DOUBLE STARS, two stars connected by gravity into a single system; the components of this system revolve around a common center of mass in elliptical orbits. Systems of stars that have several such components are called multiple stars. The orbital periods of known binary stars range from a few minutes to several million years. Most of the stars that have been sufficiently studied show the presence of at least one component gravitationally bound to them, i.e., they are binary or multiple stars. The nearest star to us - Alpha Centauri, as well as the brightest star in the sky - Sirius - are double stars. Closely spaced stars in the sky that are not connected by gravity into a single system are called optical pairs.

The reason for the wide distribution of binary stars is the formation of stars as a result of the collapse of extended rotating interstellar gas and dust clouds. The rotation prevents the accumulation of the entire matter of the original clouds by compact stars and causes the division of these clouds in the process of collapse into two (or more) parts - the future components of binary or multiple stars.

Historically, a single family of binary stars is divided into several groups that differ in the method of detecting binarity. The components of visual binary stars are separated in the field of view of the telescope. Spectral binary stars show a periodic change in the position of the spectral lines of one or both components with time, reflecting their orbital rotation due to the Doppler effect. Eclipsing binaries due to the orbital motion of the components periodically completely or partially eclipse each other if the Sun is close to the plane of their orbit. Of particular importance is the study of the properties of close binary stars, whose components, expanding in the course of their evolution, actively interact with each other and exchange matter. Binary stars also include astrometric binary stars with dark companions, stars with complex (composite) spectra, wide pairs (pairs of stars with a common proper motion).

The discoverer of double stars is considered to be W. Herschel, who made observations of double stars in the 1770-80s in an attempt to measure stellar parallaxes; at the same time, he used the idea of ​​G. Galileo about the possibility of determining the parallax of the brighter component of an optical pair relative to the weaker and therefore, probably, the more distant component. As a result of these observations, Herschel discovered the curvilinear motion of the satellites of several binary stars and estimated the magnitude of the periods of orbital motion for them. In 1803, W. Herschel published lists of several hundred double stars. V. Ya. Struve (see Struve) completed fundamental work to detect and measure the exact positions of binary and multiple stars; the results of his observations were published in three catalogs (1827, 1837, 1852). J. Herschel extended the study of binary stars to Southern Hemisphere sky. The first spectroscopic binary star was discovered in 1889 by the periodic splitting of spectral lines in its spectrum due to the Doppler effect. This method proved to be most effective in the study of close binary stars with orbital periods of less than a few years. By the beginning of the 21st century, the basic parameters of several thousand such stars are known.

The study of binary stars is the most reliable source of information about the masses, radii, structure and evolution of stars. Close binary stars have discovered a wide variety of evolutionary paths for their components, which has made it possible to widely use the duality assumption to explain the properties of many "anomalous" classes of observed stars. Some types of stars and the phenomena of their life turned out to be entirely due to the fact of their close duality. The observation of spectroscopic binary stars has become the main source of information about the structure and evolution of single and binary stars. The active interaction of the components of close binary stars during their evolution leads to the loss of matter from the shells of the components and the exposure of their cores, which makes it possible to study the late stages of the evolution of stars of various masses (white dwarfs, neutron stars and black holes).

Lit .: Masevich A. G., Tutukov A. V. Evolution of stars: theory and observations. M., 1988.

Binary stars (physical binaries)

- two stars united by gravitational forces and circulating in elliptical (in a particular case - circular) orbits around a common center of mass. There are also multiple physical. stars - triple, quadruple, etc., but their number is significantly less than physical ones. D. h. If the components of physical D. h. can be seen directly through a telescope or in photographs (obtained for this purpose using long-focus astrographs), then it is called. visual double star. Tight DZs, the duality of which cannot be detected even with the largest telescopes, may turn out to be spectroscopic binaries or eclipsing binaries (in other words, eclipsing variables, see ). The first show their duality periodic. fluctuations or bifurcation of the spectrum. lines, the second - periodic. changes in the total brightness of stars. In some cases, duality can be established by the methods , or by high-speed registration of the occultations of stars by the Moon (the photometric curves of changes in the brightness of single and binary stars turn out to be different). To D. h. They also include: astrometric stars with dark satellites (among the stars close to the Sun, about 20 astrometric stars were found); stars with complex spectra (combinations of two different spectra); wide pairs are stars with a large common eigenvalue. movement (i.e. with a large angular displacement of the star along celestial sphere expressed in arcseconds per year). In space, the components can be separated by tens of thousands of AU, and the periods of revolution can reach several. million years. Photometric D. h. sometimes called. also binary (multiple) systems, the multiplicity of which is revealed by the methods of multicolor photometry of stars based on its comparison on two-color (multicolor) diagrams (see ).

