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 across the 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 visual double star they measure the distance between the components and the position angle of the line of centers, in other words, the angle between the direction to the north pole of the world and the direction of the line connecting the main (brighter) star with its companion (Fig. 1). Long-term observations can reveal the curvilinearity of the trajectory of the relative motion of the 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. according to the apparent trajectory of the satellite around the 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) up to 59.8 years (visually D. h. 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
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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).
Binary stars are fairly common objects in the observable universe. But, despite this, they are of genuine interest to astronomers around the world.
Scientists say that double stars make up about half of all the stars in our galaxy. A binary star is a system consisting of two objects (stars) connected by gravitational forces. Both stars in the system revolve around their common center of mass. The distances between the stars can vary, as well as the mass of these stars, as well as their sizes. Both stars included in the gravitational system can have both similar and distinctive characteristics. For example, star A may have a larger mass or size than star B.
Double stars are traditionally marked with Latin letters. Usually, the letter "A" marks a brighter and more massive companion. The letter "B" is a less bright and massive star.
A striking example of a double star system is the star system closest to us - A and B. It is complete system from two stars. Alpha Centauri itself consists of three components. If you look at this star without resorting to various optical instruments, with the naked eye it will be visually perceived as one star. If we look at it through a telescope, we will clearly see two or even three components of this system. Other examples of double stars include the Beta Lyrae system, the Beta Perseus (Algol) system, and other stars.
Classification
Astronomers have long discovered that binary stars can differ in the type of their origin, physical parameters and other characteristics. For this reason, scientists have proposed to classify these objects celestial sphere. Conventionally, binary stars are divided into two types: stars between which there is no mass exchange, and stars between which it occurs, has occurred or will occur in the future. The latter, in turn, are divided into contact and semi-separated. In contact systems, both stars fill their Roche lobes. In semi-divided - only one star.
In addition to the above classification, binary stars can be divided according to the way they are observed. So, there are astrometric, obscured, spectral and visual double stars.
Astrometric binary stars are detected in the sky by observing changes and non-linearity in the movement of the visible object of the system. In this way, astronomers often discover brown dwarfs that cannot be detected in other ways. Darkened binary stars can be detected by fixing the change in brightness in a pair of stars. During rotation, the companion stars, as it were, outshine each other, and due to this they give themselves out as a double star. The method for detecting a double star is to measure over several nights. The shift of the lines of the star's spectrum over time, the large difference between the minimum and maximum speeds of the star, the change in radial velocities - all this may indicate that the observed by us heavenly body- double star. The visual method for detecting double stars is the simplest. With a powerful telescope, we can detect binary stars that are convenient for visual observation and are at a relatively close distance from us.
Phenomena and phenomena associated with double stars
An interesting phenomenon closely related to double stars, is the Algol paradox. Algol is a double star located in the constellation Perseus. According to general theory evolution heavenly bodies, the greater the mass of the star, the faster it goes through all stages of evolution. But the Algol Paradox lies in the fact that Algol B, a component of a binary star that has a smaller mass, is evolutionarily older than the more massive component of this system, Algol A. Scientists believe that this paradox is directly related to the effect of mass flow in close binary systems, due to which the smaller star could have evolved faster than the more massive component of the system.
Another interesting astronomical phenomenon inherent in binary stars is closely related to the Algol Paradox - this is the exchange of masses between them. The components of binary stars are able to exchange their masses and particles with each other. Each of the components has a Roche lobe, a region in which the gravitational forces of one companion predominate over the gravitational forces of the other. The point where the Roche lobes of the two stars meet is called the Lagrange point. Through this point, the substance of one companion can flow to another.
An interesting phenomenon associated with binary stars can also be considered symbiotic systems of binary stars. These systems usually consist of a red giant and white dwarf that revolve around a common center of mass. The lifetime of such systems is relatively short. However, they are characterized by nova-like flares that can increase the brightness of a star by 2-3 times. In addition, symbiotic binary stars have other interesting astrophysical characteristics that are captivating the minds of astronomers around the globe.
Origin and evolution of binary stars
The origin and evolution of binary stars occurs, in principle, according to the same scenario as in ordinary stars. However, there are some nuances that distinguish the origin and evolution of binary systems from the origin and evolution of single luminaries.
The evolution of a close binary system as seen by an artist
Like single stars, binary systems are formed under the influence of gravitational forces from a gas and dust cloud. There are three most popular theories of binary star formation in modern astronomy. The first of them connects the formation of binary systems with the separation at an early stage of the common core of the protocloud, which served as material for the emergence of the binary system. The second theory is related to the fragmentation of the protostellar disk, as a result of which not only binary, but also multiple systems of stars may appear. Fragmentation of the protostellar disk occurs at a later stage than the fragmentation of the core. The latest theory says that the formation of binary stars is possible through dynamic physical and chemical processes inside the protocloud, which serves as material for the formation of stars.
Exoplanets around binary stars
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 (since 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 from the point of view of luminosity and temperature, our Sun is an ordinary, average star, nothing special stands out.
(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 binarity is a very common phenomenon Statistics show that up to 30% of all stars are probably binary 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 pattern of motion 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 the total number of 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 stars that are visible in the sky close to each other for an observer from the Earth, but at the same time not connected by 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).
