The world of the stars shows great diversity, signs of which are already apparent when looking at the night sky with the naked eye. The study of stars with the help of astronomical instruments and methods of astrophysics 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 the formation of a star proceeded determine its main characteristics. These conditions can be very different. However, in general, this process is of the same nature for all stars: they are born from diffuse - scattered - gas and dust matter, which fills galaxies, by compacting it under the influence of gravity.
Composition and density of the galactic medium
Related to Earth conditions, interstellar space is the deepest vacuum. But on a galactic scale, such an extremely rarefied medium with a characteristic density of about 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 weight - 28%) and other substances in small quantities. In addition, cosmic ray fluxes 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 non-uniformly. Interstellar hydrogen, depending on the conditions in which it is located, can have different temperatures and densities: from highly rarefied plasma with a temperature of the order of tens of thousands of kelvins (the so-called HII zones) to an ultracold - just a few kelvins - molecular state.
Regions where the concentration of particles of matter is increased for any reason, are called interstellar clouds. The densest clouds, which can contain up to a million particles per cubic centimeter, are formed by cold molecular gas. They have a lot of dust that absorbs light, so they are also called dark nebulae. It is to such "cosmic refrigerators" that the places where stars originated 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 own Milky Way, molecular clouds are located not randomly, but mainly within the disk plane - in spiral arms at some distance from the galactic center. In irregularIn 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 practically does not occur there.
Clouds can be both giant - tens and hundreds of light years - molecular complexes with a complex structure and large density differences (for example, the famous Orion Cloud is only 1300 light years from us), and isolated compact formations called Bok globules.
Star formation conditions
The birth of a new star requires the indispensable development of gravitational instability in the gas and dust cloud. Due to various dynamic processes of internal and external origin (for example, different rotation rates in different regions of an irregularly shaped cloud or the passage of a shock wave during a supernova explosion in the neighborhood), the distribution density of matter in the cloud fluctuates. But not every emerging density fluctuation leads to further compression of the gas and the appearance of a star. The magnetic fields in the cloud and turbulence counteract this.
The area of increased concentration of a substance must have a length sufficient to ensure that gravity can resist the elastic force (pressure gradient) of the gas and 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 contained within the Jeansradius must also not be less than a certain value, and this value (the 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 to compact. Further, the formation of a star proceeds in several stages.
Collapse and fragmentation of a portion of the cloud
When a gas is compressed, energy is released. In the early phases of the process, it is essential that the condensing core in the cloud can effectively cool down due to radiation in the infrared range, which is carried out mainly by molecules and dust particles. Therefore, at this stage, the compaction is fast and becomes irreversible: the cloud fragment collapses.
In such a shrinking and at the same time cooling area, if it is large enough, new condensation nuclei of matter can appear, since with an increase in 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 occurs not one by one, but in groups - associations.
The duration of the stage of intense compression, according to modern concepts, is small - about 100 thousand years.
Heating up a cloud fragment and forming a protostar
At some point, the density of the collapsing region becomes too high, and it loses transparency, as a result of which the gas begins to heat up. The value of the Jeans mass increases, further fragmentation becomes impossible, and compression underonly fragments that have already formed by this time are tested by the action 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.
Features of protostars
A newborn star tends to dump the energy of gravitational contraction outward. A convection process develops inside it, and the outer layers emit intense radiation in the infrared, and then in the optical range, heating the surrounding gas, which contributes to its rarefaction. If there is a formation of a star of large mass, with a high temperature, it is able to almost completely "clear" the space around it. Its radiation will ionize the residual gas - this is how HII regions are formed.
Initially, the parent fragment of the cloud, of course, one way or another, rotated, and when it is compressed, due to the law of conservation of angular momentum, the rotation accelerates. 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 from the inner region of the disk is ejected by the protostar in the form of polar jet streams withspeeds 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-Haro objects - have already been discovered.
Hot protostars close in mass to the Sun (known as T Tauri stars) exhibit chaotic brightness variations and high luminosity associated with large radii as they continue to contract.
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 sun, as they say, "sits down on the main sequence", that is, enters the main stage of its life, during which the source of its energy is the nuclear fusion of helium from hydrogen. The release of this energy balances the gravitational contraction and stabilizes the star.
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 largely on the composition of impurities of elements heavier than helium in the initial cloud. If a star is massive enough, it will process some 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.