Relates the number of known binary (and multiple) stars is steadily increasing; it is currently believed that most (perhaps more than 70%) of the stars are combined into systems of greater or lesser multiplicity; from known D. h. about 1/3 turn out to be triple or higher multiplicity stars. Six- and seven-fold stars are known.

Of great interest are D. z., To-rykh includes physical. variable stars (e.g. ), and possibly because in this case, it is possible to estimate the masses of these objects.

When observing a visual binary star, the distance between the components and the position angle of the line of centers are measured, in other words, the angle between the direction to North Pole of the world and the direction of the line connecting the main (brighter) star with its satellite (Fig. 1). Long-term observations can reveal the curvilinearity of the trajectory relative motion satellite and make it possible to estimate the orbital periods.

The number of discovered visual binaries (including wide pairs) exceeds 60,000. Of these, only 10,000 have been measured more or less regularly. More than 500 of them have already been found to have path curvature sufficient to attempt to determine the shape of the relation. orbits. Approximately for 150 D. z. orbits are defined, i.e. along the apparent trajectory of the satellite around main star the elements of the true orbit are calculated, indicating the shape and size of the orbit, its spaces. orientation. These data can be used to predict the position of the satellite in orbit (Fig. 2). Only orbits 80 D. z. can be considered certain enough reliably to use them to try to determine the masses of stars - binary components. The application of Kepler's third law to the motion of D. z. with known distances to them makes it possible (almost the only one) to determine the masses of stars (see).

Changes in offsets or bifurcations of the spectrum. lines of spectral binary stars make it possible to determine , which is the projection of the orbital velocity onto the line of sight (Fig. 3). The radial velocity curves (Fig. 4) - one component or both, if the satellite does not differ too much in brightness from the main star and the lines of both components are visible in the spectrum and can be measured - make it possible to calculate the elements of the true orbit (of the bright component around the common center of mass , or a weaker component around the bright one, placed in focus relative to the orbit, or, finally, each component relative to the center of mass of the system, Fig. 5). Certain periods of spectral binary stars range from 0.1084 days (Ursa Minor) to 59.8 years (visually D. z. Ursa Major). The vast majority of spectroscopic binary stars have periods on the order of several. day In total, more than 3000 spectroscopic binaries have been discovered, and orbital elements have been calculated for approximately 1000 of them.

The light curve of an eclipsing D. shows periodic. brightness decreases - one or two per period and a constant brightness between minima (for stars like Algol) or its continuous change (for stars like Lyra or W Ursa Major, in the latter case the minima are almost the same depth, see). The number of open eclipsing D. z. exceeds 5 thousand

Rice. 4. Influence of the shape and orientation of the orbit on the shape radial velocity curve: 1 - circular orbit; 2 - orbital eccentricity e=- 0.5, periastron longitude; 3 - orbital eccentricity e=0,5, ; a, b, c, d - positions of the satellite star and their corresponding radial velocities.

An analysis of the curves makes it possible to determine not only the elements of the orbit of the eclipsing DZ, but also certain characteristics of the components themselves (shape, dimensions, expressed either in fractions of the major semiaxis of the orbit, or in kilometers, if radial velocity measurements are additionally available). High precision modern photoelectric light measurements in some cases makes it possible to identify and take into account the effect of the so-called light curve on the light curve. subtle effects, eg. darkening to the edge of the disk of the star, as well as to quantitatively express the degree of deviation of the shape of the components from the spherical shape for very close binaries (Lyra and W Ursa Major types). With a noticeable eccentricity of the orbit, it is possible to detect the effect of rotation of the apsidal line (i.e., the line connecting the periastron and apoaster, see), which may be due to the existence of a third, not yet discovered component of the system, or with a noticeable difference in the shape of stars from spherical due to tidal deformations of nearby components. If one of the components of the eclipsed D. z. - a hot star, and the other is a supergiant with an extended atmosphere, then it is possible to study in great detail the structure and composition of the atmosphere of the supergiant from changes in the eclipsing spectrum, when a hot star will shine through the atmosphere of the supergiant during an eclipse. The absorption lines will change as the hot star "sinks" into denser layers of the supergiant's extended atmosphere. Examples of such pairs are Charioteer (a period of 27 years, of which the eclipse lasts about 2 years!) and Auriga (period of 972 days, the eclipse lasts about 40 days).

