TO main sequence These include those stars that are in the main phase of their evolution. This, when compared with a person, is a period of maturity, a period of relative stability. All stars go through this phase, some faster (heavy stars), others longer (light stars). In the life of each star, this period is the longest.

E If we consider the Hertzsprung-Russell diagram, then the main sequence stars are located diagonally from the upper left corner (high luminosities) to the lower right (low luminosities). The position of stars on the Hertzsprung-Russell diagram depends on the mass, chemical composition of the stars and the processes of energy release in their interiors. Stars on the Main Sequence have the same source of energy (thermonuclear reactions of hydrogen combustion, so their luminosity and temperature (and therefore position on the Main Sequence) are determined mainly by mass; the most massive stars (M~50M of the Sun) are located in the upper (left) part Main Sequence, and as we move down the Main Sequence, the masses of stars decrease to M~0.08M of the Sun.

N and stars enter the main sequence after the stage gravitational compression, leading to the appearance of a thermonuclear energy source in the bowels of the star. The beginning of the Main Sequence stage is defined as the moment when the energy loss of a chemically homogeneous star through radiation is completely compensated by the release of energy in thermonuclear reactions. The stars at this moment are on the left boundary of the Main Sequence, called the initial Main Sequence or the zero-age Main Sequence. The end of the Main Sequence stage corresponds to the formation of a homogeneous helium core in the star. The star leaves the Main Sequence and becomes a giant. The scattering of stars on the observed Main Sequence is due, in addition to evolutionary effects, to differences in the initial chemical composition, rotation and possible binarity of the star.

U For stars with M<0.08M of the Sun, the time of gravitational compression exceeds the lifetime of the Galaxy, and therefore they have not reached the Main Sequence and are located somewhat to the right of it. For stars with masses 0.08 M of the Sun, the stage of thermonuclear burning of hydrogen is so long that they did not have time to leave the Main Sequence during the lifetime of the Galaxy. More massive stars have a Main Sequence lifetime of ~90% of their entire evolutionary time. This explains the predominant concentration of stars in the Main Sequence region.

A Main sequence analysis plays especially important role when studying star groups and clusters, because as their age increases, the point at which the Main Sequence of a cluster begins to deviate noticeably from the initial Main Sequence shifts to the region of lower luminosities and later spectral classes, and therefore the position of the turning point of the Main Sequence can serve as an indicator of the age of a star cluster.

The Hertzsprung-Russell Diagram (HR Diagram)

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Hertzsprung-Russell diagram

The most important physical characteristics stars are temperature and absolute magnitude. Temperature indicators are closely related to the color of the star, and the absolute magnitude is closely related to the spectral class. Let us remember that, according to the currently used classification, stars, in accordance with their spectra, as already mentioned in the “Spectral Classes” section of the site, are divided into seven main spectral classes. They are marked with Latin letters O, B, A, F, G, K, M. It is in this sequence that the temperature of stars drops from several tens of thousands of degrees for class O (very hot stars) to 2000-3000 degrees for class M stars.

Those. a measure of brilliance expressed by the amount of energy emitted by a star. It can be calculated theoretically, knowing the distance to the star.

In 1913, the Danish astronomer Einar Hertzsprung and the American Henry Norris Russell independently came up with the idea of ​​constructing a theoretical graph connecting two main stellar parameters - temperature and absolute magnitude. The result was a diagram that was given the names of two astronomers - the Hertzsprung-Russell diagram (HRD), or, more simply, G-R diagram. As we will see later, the Hertzsprung-Russell diagram helps to understand the evolution of stars. In addition, it is widely used to determine distances to star clusters.

Each point on this diagram corresponds to a star. Along the ordinate axis ( vertical axis) the luminosity of the star is plotted, and the x-axis (horizontal axis) is the temperature of its surface. If we determine its temperature by the color of a star, then we will have at our disposal one of the quantities needed to construct a G-R diagram. If the distance to a star is known, then its luminosity can be determined by its apparent brightness in the sky. Then we will have at our disposal both quantities necessary to construct the H-R diagram, and we will be able to put a point on this diagram that corresponds to our star.

The Sun is placed opposite luminosity 1 on the diagram, and since the surface temperature of the Sun is 5800 degrees, it is almost in the middle of the H-R diagram.

