Black Holes | Definition ,Theory, Formation, Facts, & Pictures

 What is a black hole?

A black hole is a place in space where gravity is so strong that no particle can escape from it. not even the light can get away from it. A black hole can be formed by the death of a massive star. They have high density because matter has been squeezed in a small place resulting in enormous gravity. Anything that ventures beyond a certain boundary will be squeezed by its enormous gravity and this boundary of no escape is called "the event horizon".

As no light can escape from it, people can't see black holes. To be able to spot them we need space telescopes with special tools. These special tools can see how other cosmic bodies (usually stars) that are very close to the black hole acts differently than other bodies which are farther from the black hole. For Example, if a black hole is a member of the binary star system, matter flowing into it from its companion becomes intensely heated and radiates X-rays copiously before entering the event horizon of the black hole and, of course, disappearing forever.

Some black holes also have non-steller origins. Various astronomers around the globe have speculated that large volumes of interstellar gas collect and collapse into supermassive black holes at the center of quasars and galaxies. The amount of energy given off by a mass of gas falling rapidly into a black hole is estimated to be more than 100 times as in a nuclear fusion by the same amount of mass.

The first black hole to be imaged directly is a supermassive black hole at the center of the M87 galaxy. This black hole has a mass equivalent to six and a half billion Suns but it is only 38 billion kilometers(24 billion miles) across. The event horizon telescope obtained an image of this black hole in 2017.

The British astrophysicist Stephen Hawking (1924-2018) proposed the existence of another kind of non-steller black hole. According to Hawking's theory, during the big bang, numerous tiny black holes, possibly with a mass equal to less than that of an asteroid might have been formed. These mini black holes, lose mass over time through Hawking radiations and eventually disappear.

Using the Event Horizon Telescope, scientists obtained an image of the black hole at the center of galaxy M87, outlined by emission from hot gas swirling around it under the influence of strong gravity near its event horizon.

Credits: Event Horizon Telescope collaboration et al.

When did we come to know about black holes?

The astronomical pioneer and English clergyman John Michell is the first person known to have proposed the idea of Black holes. He briefly proposed his idea of a body, whose gravity is so strong that even the light could not escape from it, in a letter published in 1784. His calculations assumed that such a body might have the same density as the Sun,  and also concluded that such a body would form when a star's diameter exceeds Sun's by a factor of 500, and the escape velocity exceeds the speed of light.

                                                       

The galaxy M87, imaged here by NASA's Spitzer Space Telescope, is home to a supermassive black hole that spews two jets of material out into space at nearly the speed of light. The inset shows a close-up view of the shockwaves created by the two jets.

Credits: NASA/JPL-Caltech/IPAC

General relativity and Black holes

Albert Einstein developed his general relativity in 1915. A few months later, German astronomer, Karl Schwarzchild found a solution to Einstein's field equation, which describes the gravitational field of a point mass and spherical mass, and proposed the modern version of Black holes.

The Schwarzchild solution, which makes use of Schwarzchild coordinates and Schwarzchild metric, leads to the derivation of the radius of the event horizon(no escape boundary) for non-rotating black holes(a rotating body acts slightly different) called Schwarzchild radius named after Karl Schwarzchild. The Schwarzchild is proportional to the mass of the collapsing star.

Accordingly, Schwarzchild radius of the Sun is 3.0 kilometers(1.9 miles) approximately, whereas for Earth it is only about 9mm(0.35 inches) and for the Moon is about 0.1mm(0.0039 In). Also, the Schwarzchild radius for the observable Universe's mass is 13.7 billion light-years. In other words, if the radius for an object is less than its Schwarzchild radius, then it is a black hole.

The Schwarzchild radius is given as

where G represents gravitational constant,
M represents the mass of the object,
and C is the speed of light.

Follow us on Social Media

Thanks for reading!