When we talk about accretion we are referring to the growth of a body by the aggregation of smaller bodies. It is used mainly in the field of astronomy and astrophysics and serves to explain various phenomena such as circumstellar discs, accretion discs or the accretion of a terrestrial planet. The planetary accretion theory was proposed in 1944 by the Russian geophysicist Otto Schmidt.
In this article we are going to tell you everything you need to know about accretion and its importance.
What is accretion
The accretion is used to explain how the stars, planets and certain satellites that have formed from the nebula have formed. There are many celestial objects that are have formed by the accretion of particles by condensation and inverse sublimation. In the cosmos it could be said that everything is magnetic in one way or another. Some of the most spectacular phenomena in nature are magnetic.
Accretion occurs in many different astronomical objects. This phenomenon even occurs in black holes. Normal and neutron stars also have accretion. This is the process by which mass from outside falls onto the star itself. For example, the gravitational force exerted by a white dwarf causes the mass to fall onto it. Generally, a star usually floats in the universe surrounded by a space that was practically empty. This means that there are not many circumstances that can cause the mass to fall on this celestial object. However, there are some occasions when it can.
Let's analyze the circumstances under which accretion occurs.
Circumstances of accretion
One of the situations in which accretion can occur is a celestial body is that the star has as a companion another star. These stars must be orbiting. Sometimes the companion star is so close that the mass is pulled towards the other with such force that they end up falling on it. Since the white dwarf is smaller in size than an ordinary star, the mass it must reach its surface at great speed. Let's take the example that it is not a white dwarf, but a neutron star or a black hole. In this case, the speed is close to the speed of light.
When it reaches the surface, the mass suddenly slows down so that its speed varies from almost the speed of light to a much lower value. This occurs in the case of a neutron star. This is how A large amount of energy is released, which is usually visible as X-rays.
Accretion as an efficient process
Many scientists question whether accretion is one of the most efficient ways to convert mass into energy. We know, thanks to Einstein, that energy and mass are equivalent. Our sun releases energy through nuclear reactions with an efficiency of less than 1%. Although it seems like there is a large amount of energy from the sun, it is released inefficiently. If we drop mass onto a neutron star, almost 10% of all the mass that has fallen is converted into radioactive energy. It can be said that it is the most efficient process to transform matter into energy.
Stars form through the slow accumulation of mass from their environment. Normally, this mass is composed of a molecular cloud. If accretion occurs in our solar system, it's a very different situation. Once the concentration of mass is dense enough to begin attracting itself through its own gravitational pull, it condenses to form a star. Molecular clouds rotate slightly and have a two-stage process. In the first stage, the cloud collapses into a rotating disk. After that, the disk contracts more slowly to form a star in the center.
During this process things happen inside the discs. The most interesting of all is inside the discs the formation of planets takes place. What we see as the solar system was originally an accretion disk that gave rise to the sun. However, in the process of the formation of the sun, part of the dust of the disk was compensated to give rise to the planets that belong to the solar system.
All of this means that the solar system will be a remnant of what happened long ago. The protostellar disk is of great importance for research related to the formation of planets and stars. Today, scientists are continually searching for planets around other stars that mimic other solar systems. All of this is closely related to the way accretion discs work.
Utility to discover black holes
Scientists think that all galaxies have a black hole at their center. Some of them have black holes that have the mass of billions of solar masses. However, others only have very small black holes like ours. In order to detect the presence of a black hole, it is necessary to know the existence of a source of something that can supply it with mass.
It is theorized that a black hole is a binary system that has a star orbiting around it. Einstein's theory of relativity predicts that the star companion gets closer to the black hole until it begins to give up its mass when it gets closer. But due to the rotation of the star it is possible that an accretion disk is generated and that the mass ends up in the black hole. This whole process is much slower. When some mass falls into the black hole, before disappearing, it reaches the speed of light. This is known as event horizon.
I hope that with this information you can learn more about the accretion and its characteristics.