Stars whose luminosity is greater than the Sun are located in the diagram above. For example, the number 1000 means that at this level there are stars whose luminosity is 1000 times greater than the luminosity of the Sun.

Stars with lower luminosity, such as Sirius B, a white dwarf from the Sirius system, lie lower. Stars that are hotter than the Sun, such as Sirius A and Zeta Aurigae B - hot star from the system Zeta Aurigae and Spica from the constellation Virgo, lie to the left of the Sun. Cooler stars like Betelgeuse and the red supergiant Zeta Aurigae lie to the right.

Since cool stars emit red light and hot stars emit white or blue light, the diagram shows red stars on the right and white or blue stars on the left. At the top of the diagram are stars with high luminosity, and at the bottom - with low luminosity.

Main sequence

Most of the stars on the H-R diagram are located within the diagonal stripe running from the upper left to the lower right. This strip is called "main sequence" . The stars located on it are called "main sequence stars." Our Sun belongs to the stars of the main sequence and is located in that part of it that corresponds to yellow stars. At the top of the main sequence are the brightest and hottest stars, and at the bottom right are the dimmest and, as a result, the longest-lived.

Main sequence stars are in the most “quiet” and stable phase of their existence, or, as they say, the phase of life.

The source of their energy is. According to modern estimates of the theory of stellar evolution, this phase accounts for about 90% of the life of any star. This is why most stars belong to the main sequence.

According to the theory of stellar evolution, when the supply of hydrogen in the interior of a star runs out, it leaves the main sequence, deviating to the right. In this case, the temperature of the star always falls, and its size rapidly increases. The complex, increasingly accelerating movement of the star along the diagram begins.

Red giants and white dwarfs

Separately, to the right and above the main sequence there is a group of stars with very high luminosity, and the temperature of such stars is relatively low - these are the so-called red giant stars and supergiants . These are cool stars (approximately 3000°C), which, however, are much brighter than stars with the same temperature that are in the main sequence. One square centimeter surfaces cold star emits a relatively small amount of energy per second. The high overall luminosity of a star is explained by the large surface area of ​​its surface: the star must be very large. Giants are stars whose diameter is 200 times greater than the diameter of the Sun.

In the same way we can consider the left bottom part diagrams. There are hot stars with low luminosity there. Since a square centimeter of the surface of a hot body emits a lot of energy per second, and the stars in the lower left corner of the diagram have a low luminosity, we must conclude that they are small in size. At the bottom left, therefore, are located white dwarfs , very dense and compact stars with sizes on average 100 times smaller than the Sun, with a diameter comparable to the diameter of our planet. One such star, for example, is a satellite of Sirius called Sirius B.

Star sequences of the Hertzsprung-Russell diagram in the accepted conventional numbering

On the Hertzsprung-Russell diagram, in addition to the sequences we considered above, astronomers actually identify several more sequences, and the main sequence has a conditional number V . Let's list them:

Ia - sequence of bright supergiants,
Ib - a sequence of weak supergiants,
II- sequence of bright giants,
III- sequence of weak giants,
IV - sequence of subgiants,
V - main sequence,
VI - sequence of subdwarfs,
VII - sequence of white dwarfs.

In accordance with this classification, our Sun with its spectral class G2 is designated as G2V .

Thus, from general considerations, knowing the luminosity and surface temperature, one can estimate the size of the star. Temperature tells us how much energy is emitted by one square centimeter of surface. Luminosity, equal to the energy that a star emits per unit time, allows us to find out the size of the emitting surface, and therefore the radius of the star.

It is also necessary to make a caveat that measuring the intensity of light coming to us from stars is not so easy. The Earth's atmosphere does not allow all radiation to pass through. Short-wavelength light, for example, in the ultraviolet region of the spectrum, does not reach us. It should also be noted that the apparent magnitudes of distant objects are weakened not only due to absorption by the Earth’s atmosphere, but also due to the absorption of light by dust grains present in interstellar space. It is clear that even a space telescope that operates outside the Earth’s atmosphere cannot be eliminated from this interfering factor.