Mass - one of the most important physical characteristics of stars - can be determined by its effect on the motion of other bodies. Such other bodies are the satellites of some stars (also stars), circulating with them around a common center of mass.

If you look at Ursa Major, the second star from the end of the "handle" of her "ladle", then with normal vision you will see a second faint star very close to it. She was noticed by the ancient Arabs and called Alcor (Horseman). They named the bright star Mizar. They can be called a double star. Mizar and Alcor are separated from each other by . With binoculars, you can find a lot of such stellar pairs. So, Lyra consists of two identical stars of the 4th magnitude with a distance of 5 between them.

Rice. 80. The orbit of the satellite of a double star (v Virgo) relative to the main star, whose distance from us is 10 pc. (The points mark the measured positions of the satellite in the indicated years. Their deviations from the ellipse are due to observational errors.)

Binary stars are called visual binaries if their duality can be seen through direct telescope observations.

In the Lyra telescope - a visual quadruple star. Systems with a number of stars are called multiple.

Many of the visual binaries turn out to be optical binaries, i.e., the proximity of such two stars is the result of their random projection onto the sky. In fact, they are far apart in space. And during long-term observations, one can be convinced that one of them passes by the other without changing direction at a constant speed. But sometimes, when observing stars, it turns out that a weaker companion star revolves around a brighter star. The distances between them and the direction of the line connecting them systematically change. Such stars are called physical binary, they form a single system and circulate under the action of forces of mutual attraction around a common center of mass.

Many double stars were discovered and studied by the famous Russian scientist V. Ya. Struve. The shortest known orbital period for visual binary stars is 5 years. Pairs with circulation periods of tens of years have been studied, and pairs with periods of hundreds of years will be studied in the future. The closest star to us, a Centauri, is a double star. The circulation period of its constituents (components) is 70 years. Both stars in this pair are similar in mass and temperature to the Sun.

The main star is usually not in the focus of the visible ellipse described by the satellite, because we see its orbit in a distorted projection (Fig. 80). But knowledge of geometry makes it possible to restore the true shape of the orbit and measure its semi-major axis a in seconds of arc. If the distance to the binary star is known in parsecs and the semi-major axis of the orbit of the satellite star in seconds of arc, equal to then in astronomical units(because it will be equal to:

The most important characteristic of a star, along with its luminosity, is its mass. Direct definition mass is only possible for binary stars. By analogy with § 9.4, comparing the motion of the satellite

stars with the motion of the Earth around the Sun (for which the period of revolution is 1 year, and the semi-major axis of the orbit is 1 AU), we can write according to Kepler's third law:

where are the masses of the components in a pair of stars, are the masses of the Sun and the Earth, and the orbital period of the pair in years. Neglecting the mass of the Earth in comparison with the mass of the Sun, we get the sum of the masses of the stars that make up the pair in the masses of the Sun:

To determine the mass of each star separately, it is necessary to study the motion of each of them relative to the surrounding stars and calculate their distances from the common center of mass. Then we have the second equation:

To and from the system of two equations we find both masses separately.

Double stars in a telescope are often a beautiful sight: the main star is yellow or orange, and the satellite is white or blue. Imagine a wealth of colors on a planet revolving around one of a pair of stars, where the red Sun shines in the sky, then the blue one, then both together.

The masses of stars determined by the described methods differ much less than their luminosities, approximately from 0.1 to 100 solar masses. Large masses are extremely rare. Usually stars have a mass less than five solar masses. We see that in terms of luminosity and temperature, our Sun is ordinary, middle star, nothing special.