Binary 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 there is a space object with a strong gravitational field between the system and the observer. Low-mass brown dwarfs are found using this method);
- 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, the triple system has 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 a 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.
> Double stars
– observation features: what is it with photos and videos, detection, classification, multiples and variables, how and where to look in Ursa Major.
Stars in the sky often form clusters, which can be dense or, on the contrary, scattered. But sometimes between the stars there are stronger bonds. And then it is customary to talk about binary systems or double stars. They are also called multiples. In such systems, the stars directly influence each other and always evolve together. Examples of such stars (even with the presence of variables) can be found literally in the most famous constellations, for example, Ursa Major.
Discovery of double stars
The discovery of binary stars was one of the first achievements made with astronomical binoculars. The first system of this type was the Mizar pair in the constellation Ursa Major, which was discovered by the Italian astronomer Ricciolli. Since there are an incredible number of stars in the universe, scientists decided that Mizar could not be the only binary system. And their assumption turned out to be fully justified by future observations.
In 1804, William Herschel, the famous astronomer who had made scientific observations for 24 years, published a catalog detailing 700 double stars. But even then there was no information about whether there is a physical connection between the stars in such a system.
A small component "sucks" gas from a large star
Some scientists have taken the view that binary stars depend on a common stellar association. Their argument was the inhomogeneous brilliance of the components of the pair. Therefore, it seemed that they were separated by a significant distance. To confirm or refute this hypothesis, it was necessary to measure the parallactic displacement of stars. Herschel undertook this mission and to his surprise found out the following: the trajectory of each star has a complex ellipsoidal shape, and not the form of symmetrical oscillations with a period of six months. The video shows the evolution of binary stars.
This video shows the evolution of a close binary pair of stars:
You can change subtitles by clicking on the "cc" button.
According to physical laws celestial mechanics, two bodies bound by gravity move in an elliptical orbit. The results of Herschel's research became proof of the assumption that in binary systems there is a connection between the gravitational force.
Classification of double stars
Binary stars are usually grouped into the following types: spectroscopic binaries, photometric binaries, and visual binaries. This classification allows you to get an idea of the stellar classification, but does not reflect the internal structure.
With a telescope, you can easily determine the duality of visual double stars. Today, there are data on 70,000 visual double stars. At the same time, only 1% of them definitely have their own orbit. One orbital period can last from several decades to several centuries. In turn, the alignment of the orbital path requires considerable effort, patience, the most accurate calculations and long-term observations in the conditions of the observatory.
Often, the scientific community has information only about some fragments of the orbit, and they reconstruct the missing sections of the path using the deductive method. Do not forget that the plane of the orbit may be tilted relative to the line of sight. In this case, the apparent orbit is seriously different from the real one. Of course, with a high accuracy of calculations, one can also calculate the true orbit of binary systems. For this, Kepler's first and second laws apply.
Mizar and Alcor. Mizar is a double star. On the right is the Alcor satellite. There is only one light year between them.
Once the true orbit is determined, scientists can calculate the angular distance between the binary stars, their mass and their rotation period. Often, Kepler's third law is used for this, which also helps to find the sum of the masses of the components of a pair. But for this you need to know the distance between the Earth and the double star.
Double photometric stars
The dual nature of such stars can only be known from periodic fluctuations in their brightness. During their movement, stars of this type obscure each other in turn, which is why they are often called eclipsing binaries. The orbital planes of these stars are close to the direction of the line of sight. The smaller the eclipse area, the lower the brightness of the star. By studying the light curve, the researcher can calculate the angle of inclination of the orbital plane. When fixing two eclipses, the light curve will have two minima (decreases). The period when 3 successive minima are observed on the light curve is called the orbital period.
The period of binary stars lasts from a couple of hours to several days, which makes it shorter in relation to the period of visual double stars (optical double stars).
Spectral binary stars
Through the method of spectroscopy, researchers fix the process of splitting of spectral lines, which occurs as a result of the Doppler effect. If one component is a faint star, then only periodic fluctuations in the positions of single lines can be observed in the sky. This method is used only when the components of the binary system are at a minimum distance and their identification with a telescope is complicated.
Binary stars that can be examined through the Doppler effect and a spectroscope are called spectroscopic binary. However, not every binary star has a spectral character. Both components of the system can approach and move away from each other in the radial direction.
According to the results of astronomical studies, most of the binary stars are located in the galaxy Milky Way. The ratio of single and double stars as a percentage is extremely difficult to calculate. Using subtraction, you can subtract the number of known binary stars from the total stellar population. In this case, it becomes obvious that double stars are in the minority. However, this method cannot be called very accurate. Astronomers are familiar with the term "selection effect". To fix the duality of stars, one should determine their main characteristics. This will require special equipment. In some cases, fixing double stars is extremely difficult. So, visually binary stars are often not visualized at a considerable distance from the astronomer. Sometimes it is impossible to determine the angular distance between the stars in a pair. To fix spectral-binary or photometric stars, it is necessary to carefully measure the wavelengths in the spectral lines and collect the modulations of the light fluxes. In this case, the brightness of the stars should be strong enough.
All this dramatically reduces the number of stars suitable for study.
According to theoretical developments, the proportion of binary stars in the stellar population varies from 30% to 70%.