But the intensity of light passing through the atmosphere can be measured in different ways. The human eye perceives only part of the light emitted by the Sun and stars. Light rays of different lengths, having different colour, do not have the same intense effect on the retina, photographic plate or electronic photometer. When determining the luminosity of stars, only the light that is perceived by the human eye is taken into account. Therefore, for measurements it is necessary to use instruments that, using color filters, imitate the color sensitivity of the human eye. Therefore, on H-R diagrams, instead of the true luminosity, the luminosity in visible area spectrum perceived by the eye. It is also called visual luminosity. The values ​​of true (bolometric) and visual luminosity can differ quite significantly. For example, a star whose mass is 10 times that of the Sun emits about 10 thousand times more energy than the Sun, while in the visible range of the spectrum it is only 1000 times brighter than the sun. For this reason, the spectral type of a star is often replaced today by another equivalent parameter called "color index"; or "color index" , displayed on the horizontal axis of the chart. In modern astrophysics, the color index is essentially the difference between the magnitudes of a star in different spectral ranges (it is customary to measure the difference between magnitudes in the blue and visible parts of the spectrum, called B-V or B minus V from English Blue and Visible). This parameter shows the quantitative distribution of energy that a star emits at different wavelengths, and this is directly related to the temperature of the star's surface.

The H-R diagram is usually given in the following coordinates:
1. Luminosity is the effective temperature.
2. Absolute magnitude - color indicator.
3. Absolute magnitude - spectral class.

Physical meaning of the H-R diagram

The physical meaning of the H-R diagram is that after drawing on it maximum number experimentally observed stars, by their location one can determine the patterns of their distribution in terms of the ratio of spectrum and luminosity. If there were no relationship between luminosities and their temperatures, then all the stars would be distributed evenly on such a diagram. But the diagram reveals several regularly distributed groupings of stars that we have just examined, called sequences.

The Hertzsprung-Russell diagram is of great help in studying the evolution of stars throughout their existence. If it were possible to follow the evolution of a star throughout its entire life, i.e. over several hundred million or even several billion years, we would see it slowly shift along the H-R diagram in accordance with changes in physical characteristics. The movements of stars along the diagram depending on their age are called evolutionary tracks.

In other words, the H-P diagram helps us understand how stars evolve throughout their existence. By back-calculating using this diagram, you can calculate the distances to the stars.

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Main sequence stars

Units

Most stellar characteristics are usually expressed in SI, but GHS is also used (for example, luminosity is expressed in ergs per second). Mass, luminosity and radius are usually given in relation to our Sun:

To indicate the distance to stars, units such as light year and parsec are used.

Long distances, such as radius giant stars or semimajor axis of binary star systems is often expressed using

astronomical unit(a.u.) - the average distance between the Earth and the Sun (150 million km).

Fig. 1 – Hertzsprung-Russell diagram

Types of stars

Classifications of stars began to be built immediately after their spectra began to be obtained. To a first approximation, the spectrum of a star can be described as the spectrum of a black body, but with absorption or emission lines superimposed on it. Based on the composition and strength of these lines, the star was assigned one or another specific class. This is what they do now, however, the current division of stars is much more complex: in addition, it includes absolute stellar magnitude, the presence or absence of variability in brightness and size, and the main spectral classes are divided into subclasses.

At the beginning of the 20th century, Hertzsprung and Russell plotted the ʼʼAbsolute magnitudeʼʼ - ʼʼspectral classʼʼ various stars, and it turned out that most of them are grouped along a narrow curve. Later this diagram (now called Hertzsprung-Russell diagram) turned out to be the key to understanding and researching the processes occurring inside a star.

Now that there's a theory internal structure stars and the theory of their evolution, it became possible to explain the existence of classes of stars. It turned out that the whole variety of types of stars is nothing more than a reflection quantitative characteristics stars (such as mass and chemical composition) And evolutionary stage, on which in this moment there is a star.

In catalogs and in writing, the class of stars is written in one word, and first goes alphabetic designation of the main spectral class (if the class is not precisely defined, a letter range is written, for example, O-B), then the spectral subclass is specified in Arabic numerals, then the luminosity class comes in Roman numerals (region number on the Hertzsprung-Russell diagram), and then comes Additional Information. For example, the Sun has a class G2V.

The most numerous class of stars are main sequence stars; our Sun also belongs to this type of star. From an evolutionary point of view, the main sequence is the place on the Hertzsprung-Russell diagram where the star is located most own life. At this time, energy losses due to radiation are compensated by the energy released during nuclear reactions. The lifetime on the main sequence is determined by the mass and fraction of elements heavier than helium (metallicity).

