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Stars vary greatly in size, luminosity, and temperature. Due to the huge surface area, giant stars radiate far greater power than the stars like Sun, despite the fact that their surface temperatures are much lower. The radius of red supergiant Betelgeuse (? Orionis) is many times greater than the radius of the Sun. Conversely, the size of an average red star usually does not exceed one-tenth the size of the Sun. In contrast with the giants they are called dwarfs. For example, two stars with the same spectral class M2, Betelgeuse and Lalande 21185, vary in luminosity in 600 000 times. Luminosity of Betelgeuse is in 3000 times more intensive than the luminosity of the Sun, while the Lalande 21185 luminosity is 200 times less. Giant and dwarf stars are at different stages of their evolution, and by reaching an “old age”, a giant star may turn into a white dwarf (Miller 14).
Einar Hertzsprung and Henry Russell were the first who made the comparison table of the star luminosity with their spectral classes, in the beginning of the 20th century. Thus, a spectrum-luminosity diagram is often called the Hertzsprung-Russell diagram. In this diagram, abscissa axis shows spectral classes (or effective temperature), and the vertical axis shows the luminosity (or absolute magnitudes). If there were no dependence between the luminosities and temperatures, all the stars on the diagram would be distributed uniformly. However, the diagram found several regularities that are called sequences. The following classes of luminosity are distinguished in the distribution of stars according to their luminosity and temperature in the Hertzsprung-Russell diagram:
· Supergiants - I luminosity class;
· Giants - II luminosity class;
· Main sequence stars - V luminosity class;
· Subdwarfs - VI luminosity class;
· White dwarfs - VII luminosity class (Miller 21).
Main sequence is the area in the Hertzsprung-Russell diagram, containing stars, which have thermonuclear fusion reaction of helium from hydrogen as a source of energy. The main sequence is located in the vicinity of the diagonal of the Hertzsprung-Russell diagram, and extends from the upper left corner (high luminosity, early spectral classes) to the lower right corner (low luminosity, late spectral classes) of diagram. Main sequence stars have the same energy source (“burning” of hydrogen, primarily, CNO-cycle), in connection with which their luminosity and temperature (spectral class) are defined by their content: L=M 3,9, where luminosity L and mass M are measured in units of solar luminosity and mass, respectively. Therefore, the beginning of the left side of the main sequence is represented by blue stars with masses of ~ 50 solar masses, and the end of the right side is represented by red dwarfs, with masses of ~ 0,0767 solar masses (Seggewiss 83).
The existence of the main sequence is related to the fact that the hydrogen burning phase amounts to ~ 90% of the time evolution of most stars; burning of hydrogen in the central parts of the star leads to the formation of isothermal helium core, the transition to the red giant stage and eventually leaving the main sequence. Depending on the mass, relatively brief evolution of red giants leads to the formation of white dwarfs, neutron stars or black holes. Part of the star clusters main sequence is an indicator of their age: as the rate of evolution of stars is proportional to their mass, for clusters there is a “left” break point of the main sequence in the high luminosity and early spectral classes, depending on the age of the cluster, as stars with a mass greater than certain limit, specified by age of clusters, have left the main sequence (Seggewiss 86).
Main sequence stars are in the main phase of their evolution. For instance, if to compare with human, it is a period of adulthood, or a period of relative stability. All stars undergo this phase, some faster (heavyweight stars), others longer (lightweight stars). However, the main sequence period is the longest in the life of every star. The given phase is characterized by the so-called stage of gravitational contraction, which leads to the appearance of thermonuclear energy source inside the star (Seggewiss 90).
The start of the main sequence stage is defined as the moment when the energy loss for radiation of the chemically homogeneous star is fully compensated by the energy release in thermonuclear reactions. Stars at this point are on the left edge of the main sequence, referred as the initial main sequence or main sequence of age zero. For the stars with M
Einar Hertzsprung and Henry Russell were the first who made the comparison table of the star luminosity with their spectral classes, in the beginning of the 20th century. Thus, a spectrum-luminosity diagram is often called the Hertzsprung-Russell diagram. In this diagram, abscissa axis shows spectral classes (or effective temperature), and the vertical axis shows the luminosity (or absolute magnitudes). If there were no dependence between the luminosities and temperatures, all the stars on the diagram would be distributed uniformly. However, the diagram found several regularities that are called sequences. The following classes of luminosity are distinguished in the distribution of stars according to their luminosity and temperature in the Hertzsprung-Russell diagram:
· Supergiants - I luminosity class;
· Giants - II luminosity class;
· Main sequence stars - V luminosity class;
· Subdwarfs - VI luminosity class;
· White dwarfs - VII luminosity class (Miller 21).
Main sequence is the area in the Hertzsprung-Russell diagram, containing stars, which have thermonuclear fusion reaction of helium from hydrogen as a source of energy. The main sequence is located in the vicinity of the diagonal of the Hertzsprung-Russell diagram, and extends from the upper left corner (high luminosity, early spectral classes) to the lower right corner (low luminosity, late spectral classes) of diagram. Main sequence stars have the same energy source (“burning” of hydrogen, primarily, CNO-cycle), in connection with which their luminosity and temperature (spectral class) are defined by their content: L=M 3,9, where luminosity L and mass M are measured in units of solar luminosity and mass, respectively. Therefore, the beginning of the left side of the main sequence is represented by blue stars with masses of ~ 50 solar masses, and the end of the right side is represented by red dwarfs, with masses of ~ 0,0767 solar masses (Seggewiss 83).
The existence of the main sequence is related to the fact that the hydrogen burning phase amounts to ~ 90% of the time evolution of most stars; burning of hydrogen in the central parts of the star leads to the formation of isothermal helium core, the transition to the red giant stage and eventually leaving the main sequence. Depending on the mass, relatively brief evolution of red giants leads to the formation of white dwarfs, neutron stars or black holes. Part of the star clusters main sequence is an indicator of their age: as the rate of evolution of stars is proportional to their mass, for clusters there is a “left” break point of the main sequence in the high luminosity and early spectral classes, depending on the age of the cluster, as stars with a mass greater than certain limit, specified by age of clusters, have left the main sequence (Seggewiss 86).
Main sequence stars are in the main phase of their evolution. For instance, if to compare with human, it is a period of adulthood, or a period of relative stability. All stars undergo this phase, some faster (heavyweight stars), others longer (lightweight stars). However, the main sequence period is the longest in the life of every star. The given phase is characterized by the so-called stage of gravitational contraction, which leads to the appearance of thermonuclear energy source inside the star (Seggewiss 90).
The start of the main sequence stage is defined as the moment when the energy loss for radiation of the chemically homogeneous star is fully compensated by the energy release in thermonuclear reactions. Stars at this point are on the left edge of the main sequence, referred as the initial main sequence or main sequence of age zero. For the stars with M
