 to 33 times that of the Sun. Given that massive stars lose a significant fraction of their content through violent stellar winds toward the end of their lives, and that interaction between the members of a binary system can further increase the mass loss of the heavier star, it is a challenge to theorists to explain how any star could retain enough matter to form a black hole as heavy as that of IC 10 X-1.

The microquasar V4641 Sagittarii contains the closest known black hole to Earth, with a distance of about 1,500 light-years. At the other end of the scale, a stellar black hole has been detected in the spiral galaxy NGC 300, which lies about 6 million light-years away. With a mass roughly 20 times that of the Sun the NGC 300 object joins the black holes in M33 and IC 10 mentioned above in the select band of known stellar black holes whose masses exceed 15 solar masses. Interestingly, the NGC 300 black hole is one of only two black holes known to orbit Wolf-Rayet stars (giant, hot, highly-evolved stars that are rapidly shedding mass). When the Wolf-Rayet companion eventually explodes as a supernova it will presumably leave behind another black hole; eventually the two black holes will merge give rise to the emission of copious gravitational waves (ripples in the fabric of spacetime). Merging black holes, along with supernova and merging neutron stars, are considered among the most promising targets for experiments aimed at detecting gravitational waves.

Supermassive black holes

A supermassive black hole is the largest type of black hole in a galaxy, in the order of hundreds of thousands to billions of solar masses. Most, and possibly all galaxies, including the Milky Way, are believed to contain supermassive black holes at their centers.
Supermassive black holes have properties which distinguish them from lower-mass classifications:
  • The average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be much less than the density of water (the densities are similar for 108 solar mass black holes. This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, average density decreases for larger black holes, being inversely proportional to the square of the mass.
  • The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut traveling towards the black hole center would not experience significant tidal force until very deep into the black hole.

Inside a black hole

According to the general theory of relativity, the material inside a black hole is squashed inside an infinitely dense point, known as a singularity. This is surrounded by the event horizon at which the escape velocity equals the speed of light and that thus marks the outer boundary of a black hole. Nothing from within the event horizon can travel back into the outside universe; on the other hand, matter and energy can pass through this surface-of-no-return from outside and travel deeper into the black hole.

For a non-rotating black hole, the event horizon is a spherical surface, with a radius equal to the Schwarzschild radius, centered on the singularity at the black hole's heart. For a spinning black hole (a much more likely contingency in reality), the event horizon is distorted – in effect, caused to bulge at the equator by the rotation. Within the event horizon, objects and information can only move inward, quickly reaching the singularity. A technical exception is Hawking radiation, a quantum mechanical process that is unimaginably weak for massive black holes but that would tend to cause the mini variety to explode.

Three distinct types of black hole are recognized:
  • A Schwarzschild black hole is characterized solely by its mass, lacking both rotation and charge. It possesses both an event horizon and a point singularity.

  • A Kerr black hole is formed by rotating matter, possesses a ring singularity, and is of interest in connection with time travel since it permits closed time-like paths (through the ring).

  • A Reissner-Nordstrom black hole is formed from non-rotating but electrically-charged matter. When collapsing, such an object forms a Cauchy horizon but whether it also forms closed time-like paths is uncertain.
The equations of general relativity also allow for the possibility of spacetime tunnels, or wormholes, connected to the mouths of black holes. These could act as shortcuts linking remote points of the universe. Unfortunately, they appear to be useless for travel or even for sending messages since any matter or energy attempting to pass through them would immediately cause their gravitational collapse. Yet not all is lost. Wormholes, leading to remote regions in space, might be traversable if some means can be found to hold them open long enough for a signal, or a spacecraft, to pass through.

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