The modern (Harvard) spectral classification of stars was developed at the Harvard Observatory in 1890 - 1924.

Basic (Harvard) spectral classification of stars
Class	Temperature, K	true color	Visible color	Main features
O	30 000-60 000	blue	blue	Weak lines of neutral hydrogen, helium, ionized helium, multiply ionized Si, C, N.
B	10 000-30 000	white-blue	white-blue and white	Absorption lines of helium and hydrogen. Weak H and K lines of Ca II.
A	7500-10 000	white	white	Strong Balmer series, lines H and K of Ca II intensify towards class F. Also, closer to class F, lines of metals begin to appear
F	6000-7500	yellow-white	white	The H and K lines of Ca II, the lines of metals, are strong. The hydrogen lines begin to weaken. The Ca I line appears. The G band appears and intensifies, formed by lines Fe, Ca and Ti.
G	5000-6000	yellow	yellow	The H and K lines of Ca II are intense. Ca I line and numerous metal lines. The hydrogen lines continue to weaken, and bands of CH and CN molecules appear.
K	3500-5000	orange	yellowish orange	Metal lines and G band are intense. The hydrogen line is almost invisible. TiO absorption bands appear.
M	2000-3500	red	orange-red	The bands of TiO and other molecules are intense. The G band is weakening. Metal lines are still visible.

Brown dwarfs

Brown dwarfs are a type of star in which nuclear reactions could never compensate for energy losses due to radiation. For a long time brown dwarfs were hypothetical objects. Their existence was predicted in the mid-20th century, based on ideas about the processes occurring during the formation of stars. At the same time, a brown dwarf was discovered for the first time in 2004. To date, quite a lot of stars of this type have been discovered. Their spectral class is M - T. In theory, another class is distinguished - designated Y.

Main sequence stars - concept and types. Classification and features of the category "Main Sequence Stars" 2017, 2018.

Stars are the most interesting astronomical objects, and represent the most fundamental building blocks galaxies. The age, distribution and composition of stars in a galaxy allows us to determine its history, dynamics and evolution. In addition, the stars are responsible for the production and distribution of outer space heavy elements such as carbon, nitrogen, oxygen, and their characteristics are closely related to planetary systems which they form. Therefore, studying the process of birth, life and death of stars takes central place in the astronomical field.

The Birth of Stars

Stars are born in clouds of dust and gas that are scattered throughout most galaxies. A striking example The distribution of such a cloud is the Orion Nebula.

The presented image combines images in visible and infrared range waves received from space telescope Hubble and Spitzer. Turbulence in the depths of these clouds leads to the creation of nodes with sufficient mass to begin the process of heating the material in the center of this node. It is this hot core, better known as a protostar, that could one day become a star.

three-dimensional computer modelling the process of star formation shows that rotating clouds of gas and dust can collapse into two or three parts; this explains why most stars in Milky Way are in pairs or small groups.

Not all the material from the gas and dust cloud ends up in the future star. The remaining material may form planets, asteroids, comets, or simply remain as dust.

Main sequence of stars

A star the size of our Sun takes about 50 million years to mature from formation to adulthood. Our Sun will remain in this phase of maturity for approximately 10 billion years.

Stars feed on the energy released in the process nuclear fusion hydrogen with the formation of helium in its depths. The outflow of energy from their central regions of the star provides the necessary pressure to prevent the star from collapsing under the influence of gravity.

As shown in the Hertzsprung-Russell diagram, the main sequence of stars covers wide range luminosity and color of stars, which can be classified according to these characteristics. The smallest stars are known as red dwarfs, have a mass of about 10% of the mass of the Sun and emit only 0.01% of the energy compared to our star. Their surface temperature does not exceed 3000-4000 K. Despite their miniature size, red dwarfs are by far the most numerous type of stars in the Universe and are tens of billions of years old.

On the other hand, most massive stars, known as hypergiants, can have a mass of 100 times or more, more mass Suns and surface temperatures of more than 30,000 K. Hypergiants release hundreds of thousands of times more energy than the Sun, but have a lifetime of only a few million years. Such extreme stars, scientists believe, were widespread in the early Universe, but today they are extremely rare - only a few hypergiants are known throughout the Milky Way.

