The question for the stellar evolution is one of the most important for astrophysics because its answer would reveal the Universe secrets. The theory of stellar evolution tries to give an answer.

The stellar evolution means the changes of the physical haracteristics, the internal structure and the chemical composition of the stars during the time.

Stellar evolution: Basic concept

The stars are enormous globes of hot gas. When we observe them they look like spots on the sky because they are located at huge distances from us. The stars shine because nuclear reactions occur in their cores.

Similarly as for the human beings we divide the stellar life in different stages: birth, maturity and dead.

The stars are born in giant clouds of dust and gas as the the main building element is hydrogen.

Yet Isaac Newton (1643-1727) has discussed the idea for stellar forming from the diluted interstellar medium but the evidences for this have been obtained only at the beginning of 20th century: through the infrared observations of suitable clouds of interstellar gas and it has been found out that they have lost their stability and colapsed under the gravity and have become stars. The most popular example for such a cloud, incubator of stars, is the Orion nebula (the figure below).

The turbulence deep inside these clouds creates fragments with a mass enough to start collapsing because of the gravity. Just before the beginning of collapsing the gas temperature inside the cloud is only 10-30 K i.e.these are one of the coolest objects in the Universe. The matter inside the cloud is dense – there are 2 billion molecules in 1m3 which is 1016 times lower than in the air in normal conditions. The mass of these clouds is enormous – it reaches to million of solar masses which makes the gravity the main factor in their evolution. When the cloud collapses the matter into the center begins to heat. That is the moment when the protostar appears. As a result of the collapse the internal gas pressure is increasing and this slows down the collapse process. At the next stage the gas gradually heats up and the protostar begins to glow dimly. The collapse is ongoing inhomogeneously and the density is growing more in the central parts of the cloud. When the temperature increases enough the collapsing gas becomes ionised and it become opaque for the radiation from the central regions of the star. This leads to the collapse of the outer layers and the temperature and density in the protostar center increase. Soon the density becomes so high that the collapse stops and the hydrostatic stable core is created inside the cloud. Outside the cloud however the gas is transparent for the infrared radiation and it continues to fall towards the center. Falling to it the kinetic energy converts into heat as about 50% of it goes for gas heating and the rest is radiating outside. When the envelope fаlls entirely on the core and becomes transparent, the core ‘emerges from the dust cocoon’ and the young STAR is born! It continues to collapse slowly and the heat is released because of the gravity. Part of this heat is radiated and other part heats the internal layers keeping in such a way a hydrostatic quasi equilibrium. When the temperature in the stellar center exceeds a few million degrees the thermonuclear reactions start and hydrogen becomes to helium as a result.

The models predict that the rotating clouds of gas and dust may be separated in two or three parts. This would explain why the majority of stars are not single but double, triple or more multiple stellar systems.

When the cloud collapses, a dense hot core is forming and a dust and gas begin to collect around it. But not all of this matter ends as a part of a star – dust remnants could become planets, asteroids or comets, or simply remain as interstellar dust.

If the collapsed cloud mass is lower than 0,08M, where M denotes the solar mass, than the gravity collapse cannot lead to temperatures high enough to ignite thermonuclear reactions. Such failed stars are called brown dwarfs. The only source of the internal energy of brown dwarfs is the gravitational potential energy. If the mass of such an object is below 0,002M, is called a planet. And if it is above 0,08M, its mass is large enough to keep thermonuclear reactions and this is already a real star.

When the thermonuclear reactions ignite in the stellar core they create enough heating that prevents further contraction of the star. The balance between gravity which tries to collapse the star and heating which tries to expand it is called THERMODYNAMIC EQUILIBRIUM.

From that moment the star does not change for a very long time. This is the time when the star lives on the Main sequence of the Hertzsprung-Russell diagram.

For a star like our Sun this period is about 10 billion years.

Although during this ‘mature’ part of their life one and the same processes in the their cores are taking place, the stars differ in temperature (color), size, mass, brightness, age.

The larger a star is, the hotter and brighter it is. The hot stars are blue in color. The smaller stars are a little bit brighter, cooler and red in color.

As a whole the more massive a star is, the shorter life it has although the stars live billion of years.

Here is the place to pay more attention to the Hertzsprung-Russell diagram (abbreviated as H- R diagram). This is a scatter plot which gives the relationship between fundamental parameters of stars. At the beginning of 20th century two astronomers, the Dane Ejnar Hertzsprung and the American Henry Norris Russell, independantly established that the stars are arranged in a certain way on a diagram which connects the spectral class of the stars and their absolute apparent magnitudes. As the spectral class is associated with the color and the surface temperature of the star, and the absolute magnitude with the luminosity, then the X-R diagram is often presented also as a color-magnitude diagram or temperature-luminosity diagram.

On these diagrams the stars are not scattered uniformly but occupy certain sequences. There is a Main sequence which begins with high luminosity and high temperature stars, crosses the diagram diagonally and ends with low luminosity and lowest temperatures stars. Almost 90% of the stars occupy that sequence. Our Sun is placed also on the Main sequence. It is a spectral class G2 star.

Above the Main sequence there is a region where the red giants are concentrated and even higher are the stars with higher luminosity called supergiants. Below the Main sequence is the area of the white dwarfs.

Astronomers find out that the H-R diagram has a deep meaning. It is not just a relationship between some parameters but an evolutionary relationship. During its evolution the star changes its temperature and luminosity and moves on the diagram. If we know the place of a star on the diagram and its mass in a given moment we can predict its evolution, i.e. what phases of its evolution it will go through and how it will end its life.