Monday / 5 / Jumada Al-Awwal / 1445 AH – 12:39 – Monday, November 20, 2023 12:39
Translated by – Hafez Edokhraz –
Black holes: They are amazing astronomical objects. For a century, it has aroused the passion of the general public as much as the curiosity of physicists around the world, who still study it with great interest. The formation of these holes remains one of the remaining mysteries, which is still not fully understood today.
What is a black hole?
In France, the concept of a black hole dates back to Pierre-Simon de Laplace (1796), who wondered whether it was possible for an object to be so dense that its escape velocity would be greater than the speed of light (which is about 300,000 km). /second).
Escape Velocity is defined as the speed that an object needs in order to escape the gravity of an astronomical body. The escape velocity on Earth is 11.3 km/s (about thirty thousand times less than the speed of light), which means that an object such as a rocket must reach this speed in order to escape Earth’s gravity and exit into space.
If the Earth’s mass were concentrated into a sphere with a radius of about 9 mm, gravity would be much greater than it is now, which would thus trap light. Therefore, it is necessary for the density of the astronomical object to be much greater than that of the Earth in order for it to trap the light.
German physicist Karl Schwarzschild revisited this idea in 1916, a year after Einstein published his theory of general relativity, and demonstrated that a sufficiently dense object could prevent light from escaping from it.
If matter or light falls within a black hole’s event horizon, they will not be able to escape the hole’s gravity: this horizon is the black hole’s “maximum.” It is the surface of a sphere whose ray is called the Schwarzschild ray (which is therefore the “volume of a black hole”). This beam is proportional to the mass of the object and is very small: a black hole with a mass similar to the mass of the Sun will have a beam of about 3 km (compared to 700 thousand km for the Sun).
Today, thanks to the development of astronomical observation techniques, it has become possible to photograph a black hole. For example, this is a picture of the black hole in the center of our galaxy (the Milky Way), called “Sagittarius A*”.
Different scenarios for black hole formation
It remains difficult to know how a black hole formed. However, there are different scenarios that explain the formation of black holes, and they depend on the category to which the black hole’s mass belongs.
In the case of a black hole with a mass similar to that of the Sun, we are talking about a stellar black hole. These black holes are formed as a result of the collapse of a star at the end of its life. When a star converts all the hydrogen in its core into helium, gravity becomes stronger than the pressure that pushes the material outward, and the material collapses in on itself under its own weight. The core density becomes much greater than it was before, and the star can thus turn into a white dwarf, a neutron star, or a black hole (depending on its mass). Likewise, a white dwarf can collapse due to gravity and turn into a neutron star or black hole when its mass exceeds the Chandrasekhar limit (about 1.4 times the mass of the Sun). The neutron star, in turn, can collapse into a black hole when its mass reaches the Tolman-Oppenheimer-Volkoff limit (about 2.5 times the mass of the Sun).
It is possible for two stellar black holes to merge to form a more massive black hole. This is what happens with what we call binary systems. A binary system consists of two stars orbiting each other, instead of a single star as in the solar system. A well-known example in popular culture is the planet Tatooine from the science fiction movie “Star Wars,” which has two suns.
Some binary systems consist of a black hole and a star, or of two black holes. In this case, these two stars orbit each other, approaching each other until they merge. This subsequently leads to the formation of a black hole larger than the previous hole. Scientists have already observed the merger of two black holes through gravitational waves.
There is another type of black hole, and it is related to supermassive black holes, whose mass ranges from a million to several billion times the mass of the sun. The black hole at the center of the Milky Way galaxy, called Sagittarius A*, is a supermassive black hole. Its mass is more than four million times greater than the mass of the Sun. Supermassive black holes have also been observed in the centers of other galaxies in the universe. The origin of these black holes is still a matter of great debate among researchers. But one of the current favorite hypotheses in specialized scientific circles is the gravitational collapse of a huge gas cloud in the early universe.
After being merely a subject of scientific curiosity that did not go beyond assumptions for a long time, black holes are now being monitored by the scientific community. But many mysteries remain, such as its formation and what lies within the horizon…
Roman Codor – Professor of Physics and Fluid Dynamics
Source: The Conversation website
