Black hole

:This article is about an object in astrophysics. For other uses, see Black hole (disambiguation).

Evidence

Formation

General relativity (as well as most other metric theories of gravity) not only says that black holes can exist, but in fact predicts that they will be formed in nature whenever a sufficient amount of mass gets packed in a given region of space, through a process called gravitational collapse. For example, if you compressed the Sun to a radius of three kilometers, about four millionths of its present size, it would become a black hole. As the mass inside the given region of space increases, its gravity becomes stronger — or, in the language of relativity, the space around it becomes increasingly deformed. When the escape velocity at a certain distance from the center reaches the speed of light, an event horizon is formed within which matter must inevitably collapse onto a single point, forming a singularity.

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A quantitative analysis of this idea led to the prediction that a star remaining about three times the mass of the Sun at the end of its evolution (usually as a neutron star), will almost inevitably shrink to the critical size needed to undergo a gravitational collapse. Once it starts, the collapse cannot be stopped by any physical force, and a black hole is created.

Related Topics:
Evolution - Neutron star

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Stellar collapse will generate black holes containing at least three solar masses. Black holes smaller than this limit can only be created if their matter is subjected to sufficient pressure from some source other than self-gravitation. The enormous pressures needed for this are thought to have existed in the very early stages of the universe, possibly creating primordial black holes which could have masses smaller than that of the Sun.

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Supermassive black holes containing millions to billions of solar masses could also form wherever a large number of stars are packed in a relatively small region of space, or by large amounts of mass falling into a "seed" black hole, or by repeated fusion of smaller black holes. The necessary conditions are believed to exist in the centers of some (if not most) galaxies, including our own Milky Way .

Related Topics:
Supermassive black hole - Galaxies - Milky Way

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Observation

In theory, no object past the event horizon could ever have enough velocity to escape a black hole, including light; due to this, black holes cannot "emit" any kind of light or evidence that would otherwise confirm its existence. However, black holes can be inductively detected from observation of phenomena near them, such as gravitational lensing and stars that appear to be in orbit around space where there is no visible matter.

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The most conspicuous effects are believed to come from matter falling into a black hole, which (like water flowing into a drain) is predicted to collect into an extremely hot and fast-spinning accretion disk around the object before being swallowed by it. Friction between adjacent zones of the disk causes it to become extremely hot and emit large amounts of X-rays. This heating is extremely efficient and can convert about 50% of the mass energy of an object into radiation, as opposed to nuclear fusion which can only convert a few percent of the mass to energy. Other predicted effects are narrow jets of particles at relativistic speeds squirting off along the disk's axis.

Related Topics:
Accretion disk - X-ray - Jet

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However, accretion disks, jets, and orbiting objects are found not only around black holes, but also around other objects such as neutron stars; and the dynamics of bodies near these non-black hole attractors is largely similar to the dynamics of bodies around black holes, and is currently a very complex and active field of research involving magnetic fields and plasma physics. Hence, for the most part, observations of accretion disks and orbital motions merely indicate that there is a compact object of a certain mass, and says very little about the nature of that object. The identification of an object as a black hole requires the further assumption that no other object (or bound system of objects) could be so massive and compact. Most astrophysicists accept that this is the case, since according to general relativity, any concentration of matter of sufficient density must necessarily collapse into a black hole.

Related Topics:
Neutron star - Magnetic field - Plasma physics

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One important observable difference between black holes and other compact massive objects is that any infalling matter will eventually collide with the latter, at relativistic speeds, leading to irregular intense flares of X-rays and other hard radiation. Thus the lack of such flare-ups around a compact concentration of mass is taken as evidence that the object is a black hole, with no surface onto which matter can be suddenly dumped.

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Have we found them?

There is now a great deal of indirect astronomical observational evidence for black holes in two mass ranges:

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• stellar mass black holes with masses of a typical star (4–15 times the mass of our Sun), and
• supermassive black holes with masses perhaps 1% that of a typical galaxy

Additionally, there is some evidence for intermediate-mass black holes (IMBHs), those with masses of a few thousand times that of the Sun. These black holes may be responsible for the formation of supermassive black holes.

