A black hole is a star from which nothing can escape. No matter, no light! … The gravitational force keeps us on the surface of the Earth. This force of attraction is proportional to the mass of the Earth, and at the distance that separates us from the center. To leave Earth or another star, you just have to acquire what is called the speed of liberation. It only depends on the mass of the star on which we are, and the distance from the center of the star. For example, if we are on the surface of the Earth, that is to say, 6400 km from the center of the Earth, release speed to acquire to leave the earth is 11.2 km / s. A star having the same weight as the earth is not necessarily the same size. It all depends on the density of the materials that make up this star. To know the density of a material, we compare the weight of the material to an identical volume of water. For example, a cubic meter of water weighs 1000 kg or a ton, while one cubic meter of pine wood weighs 500kg. So the wood has a density of 0.5. Of course, wood floats on water. However, a concrete block has a density of 2.2: it is more than twice as heavy as water, and therefore it sinks Let us return to our star which has the same mass as the Earth. The density of the Earth is 5.5, but suppose that our star is in osmium, the densest metal that exists. Its density is 23, which is to say more than 4 times that of Earth. For the same mass as Earth, our star would only have a radius of 4000 km. On the surface of our earth in osmium, the speed to acquire to free yourself from gravitational pull has increased: it is now 14 km / s. We know that black holes are stars, and the stars are mostly composed of gas, more precisely hydrogen and helium. At the surface of the earth, gases have a very low density. For example, the air has a density a thousand times lower than water. But in a star-like a sun, pressure and temperature conditions are such that the density of the gases becomes comparable to that of water on earth: the sun has a density of 1.4, so it’s a little heavier than water. But there are much denser stars. This is the case with white dwarfs, little stars the size of the earth, much hotter than the sun: their density reaches a million times that of water! A 1 dollar coin with the same density would weigh a ton! And if the Earth had this density, it would only be 113 km in diameter, and its release speed would be 843 km / s. Let’s multiply this density again by a trillion billion. Our euro coin now weighs the same weight as the whole earth! And so the Earth, with this density, is reduced to a small ball less than a centimeter in radius. The speed of release now reaches 300,000 km / s, that is to say, the speed of light! Physics tells us that this speed cannot be exceeded. Any matter landing on the surface of the star is therefore trapped. Even the light emitted on the surface of this star ends up falling: the star, therefore, appears all black, because nothing comes out:
it’s a black hole. In fact, the gravitational forces are so high at these densities that the black hole continues to collapse on itself, to the point that all of its matter is finally concentrated in its center. But it does not change anything : the radius for which the release speed is greater than the speed of light remains the same. We call it the horizon of the black hole, and everything that passes this horizon no longer stands out. We cannot observe a black hole since nothing can come from it. But fortunately, in the immediate suburbs of a black hole, phenomena are happening that you don’t find anywhere else: for example, the light is deflected, gravitational mirages are formed which can be observed; or the matter, before being swallowed by the black hole, emits X-rays that can be detected. So we know that in the center of our galaxy, there is a huge black hole, with a mass equal to 4 million times that of the sun, and whose radius of the horizon is equal to 45 times the distance between the earth and the sun!
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