The world of stars shows a great variety, the signs of which are already evident when looking at the night sky with the naked eye. Studying stars using astronomical instruments and methods of astrophysics has made it possible to systematize them in a certain way and, thanks to this, gradually come to an understanding of the processes that govern stellar evolution.
In the general case, the conditions under which star formation took place determine its main characteristics. These conditions can be very different. However, in general, this process has a single nature for all stars: they are born from the diffuse - scattered - gas and dust matter, which fill the galaxies, by its compaction under the influence of gravity.
The composition and density of the galactic medium
Regarding terrestrial conditions, interstellar space is the deepest vacuum. But on a galactic scale, such an extremely rarefied medium with a characteristic density of the order of 1 atom per cubic centimeter is gas and dust, and their ratio in the composition of the interstellar medium is 99 to 1.
The main component of the gas is hydrogen (about 90% of the composition, or 70% of the mass), there is also helium (approximately 9%, and by mass - 28%) and other substances in small quantities. In addition, cosmic rays and magnetic fields are referred to the interstellar galactic medium.
Where stars are born
Gas and dust in the space of galaxies are distributed very nonuniformly. Interstellar hydrogen, depending on the conditions in which it is located, can have different temperatures and densities: from a very rarefied plasma with a temperature of the order of tens of thousands of kelvin (the so-called HII zones) to ultracold - only a few kelvin - molecular state.
Areas where the concentration of particles of matter due to any reason is increased are called interstellar clouds. The densest clouds, in a cubic centimeter of which can contain up to a million particles, are formed by a cold molecular gas. They have a lot of dust that absorbs light, so they are also called dark nebulae. It is precisely to such "space refrigerators" that the places of the origin of stars are confined. HII regions are also associated with this phenomenon, but stars do not form directly in them.
Localization and types of "star cradles"
In spiral galaxies, including our Milky Way, molecular clouds are not located randomly, but mainly within the plane of the disk - in spiral arms at some distance from the galactic center. In irregular galaxies, the localization of such zones is random. As for elliptical galaxies, gas and dust structures and young stars are not observed in them, and it is generally accepted that this process is practically not going on there.
Clouds can be either giant (tens or hundreds of light years) molecular complexes with a complex structure and large density drops (for example, the famous Orion Cloud just 1300 light years from us), or isolated compact formations called Bock globules.
Star formation conditions
The birth of a new luminary requires the indispensable development of gravitational instability in a gas-dust cloud. Due to various dynamic processes of internal and external origin (for example, different speeds of rotation in different regions of the cloud of irregular shape or the passage of a shock wave during a supernova explosion in the neighborhood), the distribution density of the substance in the cloud fluctuates. But not every density fluctuation that arises leads to further compression of the gas and the appearance of a star. The magnetic fields in the cloud and turbulence counteract this.
The region of increased concentration of the substance must have a length sufficient so that gravity can withstand the elastic force (pressure gradient) of the gas-dust medium. Such a critical size is called the Jeans radius (an English physicist and astronomer who laid the foundations of the theory of gravitational instability at the beginning of the 20th century). The mass enclosed within the jeans radius should also not be less than a certain value, and this quantity (Jeans mass) is proportional to the temperature.
It is clear that the colder and denser the medium, the smaller the critical radius at which the fluctuation does not smooth out, but continues compaction. Further, star formation proceeds in several stages.
Cloud collapse and fragmentation
When gas is compressed, energy is released. In the early phases of the process, it is essential that the condensed core in the cloud can effectively cool down due to radiation in the infrared range, which is carried out mainly due to molecules and dust particles. Therefore, at this stage, compaction proceeds quickly and becomes irreversible: a fragment of the cloud collapses.
In such a contracting and at the same time cooling section, if it is sufficiently large, new nuclei of condensation of the substance may appear, since with increasing density the critical jeans mass decreases if the temperature does not increase. This phenomenon is called fragmentation; thanks to him, the formation of stars most often does not occur singly, but in groups - associations.
The duration of the stage of intense compression, according to modern concepts, is small - about 100 thousand years.
Warming up a cloud fragment and the formation of a protostar
At some stage, the density of the collapsing region becomes too high, and it loses its transparency, as a result of which the gas begins to heat up. The size of the Jeans mass increases, further fragmentation becomes impossible, and only fragments already formed by this time undergo compression under the influence of their own gravity. Unlike the previous stage, due to the steady increase in temperature and, accordingly, gas pressure, this stage takes much longer - about 50 million years.
The object formed during this process is called a protostar. It is distinguished by active interaction with the residual gas and dust matter of the parent cloud.
Protostar Features
A born star seeks to dump the energy of gravitational compression outward. A convection process develops inside it, and the outer layers intensively emit in the infrared, and then in the optical range, heating the surrounding gas, which contributes to its rarefaction. If a star of large mass with high temperature is formed, it is able to almost completely “clear” the space around it. Its radiation will ionize the residual gas - this is how the HII regions are formed.
Initially, the parent cloud fragment, of course, one way or another, rotated, and when it is compressed, due to the law of conservation of angular momentum, the rotation is accelerated. If a star comparable to the Sun is born, the surrounding gas and dust will continue to fall on it in accordance with the angular momentum, and a protoplanetary accretion disk will form in the equatorial plane. Due to the high rotation speed, hot, partially ionized gas is ejected from the inner region of the disk by a protostar in the form of polar jet flows at a speed of hundreds of kilometers per second. These jets, colliding with interstellar gas, form shock waves visible in the optical part of the spectrum. To date, several hundred such phenomena — Herbig – Aro objects — have been discovered.
Hot protostars close in mass to the Sun (known as T Taurus stars) exhibit chaotic changes in brightness and high luminosity associated with a large radius, because they still continue to shrink.
The beginning of nuclear fusion. Young star
When the temperature in the central regions of the protostar reaches several million degrees, thermonuclear reactions begin there. The process of the birth of a new star at this stage can be considered completed. The young star, as they say, "sits on the main sequence", that is, enters the main stage of his life, during which the source of its energy is the nuclear synthesis of helium from hydrogen. The release of this energy balances gravitational compression and stabilizes the star.
The features of the course of all further stages of the evolution of stars are determined by the mass with which they were born and the chemical composition (metallicity), which depends to a large extent on the composition of impurities of elements heavier than helium in the initial cloud. If the star is massive enough, it will process part of the helium into heavier elements - carbon, oxygen, silicon and others - which, at the end of its life, will become part of interstellar gas and dust and serve as material for the formation of new stars.