To arise
Theoretically, the following four scenarios have been proposed for the formation of black holes:
“Direct, isolated”: a massive star collapses, almost spherically symmetrical, producing a neutron star core that is too massive to sustain itself against gravity;
"Indirect, isolated": a star with a white dwarf core collapses and loses some of its mass through rotation; the ejected fragments lose energy and torque by emitting gravitational waves, and are captured one by one by the center;
"Densely packed": stars in a dense cluster happen to exchange gravitational energy, with some stars escaping the halo and the others being driven closer together;
"Primordial": Small density differences in the early universe create primordial black holes, which gradually become heavier by trapping radiation and matter.
Most black holes we know correspond to the first scenario; they are the remains of hypernova or supernova implosions. If the core of the imploding star is more than about five times as massive as the Sun (the Oppenheimer-Volkoff limit), the star's core eventually implodes into a black hole, due to the enormous gravity pushing down on the star and ultimately wins over the star's internal pressure.
According to general relativity, this mass can be understood as concentrated in a singularity. This can be a point, a ring or a sphere - scientists do not agree in all cases.
The very massive black holes that can be found in the centers of many galaxies and have a mass of several million solar masses probably formed shortly after the Big Bang. Black holes with a mass of several thousand solar masses are now also known, but how they formed is not yet clear. It is also not known what exactly happens in and around the singularity, since general relativity is no longer exactly valid at such small distances.
According to estimates, our own galaxy also contains several million black holes, each about ten times the mass of the Sun, which were formed from massive stars or from collisions between stars. When these black holes are in an isolated place, that is, without other stars nearby, they are much more difficult to detect than when they form a binary system with a star orbiting it; The presence of a dark and heavier object can be deduced from the path that the star then follows.
Evaporation by hawking radiation
The English physicist Stephen Hawking theoretically demonstrated in 1974 that black holes must evaporate slowly. According to the uncertainty laws in the quantum world, pairs of particles and antiparticles are continuously created on the event horizon. Normally these particles cancel themselves out almost immediately by mutual annihilation so that the energy effect is zero again. However, with a black hole it sometimes happens that one particle falls into the black hole and the other escapes into space. In other words: radiation comes out. This is also called 'hawking radiation'. The energy for this is extracted from the black hole. This will be slightly smaller. The smaller the black hole, the faster this will happen. After a while, a black hole can completely 'evaporate'. Initially, some physicists thought that the information lost when matter fell into it when a black hole was formed had disappeared forever.
Sagittarius-A*
At the center of the Milky Way is a supermassive black hole: Sagittarius A*. Its mass can be determined by studying the orbits of stars near the center of the Milky Way, and is found to be 4.15 million times the mass of the Sun. The Schwarzschild radius for such a mass is 0.08 astronomical units. The maximum diameter of Sagittarius-A* that can be reconciled with observations of nearby stars is 45 astronomical units, so it is not absolutely certain for the time being that this is a black hole.
White holes
A white hole is a hypothetical celestial body that spews energy, stars and other matter. In other words, a kind of inverted black hole. But if white holes exist, why don't we see them? A University of Oregon physicist may know why!
To know how it works, it is useful to know how a wormhole is created. Imagine this: a heavy bowling ball rests on a rubber sheet. As the bowling ball creates a hole in the skin, the ball becomes smaller and smaller. This pushes the mass into an increasingly smaller space.
At some point the ball is as small as a pinhead. The density is high that the bowling ball creates a hole and falls through the rubber sheet. That's what happens when a wormhole forms in the center of a black hole.
But what happens on the other side, where the hole appears? According to Einstein, that's where the bowling ball (or matter) comes out of the wormhole. In other words, a white hole is created there.
If white holes exist in this universe, they are probably difficult to identify. Stephen Hawking showed in the 1970s that white holes in thermal equilibrium with their surroundings absorb and emit as much radiation as black holes. Perhaps white holes have long been discovered, but they are described in the books as black holes.
But what if white holes are not in thermal equilibrium with their surroundings? What if they are on their own? Stable white holes cannot arise in an empty universe. That's why we don't see any evidence. White holes probably explode so quickly that we cannot observe them.
So if white holes exist, it is difficult to discover them. So keep dreaming!