(see scan)

2. Spectral binary stars.

If the stars in mutual circulation come close to each other, then even in the most powerful telescope they cannot be seen separately, in this case the duality can be determined from the spectrum. If the plane of the orbit of such a pair almost coincides with the line of sight, and the rotation velocity is high, then the velocity of each star in the projection onto the line of sight will change rapidly. In this case, the spectra of binary stars are superimposed on each other, and since the difference in the velocities of these

Rice. 81. Explanation of the bifurcation, or fluctuation, of lines in the spectra of spectral binary stars.

stars is large, then the lines in the spectrum of each of them will shift in opposite directions. The shift value changes with a period equal to the period of rotation of the pair. If the brightness and spectra of the stars that make up the pair are similar, then a periodically repeating splitting of spectral lines is observed in the spectrum of the binary star (Fig. 81). Let the components occupy positions, or then one of them moves towards the observer, and the other - away from him (Fig. 81, I, III). In this case, a splitting of the spectral lines is observed. In an approaching star, the spectral lines will shift to the blue end of the spectrum, and in a receding star, to the red. When the components of a binary star occupy positions or (Fig. 81, II, IV), then both of them move at right angles to the line of sight and there will be no bifurcation of the spectral lines.

If one of the stars glows weakly, then only the lines of the other star will be visible, shifting periodically.

One of Mizar's components is itself a spectroscopic binary.

3. Eclipsing binary stars - Algols.

If the line of sight lies almost in the plane of the orbit of a spectral binary, then the stars of such a pair will alternately block each other. During eclipses, the overall brightness of a pair whose components we cannot see individually will weaken (positions B and D in Fig. 82). In the rest of the time, in the intervals between eclipses, it is almost constant (positions A and C) and the longer, the shorter the duration of the eclipses and the greater the radius of the orbit. If the satellite is large, but itself gives little light, then when the bright

the star eclipses it, the total brightness of the system will decrease only slightly.

The brightness minima of eclipsing binary stars occur when their components move across the line of sight. An analysis of the apparent magnitude curve as a function of time makes it possible to determine the size and brightness of the stars, the size of the orbit, its shape and inclination to the line of sight, as well as the masses of the stars. Thus, eclipsing binaries, also observed as spectroscopic binaries, are the most well studied systems. Unfortunately, relatively few such systems are known so far.

Eclipsing binary stars are also called Algols, after the name of their typical representative Perseus. The ancient Arabs called Perseus Algol (spoiled el gul), which means "devil". It is possible that they noticed her strange behavior: for 2 days 11 hours, the brightness of Algol is constant, then in 5 hours it weakens from 2.3 to 3.5 magnitudes, and then in 5 hours its brightness returns to its previous value.

The periods of known spectroscopic binary stars and Algols are mostly short, about a few days. Altogether, stellar binaries are very common Statistics show that up to 30% of all stars are probably binaries Obtaining a variety of data on individual stars and their systems from the analysis of spectroscopic binaries and eclipsing binaries are examples of the unlimited possibility of human cognition

Rice. 82. Changes in the apparent brightness of Lyra and the motion pattern of its satellite (The shape of stars that are close to each other, due to their tidal effect, can differ greatly from spherical)

A large number of stars visible in our galaxy and beyond belong to double and more multiple. That is, we can say with confidence that our single star the Sun belongs to the minority in the classification of stellar systems. Let's talk about what these systems are.

Some sources say that only 30% of total number stars are single, in others you can find the number 25. But with the improvement of methods for measuring and studying double and multiple stars, the percentage of single stars changes. This is primarily due to the difficulty of detecting small (in size, but not mass) stars. To date, astronomers have discovered many that, when first discovered, may fit the description of secondary stars in a system of two or more stars, only after a detailed study and many calculations, the option is excluded that this is a star, and the found object is classified as a planet (this is determined by mass, by gravitational attraction, by relative position, behavior, and many other factors).

double stars

Kappa Bootes

A system of two stars bound by gravity is called double star system or simply double star.

First of all, it should be emphasized that not all optically adjacent two stars are binary. It follows that the stars that are visible in the sky are close to each other for an observer from the Earth, but are not connected gravitational forces and not having a common center of mass are called optical double. A good example is α Capricornus - a pair of stars are at a great distance from each other (about 580 light years), but it seems to us that they are nearby.