Evolution of a star

IN general outline, how more star, the shorter her life expectancy, although everything except supermassive stars live for billions of years. When a star has completely produced hydrogen in its core, the nuclear reactions in its core stop. Deprived of the energy needed to sustain itself, the core begins to collapse into itself and become much hotter. The remaining hydrogen outside the nucleus continues to fuel the nuclear reaction outside the nucleus. The hotter and hotter core begins to push the star's outer layers outward, causing the star to expand and cool, turning it into a red giant.

If the star is massive enough, the process of core collapse can raise its temperature enough to support more exotic nuclear reactions that consume helium and produce various heavy elements, including iron. However, such reactions provide only a temporary reprieve from global catastrophe stars. Gradually, internal nuclear processes stars become increasingly unstable. These changes cause a pulsation inside the star, which will subsequently lead to the shedding of its outer shell, surrounding itself with a cloud of gas and dust. What happens next depends on the size of the kernel.

The further fate of a star depending on the mass of its core

	For medium-sized stars like the Sun, the process of stripping the core from its outer layers continues until all the surrounding material is ejected. The remaining, highly heated core is called a white dwarf. White dwarfs are comparable in size to Earth and have the mass of a full-fledged star. Until recently, they remained a mystery to astronomers - why further destruction of the core does not occur. Quantum mechanics solved this riddle. The pressure of fast-moving electrons saves the star from collapse. The more massive the core, the denser the dwarf is formed. Thus, than smaller size white dwarf, the more massive it is. These paradoxical stars are quite common in the Universe - our Sun will also turn into a white dwarf in a few billion years. Due to the lack internal source energy, white dwarfs cool down over time and disappear into the vast expanses of outer space.
	If a white dwarf formed in a binary or multiple star system, the end of his life may be more eventful known as education nova. When astronomers this event They gave it this name, they really thought that a new star was forming. However, today it is known that in fact we're talking about about very old stars - white dwarfs. If a white dwarf is close enough to a companion star, its gravity can pull hydrogen from the outer layers of its neighbor's atmosphere and create its own surface layer. When enough hydrogen accumulates on the surface of a white dwarf, an explosion occurs nuclear fuel. This causes its brightness to increase and the remaining material to be shed from the surface. Within a few days, the star's brightness decreases and the cycle begins again. Sometimes, especially in massive white dwarfs (whose mass is more than 1.4 solar masses), it can become so overgrown big amount material so that during an explosion they are completely destroyed. This process is known as birth supernova.
	Main sequence stars with masses of about 8 or more solar masses are destined to die as a result powerful explosion. This process is called the birth of a supernova. A supernova is not just a big nova. In a nova, only the surface layers explode, while in a supernova, the core of the star itself collapses. As a result, a colossal amount of energy is released. In a period of several days to several weeks, a supernova can eclipse an entire galaxy with its light. The terms Nova and Supernova do not accurately describe the essence of the process. As we already know, physically, the formation of new stars does not occur. The destruction of existing stars occurs. There are several explanations for this misconception historical cases when they appeared in the sky bright stars, which until that time were practically or completely invisible. This effect and the appearance of a new star influenced the terminology.
	If at the center of a supernova there is a core with a mass of 1.4 to 3 solar masses, the destruction of the core will continue until electrons and protons combine and create neutrons, which subsequently form a neutron star. Neutron stars are incredibly dense space objects- their density is comparable to the density atomic nucleus. Because a large number of mass packed in a small volume, gravity on the surface neutron star just incredible Neutron stars have large magnetic fields which can speed up atomic particles around her magnetic poles producing powerful beams of radiation. If such a beam is oriented towards the Earth, then we can detect regular pulses in the X-ray range from this star. In this case, it is called a pulsar.
	If the core of a star is more than 3 solar masses, then in the process of its collapse a black hole is formed: an infinitely dense object whose gravity is so strong that even light cannot escape it. Since photons are the only tool through which we can study the universe, detecting black holes directly is impossible. Their existence can only be known indirectly. One of the main indirect factors indicating the existence of a black hole in a certain area is its enormous gravity. If there is any material near the black hole - most often companion stars - it will be captured by the black hole and pulled towards it. The attracted matter will move towards the black hole in a spiral, forming a disk around it, which heats up to enormous temperatures, emitting copious amounts of X-rays and gamma rays. It is their detection that indirectly indicates the existence of a black hole next to the star.

Useful articles that will answer most interesting questions about the stars.

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