Black holes

Black holes are regions of spacetime from which gravity prevents anything, including light, from escaping.

The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole.

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Artists impression of a black hole with an associated magnetic field. image: NASA

The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. 

Black_hole_-_Messier_87

The supermassive black hole inside the core of the supergiant elliptical galaxy Messier 87 in the constellation Virgo. Its mass, estimated at 7.22+0.34 −0.40×109 M, is billions of times that of the Sun. It was the first black hole to be directly imaged by the Event Horizon Telescope


Origin of the word black hole

John Michell used the term "dark star”, and in the early 20th century, physicists used the term "gravitationally collapsed object". Science writer Marcia Bartusiak traces the term "black hole" to physicist Robert H. Dicke, who in the early 1960s reportedly compared the phenomenon to the Black Hole of Calcutta, notorious as a prison where people entered but never left alive.

The term "black hole" was used in print by Life and Science News magazines in 1963, and by science journalist Ann Ewing in her article "'Black Holes' in Space", dated 18 January 1964, which was a report on a meeting of the American Association for the Advancement of Science held in Cleveland, Ohio.

In December 1967, a student reportedly suggested the phrase "black hole" at a lecture by John Wheeler; Wheeler adopted the term for its brevity and "advertising value", and it quickly caught on,[56] leading some to credit Wheeler with coining the phrase

In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. 

This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it all but impossible to observe.


Brief history of the concept of black holes

Objects whose gravitational fields are 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 that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. 



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a  ‘naked’ black hole creating a lensing effect of background stars. image: Urbane Legend (optimised for web use by Alain r)

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 very massive stars collapse 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 form. There is general consensus that supermassive black holes exist in the centers of most galaxies.

How black holes are detected

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter and with electromagnetic radiation such as light. Matter falling onto a black hole can form an accretion disk heated by friction, forming some of the brightest objects in the universe. 

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This artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. image: NASA/JPL

If there are other stars orbiting a black hole, their orbit can be used to determine its mass and location. Such observations can be used to exclude possible alternatives (such as neutron stars). 

In this way, astronomers have identified numerous stellar black hole candidates in binary systems, and established that the core of the Milky Way contains a supermassive black hole of about 4.3 million solar masses.


Adapted from

Black hole. (2014, October 25). In Wikipedia, The Free Encyclopedia. Retrieved 08:56, October 27, 2014, from http://en.wikipedia.org/w/index.php?title=Black_hole&oldid=631077036


Visualisation of black holes

Some recent visual calculations developed for the sci-fi movie Interstellar ,  suggest that they have more unusual gravitational lensing effects around the accretion disc as shown in the video below.


Here is a nice explanation of why black holes look the way they do when imaged using a giant interferometer of telescopes.