Physical binary stars revolve around a common center of mass and are interconnected by gravitational forces. An example is η () of Cassiopeia. From the period of rotation and the mutual distance, one can determine the mass of each of the stars. The rotation period has an impressive range: from several minutes, when it comes to the rotation of dwarf stars around neutron stars, to several million years. The distances between the stars can approximately be from 10 10 to 10 16 m (about 1 light year).

double stars have a very broad classification. Here are just the main points:

• Astrometric(you can see the movement of two objects at once);
• Spectral(duality is determined by spectral lines);
• eclipsing binaries(due to the different angle of inclination to the orbit, a dimming of one star by another is periodically observed);
• Microlensed(when between the system and the observer there is a space object with a strong gravitational field. Using this method, low-mass brown dwarfs are found);
• Speckle interferometric(according to the diffraction limit of the resolution of stars, binary stars are found);
• X-ray.

Multiple stars

As the name implies, if the number of interconnected stars exceeds two, then this multiple star systems or . They are also divided into optically and physically multiple stars. If the number of stars in the system can be seen with the naked eye, through binoculars or a telescope, then such stars are called visually multiple. If additional spectral measurements are required to determine the multiplicity of the system, then this spectral multiple system. And, if the multiplicity of the system is determined by the change in brightness, then this eclipsing multiple system. A simple example of a triple star is shown below - this is a star HD 188753 in the constellation Cygnus:

Triple star HD 188753

As you can see in the image above, in the triple system there are a pair of closely related stars and one distant one with a larger mass, around which the pair rotates. But more often, a distant star revolves around a pair of closely related stars that are a single whole. Such a pair is called main.

Of course, the multiplicity is not limited to three stars. There are systems of four, five and six stars. The higher the multiplicity, the fewer such systems. For example, the star ε Lyra is two pairs of interconnected, remote from each other at a great distance. Scientists have roughly calculated that the distance between pairs should be 5 or more times greater than the distance between stars within one pair.

The best example of a sixfold system of stars is Castor in the constellation. In it, three pairs of stars interact in an organized manner with each other. More than 6 stars in the system have not yet been discovered.

Multiple stars occupy astronomers-observers no less than deep sky objects. Star systems look especially beautiful when the components in them have a different color shade, for example, one of them is cold red, and the other is hot bright blue Star. There are many reference books with detailed characteristics of the most famous and interesting binary and multiple stars for observation. I will introduce you to some of the systems in a separate article.

Sometimes two or more closely spaced stars can be seen in the night sky. Those that are actually far apart and do not have any physical connection with each other are called optical double stars. Visually, they seem close, because they are projected into very close points on the celestial sphere. Unlike them, physical double called stars that form a single dynamic system and revolve around a common center of mass under the action of forces of mutual attraction. Sometimes you can observe associations of three or even more stars (the so-called triple and multiple systems). If both components of a binary star are sufficiently distant from each other so that they are visible separately, then such binaries are called visually double. The duality of pairs whose components are not visible separately can be detected either photometrically (for example, eclipsing variable stars), or spectroscopically (for example, spectral doubles).

In nature, double stars are quite common. To determine whether there is a physical connection between a pair of stars, and whether this pair is not an optical binary, astronomers make long-term observations, with the help of which they determine the orbital motion relative to the other. The physical duality of such stars can be detected with a high probability from their own motions, because the stars that form a physical pair have almost the same proper motion. In some cases, only one of the stars is visible, making mutual orbital motion, while its path in the sky looks like a wavy line.

photo: Visually double star Sirius (Sirius A and Sirius B)

At present, several tens of thousands of visually close binary stars have been discovered. Only a tenth of them confidently detect relative orbital motions, and only for 1% (about 500 stars) it is possible to calculate orbits. The motion of stars in a pair occurs in accordance with Kepler's laws: around a common center of mass, both components describe in space similar (i.e. with the same eccentricity) elliptical orbits. The orbit of the satellite star relative to the main star has the same eccentricity, if the latter is considered stationary. If the orbit of relative motion is known from observations, then the sum of the masses of the components of the binary star can be determined. If the ratios of the semi-axes of the orbits of the motion of stars relative to the center of mass are known, then the ratio of masses and, consequently, the mass of each star separately can also be found. This is great value study of binary stars in astronomy, which allows to determine an important characteristic of a star - the mass, the knowledge of which is necessary for research internal structure stars and their atmospheres. Sometimes on the basis of complex own movement a single star relative to the background stars, one can judge that it has a companion that cannot be seen either because of its proximity to the main star, or because of its much lower luminosity (dark companion). It was in this way that the first white dwarfs were discovered - satellites of Sirius and Procyon, subsequently discovered visually.

