What Is a Black Hole or a Dark Star?

John Michell and dark stars.

During 1783, Geologist John Michell wrote a letter to Henry Cavendish outlining the expected properties of dark stars, published by The Royal Society in their 1784 volume. Michell calculated that when the escape velocity at the surface of a star was equal to or greater than the speed of light, the generated light would be gravitationally trapped so that the star would not be visible to a distant astronomer. 

"If the semi-diameter of a sphere of the same density as the Sun was to exceed that of the Sun is the proportion of 500 to 1, a body falling from an infinite height towards it would have acquired at its surface greater velocity than that of light, and consequently supposing light to be attracted by the same force in proportion to its vis inertia(A want of power in a body to move itself when at rest or to come to rest when in motion.), with other bodies, all light emitted from such a body would be made to return towards it by its own proper gravity. This assumes that gravity influences light in the same way as massive objects." 

Michell's idea for calculating the number of such "invisible" stars anticipated 20th-century astronomers' work: "He suggested that since a certain proportion of double-star systems might be expected to contain at least one "dark" star, we could search for and catalogue as many double-star systems as possible, and identify cases where only a single circling star was visible. This would then provide a statistical baseline for calculating the amount of other unseen stellar matter that might exist in addition to the visible stars."

"A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because the matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can't see black holes".

How Big Are Black Holes?

Black holes can be big or small. These black holes are very tiny but have the mass of a large mountain. Another kind of black hole is called stellar. Its mass can be up to 20 times more than the mass of the sun. There may be many, many stellar-mass black holes in Earth's galaxy.

Earth's galaxy is called the Milky Way(Everybody knows it). The largest black holes are called supermassive black holes. These black holes have masses that are more than 1 million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. 


How Do Black Holes Form?

Stellar black holes are made when the center of a very big star falls in upon itself or collapses. A supernova is an exploding star that blasts part of the star into space. Scientists think supermassive black holes were made at the same time as the galaxy they are in. Black holes are volumes of space where gravity is extreme enough to prevent the escape of even the fastest moving particles.

Karl Schwarzschild realized it was possible for mass to be squeezed into an infinitely small point. This would make spacetime around it bend so that nothing – not even massless photons of light – could escape its curvature. The cusp of the black hole's slide into oblivion is today referred to as its event horizon and the distance between this boundary and the infinitely dense core – or singularity – is named after Schwarzschild. Theoretically, all masses have a The Schwarzschild radius that can be calculated.

If the Sun's mass was squeezed into an infinitely small point, it would form a black hole with a radius of just under 3 kilometers. Similarly, Earth's mass would have a Schwarzschild radius of just a few millimeters, making a black hole no bigger than a marble. Physicists have become increasingly confident in their existence as other extreme astronomical objects, such as neutron stars, were discovered. Today it's believed most galaxies have monstrous black holes at their core.

It's generally accepted that stars with a mass at least three times greater than that of our Sun's can undergo extreme gravitational collapse once their fuel depletes. With so much mass in a confined volume, the collective force of gravity overcomes the rule that usually keeps the building blocks of atoms from occupying the same space.

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole. But scientists can see how strong gravity affects the stars and gas around the black hole. When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes.

 
If Black Holes Are "Black," Than How Do Scientists Know That They Are There?

A black hole can not be seen because strong gravity pulls all of the light into the middle of the black hole(That's why John Michell called it Dark Star in his letter to Henry Cavendish). 

Scientists can see how strong gravity affects the stars and gas around the black holes. Scientists can study stars to find out if they are flying around, or orbiting, a black hole.

When a black hole and a star are close together, high-energy light is made. This kind of light can not be seen with human eyes. Scientists use satellites and telescopes in space to see the high-energy light.

Space telescopes with special tools can help find black holes. The special tools can see how stars that are very close to black holes act differently than other stars.
 

Could a Black Hole Destroy Earth?

Black holes do not go around in space eating stars, moons, and planets(Well Some scientists say that black holes move around the universe eating stars, moons, and planets, but I don't think so.). 

Black holes can indeed move through space. Rapidly moving black holes that are flying solo will be essentially impossible to detect, since space is very big and they will encounter other objects only very rarely.
 
Earth will not fall into a black hole because no black hole is close enough to the solar system for Earth to do that. 

Even if a black hole with the same mass as the sun were to take the place of the sun, Earth still would not fall in. The black hole would have the same gravity as the sun. Earth and the other planets would orbit the black hole as they orbit the sun now.

The sun will never turn into a black hole. The sun is not a big enough star to make a black hole.


How Is Space Agencies Studying Black Holes?

Space Agencies are using satellites and telescopes that are traveling in space to learn more about black holes. These spacecraft help scientists answer questions about the black holes and about the universe.

 

Reference:- 

Wikipedia

NASA

Sciencealert