A black hole is simply an object from which light can’t escape. If particles of light (photons) cannot escape the surface of the object, then we can’t see the object - hence the name.
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| A black hole. |
They can be formed from the gravitational collapse of massive stars but are also of interest in their own right, independently of how they were formed.
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| Life cycle of a star. |
Far from being hungry beasts devouring everything in their vicinity as shown in popular culture, we now believe certain (supermassive) black holes actually drive the evolution of galaxies.
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| The first ever image of a black hole at the heart of the M87 galaxy taken in April 2019. |
At the centre of our own galaxy, the milky way, the supermassive black hole is called Sagitarrius A* and has a mass of 2.6 million times the mass of our sun.
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| Orbit of stars around the central black hole, Sagittarius A.* |
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| The second ever image of a black hole, Sagittarius A, at the heart of the our galaxy taken in May 2022. |
The first person to come up with the idea of a black hole was John Mitchell in 1783, followed (independently) by Pierre-Simon Laplace in 1796. These prototypical black objects, called ‘dark stars’, were considered from the point of view of Newton’s law of motion and gravitation.
In particular, Mitchell calculated that when the escape velocity at the surface of a star was equal to or greater than lightspeed, the generated light would be gravitationally trapped, so that the star would not be visible to a distant astronomer.
The event horizon is the boundary of this region, also known as the ‘point of no return’ because once you go past this point it is impossible to turn back.
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| The black hole region. |
Black holes also arise as solutions in Einstein’s theory of general relativity. The first such solution was found by Karl Schwarzchild in 1916 and called the ‘Schwarzchild solution’. It was originally formulated in order to describe the gravitational field of the solar system, and to understand the motion of objects passing through this field, but the idea of a black hole remained as an intriguing possibility.
The first black hole was discovered in 1964 called Cygnus X1 and it is estimated to have a mass of about 14.8 times the mass of the sun.
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| Cygnus X1 is a galactic X-ray source discovered during a rocket flight. |