Black hole

History

The idea of a body so massive that even light could not escape was first proposed in the late 18th century by English astronomer and clergyman John Michell and independently by French scientist Pierre-Simon Laplace. Both scholars proposed very large stars in contrast to the modern concept of an extremely dense object.

Michell's idea, in a short part of a letter published in 1784, calculated that a star with the same density but 500 times the radius of the Sun would not let any emitted light escape; the surface escape velocity would exceed the speed of light. Michell correctly hypothesized that such non-radiating bodies might be detectable through their gravitational effects on nearby visible bodies. In 1796, while speculating on the origin of the Solar System in his book Exposition du Système du Monde, Laplace made a qualitative suggestion that a star could be invisible if it were sufficiently large. Franz Xaver von Zach asked Laplace for a mathematical analysis, which Laplace provided and published in vo

In 1905, Albert Einstein showed that the laws of electromagnetism are identical for observers travelling at different velocities relative to each other. The laws of mechanics had already been shown to be invariant in this way. However, the theory of gravitation was yet to be included.

Definition

A black hole is generally defined as a region of spacetime from which no information-carrying signals or objects can escape. However, verifying an object as a black hole by this definition would require waiting for an infinite time and at an infinite distance from the black hole to verify that nothing has escaped, and thus cannot be used to identify a physical black hole. There are several other definitions that can be used to describe or identify black holes, leading to a variety of ways to study them. Astronomical observations measure the mass of objects, and gravitational collapse theories predict that a compact object with a mass larger than three solar masses can only be a black hole: t

Properties

The no-hair theorem establishes that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, electric charge, and angular momentum; the black hole is otherwise featureless. Any two black holes that share the same values for these properties, or parameters, are indistinguishable from one another. A related no-hair conjecture proposes that dynamic gravitational collapse always results in an object characterized with only these three properties. The conjecture is currently an unsolved problem. The no-hair theorem also makes idealized assumptions in addition to equilibrium that may not apply to astrophysical objects.

The simplest equilibrium black hole model with only mass but neither electric charge nor angular momentum is called a Schwarzschild black hole. Non-rotating charged black holes are described by the Reissner–Nordström metric, while the Kerr metric describes a non-charged rotating black hole. The most general stationary black hole solution known is the Kerr–Newman metric, which describes a black hole with both charge and angular momentum.

The simplest static black holes have mass but neither electric charge nor angular momentum. Contrary to the popular notion of a black hole "sucking in everything" in its surroundings, from far away, the external gravitational field of a black hole is identical to that of any other body of the same mass.

Classification

Black holes are classified by the theory of their formation and by their mass (expressed in terms of M☉, the mass of the Sun), but these criteria are intertwined. Stellar black holes are formed by stellar collapse. The minimum mass of a black hole formed by stellar gravitational collapse is governed by the maximum mass of a neutron star and is believed to be 2-4 M☉. Hypothetical primordial black holes, believed to have formed soon after the Big Bang, could be far smaller, with masses as little as 10−5 grams at formation. These very small black holes are sometimes called micro black holes.

Stellar black holes can have a wide range of masses. Estimates of their maximum mass at formation vary, but generally range from 10-100 M☉, with higher estimates for black holes progenated by low-metallicity stars. Stellar black holes can gain mass via accretion of nearby matter, often from a companion object such as a star or by merger with another black hole.

A small number of candidate black holes with masses in the range 102-104 M☉ have been reported. These are larger than stellar black holes but smaller than supermassive black holes, are rarer than either extreme, and are called intermediate-mass black holes. Physicists have speculated that such black holes may form from collisions in globular and star clusters or at the centre of low-mass galaxies. They may also form as the result of mergers of smaller black holes, with several gravitational wave measurements consistent with merged black holes within 110–350 M☉.

Structure

While black holes are conceptually invisible sinks of all matter and light, in astronomical settings, their enormous gravity alters the motion of surrounding objects and pulls nearby gas inwards at near-light speed, making the area around black holes the brightest objects in the universe.

Some black holes have relativistic jets—thin streams of plasma travelling away from the black hole at more than 90% of the speed of light. A small fraction of the matter falling towards the black hole gets accelerated away along the hole rotation axis. These jets can extend as far as millions of light-years from the black hole itself.

Black holes of any mass can have jets. However, they are typically observed around spinning black holes with strongly-magnetized accretion disks. Relativistic jets were more common in the early universe, when galaxies and their corresponding supermassive black holes were rapidly gaining mass. All black holes with jets also have an accretion disk, but the jets are usually brighter than the disk. Quasars, typically found in other galaxies, are believed to be supermassive black holes with jets; microquasars are believed to be stellar-mass objects with jets, typically observed in the Milky Way.

Formation

Black holes are formed by gravitational collapse of massive stars, either by direct collapse or during a supernova explosion in a process called fallback. Black holes can result from the merger of two neutron stars or a neutron star and a black hole. Other more speculative mechanisms include primordial black holes created from density fluctuations in the early universe, the collapse of dark stars, a hypothetical object powered by annihilation of dark matter, or from hypothetical self-interacting dark matter.

Gravitational collapse occurs when an object's internal pressure is insufficient to resist the object's own gravity. At the end of a star's life, it will run out of hydrogen to fuse, and will start fusing more and more massive elements, until it gets to iron. Since the fusion of elements heavier than iron would require more energy than it would release, nuclear fusion ceases. If the iron core of the star is too massive, the star will no longer be able to support itself and will undergo gravitational collapse.

The mass of a black hole formed via a supernova has a lower bound: if the progenitor star is too small, the collapse may be stopped by the degeneracy pressure of the star's constituents, allowing the condensation of matter into an exotic denser state. Degeneracy pressure occurs from the Pauli exclusion principle: particles will resist being in the same place as each other. Progenitor stars with masses less than about 8 M☉ will become white dwarfs, where the degeneracy pressure of electrons balances gravity. For more massive progenitor stars, the force of gravity overcomes electron degeneracy pressure and the star compresses until neutron degeneracy pressure resists gravity, forming a neutron