A black hole is a theoretical region of space in which the gravitational field is so powerful that nothing, not even electromagnetic radiation (e. g. visible light), can escape its pull after having fallen past its event horizon. Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole. On the other hand, a black hole exerts the same force on something far away from it as any other object of the same mass would.
For example, if our Sun was magically crushed until it was about 1 mile in size, it would become a black hole, but the Earth would remain in its same orbit. Even back in Isaac Newton’s time, scientists speculated that such objects could exist, even though we now know they are more accurately described using Einstein’s General Theory of Relativity. Using this theory, black holes are fascinating objects where space and time become so warped that time practically stops in the vicinity of a black hole.
The former types have measured masses ranging from 4 to 15 Suns, and are believed to be formed during supernova explosions. The after-effects are observed in some X-ray binaries known as black hole candidates. On the other hand, galaxy-mass black holes are found in Active Galactic Nuclei (AGN). These are thought to have the mass of about 10 to 100 billion Suns. The mass of one of these super massive black holes has recently been measured using radio astronomy.
X-ray observations of iron in the accretion disks may actually be showing the effects of such a massive black hole as well. Formation and Evolution The primary formation process for black holes is expected to be the gravitational collapse of heavy objects such as stars, but there are also more exotic processes that can lead to the production of black holes. Gravitational collapse occurs when an object’s internal pressure is insufficient to resist the object’s own gravity.
For stars this usually occurs either because a star has too little “fuel” left to maintain its temperature, or because a star which would have been stable receives a lot of extra matter in a way which does not raise its core temperature. In either case the star’s temperature is no longer high enough to prevent it from collapsing under its own weight. The result is one of the various types of compact star. Which type of compact star is formed depends on the mass of the remnant – the matter left over after changes triggered by the collapse (such as supernova or pulsations leading to a planetary nebula) have blown away the outer layers.
If the mass of the remnant exceeds ~3-4 solar masses (the Tolman-Oppenheimer-Volkoff limit)—either because the original star was very heavy or because the remnant collected additional mass through accretion of matter)—even the degeneracy pressure of neutrons is insufficient to stop the collapse. After this no known mechanism (except maybe the quark degeneracy pressure, see quark star) is powerful enough to stop the collapse and the object will inevitably collapse to a black hole.
This gravitational collapse of heavy stars is assumed to be responsible for the formation of most (if not all) stellar mass black holes. Once a black hole has formed, it can continue to grow by absorbing additional matter. Any black hole will continually absorb interstellar dust from its direct surroundings and omnipresent cosmic background radiation, but neither of these processes should significantly affect the mass of a stellar black hole. Properties of Black Holes According to the “No Hair” theorem a black hole has only three independent physical properties: mass, charge and angular momentum.
Any two black holes that share the same values for these properties are indistinguishable. This contrasts with other astrophysical objects such as stars, which have very many—possibly infinitely many—parameters. Consequently, a great deal of information is lost when a star collapses to form a black hole. Since in most physical theories information is preserved (in some sense), this loss of information in black holes is puzzling. Black Hole Types The simplest possible black hole is one that has mass but neither charge nor angular momentum.
These black holes are often referred to as Schwarzschild black holes after the physicist Karl Schwarzschild who discovered this solution in 1915. It was the first (non-trivial) exact solution to the Einstein equations to be discovered, and according to Birkhoff’s theorem, the only vacuum solution that is spherically symmetric. The Reissner-Nordstrom solution describes a black hole with electric charge, while the Kerr solution yields a rotating black hole. The most general known stationary black hole solution is the Kerr-Newman metric having both charge and angular momentum. Sizes
Black holes occurring in nature are commonly classified according to their mass, independent of angular momentum J. The size of a black hole, as determined by the radius of the event horizon, or Schwarzschild radius, is proportional to the mass through where is the Schwarzschild radius and is the mass of the Sun. Thus, size and mass have a simple relationship, which is independent of rotation. According to this mass/size criterion then, black holes are commonly classified as Super massive black holes, Intermediate-mass black holes, Stellar-mass black holes, Micro black holes Conclusion
There is very good evidence from astronomical observations that the universe is full of black holes with sizes ranging from six to a billion solar masses in size. Black hole accretion power is responsible for some of the most spectacular phenomena in the universe. These phenomena are NOT well understood, however, largely because of the complexity of the physics of the central accretion flow. There is little doubt, though, that black holes exist at the heart of active galactic nuclei, quasars, and certain X-ray binaries. Accretion power is an important contributor to the overall evolution and ecology of the universe.
Electromagnetic observations are currently probing the innermost parts of accretion flows, and revealing interesting effects of the relativistic space-time – assuming our models of the flows are not WAY wrong. How can one prove the existence of black holes, short of a suicidal leap across an event horizon? Detection of gravitational waves – ripples in the fabric of space-time itself, is perhaps the only way. Currently astronomers view black holes if they are lit up electromagnetically – a very biased view! Black holes which are “lit up” gravitationally may offer a very differently biased view. Works cited Kraus, Ute.
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