Related Topics:
Intermediate-mass black hole - Supermassive black holes

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Candidates for stellar-mass black holes were identified mainly by the presence of accretion disks of the right size and speed, without the irregular flare-ups that are expected from disks around other compact objects. Stellar-mass black holes may be involved in gamma ray bursts (GRBs), although observations of GRBs in association with supernovae or other objects that are not black holes http://liftoff.msfc.nasa.gov/academy/universe/plasma_univ.html http://www.wkap.nl/prod/b/0-7923-3784-0 have reduced the possibility of a link.

Related Topics:
Gamma ray burst - Supernovae

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Candidates for more massive black holes were first provided by the active galactic nuclei and quasars, discovered by radioastronomers in the 1960s. The efficient conversion of mass into energy by friction in the accretion disk of a black hole seems to be the only explanation for the copious amounts of energy generated by such objects. Indeed the introduction of this theory in the 1970s removed a major objection to the belief that quasars were distant galaxies — namely, that no physical mechanism could generate that much energy.

Related Topics:
Active galactic nuclei - Quasar - Radioastronomers - 1960s - 1970s

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From observations in the 1980s of motions of stars around the galactic center, it is now believed that such supermassive black holes exist in the center of most galaxies, including our own Milky Way. Sagittarius A* is now generally agreed to be the location of a supermassive black hole at the center of the Milky Way galaxy. Recent research has shown that black holes may play a part in the birth and creation of galaxies. The orbits of stars within a few AU of Sagittarius A* rule out any object other than a black hole at the centre of the Milky Way assuming the current standard laws of physics are correct.

Related Topics:
1980s - Milky Way - Sagittarius A* - AU

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The current picture is that all galaxies may have a supermassive black hole in their center, and that this black hole swallows gas and dust in the middle of the galaxies generating huge amounts of radiation — until all the nearby mass has been swallowed and the process shuts off. This picture also nicely explains why there are no nearby quasars. Though the details are still not clear, it seems that the growth of the black hole is intimately related to the growth of the spheroidal component — an elliptical galaxy, or the bulge of a spiral galaxy — in which it lives. Interestingly, there is no evidence for massive black holes in the center of globular clusters, suggesting that these are fundamentally different from galaxies.

Related Topics:
Quasar - Elliptical galaxy - Bulge - Spiral galaxy - Globular clusters

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Recent discoveries

In 2004 a cluster of black holes was detected, broadening our understanding of the distribution of black holes throughout our universe. This has led scientists' inferences of how many black holes are in our universe to be significantly revised. Due to these finds, it is believed that there are close to five fold the number of black holes than were previously predicted.

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In July 2004 astronomers found a giant black hole, Q0906+6930, at the center of a distant galaxy in the Ursa Major constellation. The size and presumed age of the black hole has implications that may determine the age of the universe

Related Topics:
Q0906+6930 - Galaxy - Ursa Major - Constellation - Age of the universe

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http://www.space.com/scienceastronomy/heavy_blazar_040628.html.

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In November 2004 a team of astronomers reported the discovery of the first intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre (Nature News) (original article). This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.

Related Topics:
Intermediate-mass black hole - Sagittarius A*

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In February 2005, a blue giant star SDSS J090745.0+24507 was found to be leaving the Milky Way at twice the escape velocity (0.0022 of the speed of light). The path of the star can be traced back to the galactic core. The high velocity of this star supports the hypothesis of a super-massive black hole in the center of the galaxy.

Related Topics:
February 2005 - Blue giant - Star - SDSS J090745.0+24507 - Milky Way

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The formation of micro black holes on Earth in particle accelerators has been tentatively reported, (see, for example, http://news.bbc.co.uk/2/hi/science/nature/4357613.stm) but not yet confirmed. So far there are no observed candidates for primordial black holes.

Related Topics:
Micro black hole - Particle accelerators - Primordial black holes

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