eclipsing variables are called such close pairs of stars that are inseparable during observation, in which the visible stellar star changes due to periodically occurring eclipses for the observer of one component of the system by another. In such a pair, a star with a higher luminosity is called the main one, and a star with a smaller one is called a companion. Bright representatives of this type of stars are the stars Algol (β Perseus) and β Lyrae. Due to regularly occurring eclipses of the main star by the companion, as well as the satellite by the main star, the total apparent magnitude of the eclipsing stars periodically changes. A graph depicting the change in the radiative flux of a star over time is called a light curve. The point in time at which the star has the smallest apparent stellar magnitude is called the epoch of maximum, and the largest - the epoch of minimum. The amplitude is the difference between the magnitudes at minimum and maximum, and the period of variability is the time interval between two successive maxima or minima. For Algol, for example, the period of variability is slightly less than 3 days, while for β Lyra it is more than 12 days. By the nature of the light curve of an eclipsing variable star, one can find the elements of the orbit of one star relative to another, the relative sizes of the components, and sometimes even get an idea of ​​their shape. Currently, more than 4000 eclipsing variable stars of various types are known. The minimum known period is less than an hour, the largest is 57 years.

photo: Eclipsing variable star Algol (β Perseus)

In the spectra of some stars, one can see a periodic bifurcation or fluctuation in the position of the lines of the spectrum. If such stars are eclipsing variables, then the oscillations of the spectral lines occur with the same period as the brightness change. In addition, at the moments of conjunctions, when the motion of both stars is perpendicular to the line of sight, the deviation of the spectral lines from the mean position is equal to zero. For the rest of the time, there is a bifurcation of the spectral lines common to both stars, reaching its maximum value at the highest radial velocity of the components, one in the direction towards the observer, and the other away from it. If the observed spectrum belongs to only one of the two stars (and the spectrum of the second is not visible due to its weakness), then instead of bifurcation of the lines, their shift is observed either to the red or to the blue part of the spectrum. The time dependence of the radial velocity determined from the line shifts is called the radial velocity curve. Stars whose duality can only be established on the basis of spectral observations are called spectral double. Unlike eclipsing variable stars, whose orbital planes form a rather small angle with the line of sight, spectroscopic binary stars can also be observed when this angle is much larger. And only if the plane of the orbit is close to the plane of the sky, the motion of the stars does not cause a noticeable shift of the lines, and then the duality of the star cannot be detected. If the plane of the orbit passes through the line of sight, then the largest shift of the spectral lines makes it possible to determine the value of the total velocity V of the motion of stars relative to the center of mass of the system at two diametrically opposite points of the orbit.

In cases where the radial velocity curve for an eclipsing variable star is known, it is possible to determine the most complete and reliable elements of the orbit, as well as such characteristics as the size and shape of the stars, and even their masses. All linear quantities are determined in kilometers. Approximately 2500 stars have been discovered so far, the binary nature of which has been established only on the basis of spectral observations. For about 750 of them, it was possible to obtain radial velocity curves that make it possible to find the periods of revolution and the shape of the orbit. The study of spectroscopic binary stars is especially important, since it allows one to get an idea of ​​the masses remote objects high luminosity and, consequently, sufficiently massive stars.

rice. β Lyrae close spectroscopic binary

close binary systems are such star couples, the distance between which can be compared with their sizes. In this case, tidal interactions between the components of the system begin to play a significant role. The surfaces of both stars under the action of tidal forces cease to be spherical, the stars acquire an ellipsoidal shape and they have tidal humps directed towards each other, like lunar tides in the Earth's ocean. The shape taken by a body consisting of gas is determined by the surface passing through points with the same values ​​of the gravitational potential. Such stellar surfaces are called equipotential. If the outer layers of stars go beyond the inner Roche lobe, then, spreading along the equipotential surfaces, the gas can, firstly, flow from one star to another, and, secondly, form a shell that encloses both stars. A classic example of such a system is the star β Lyrae, whose spectral observations make it possible to detect both the common envelope of a close binary and the gas flow from the companion to the main star.