What are variably pulsating stars called? Close binary eclipsing systems

Any star can be called a variable - its brightness and even color change over time. But these changes occur so slowly that no human life will be enough to detect them. It is not without reason that since ancient times the starry sky has been considered a symbol of immutability and eternity.

But even in the seemingly constant stellar world there are many exceptions. This is a large group of stars whose brightness changes over relatively short periods of time and these changes can be recorded using astronomical instruments.

Variables are “blinking” stars that have changed their brightness at least once. But most variables change their brightness periodically, and this indicates that unusual physical processes are occurring in the vicinity of such a star or in its interior.

Changes in the brightness of stars should not be confused with their flickering, which occurs due to the movement of air masses having different temperatures in the earth's atmosphere. When observed from space, stars do not twinkle, and if fluctuations in their brightness are recorded, we have a variable in front of us.

Star monster

In the constellation Perseus there is a bright second magnitude star, Algol, well known to astronomers. This name is translated from Arabic as “monster”, and in medieval images of Perseus this star played the role of the “eye” of the severed head of Medusa the Gorgon. And it’s not without reason - it was noticed a long time ago that Algol, with a periodicity of about three Earth days, suddenly sharply reduces its brightness by almost one and a half magnitudes - that is, three and a half times!

Only in our days has it been possible to find out exactly the reason for this “wink.” Algol turned out to be an unusually close system of two stars - Algol A and Algol B, the distance between which is 16 times less than the distance from the Earth to the Sun. The less massive Algol B is larger than Algol A, but the subgiant is much fainter in brilliance than its main sequence star partner Algol A. When a brighter star is “eclipsed” by a less bright star for an observer on Earth, the total amount of light coming from the system becomes significantly less.

Such variables - and there were quite a lot of them among double stars - are called optical, or eclipsing variables.

The Mystery of Delta Cepheus

Another thing is stars that are not binary, but periodically change their brightness greatly. Obviously, the point here is not the nature of the star’s movement, but the complex processes occurring in their depths. The first of these stars studied by astronomers was Delta Cephei - it changes its brightness by an entire stellar magnitude in 5 days and 9 hours. Studies of the spectrum of this star have shown that its lines periodically shift either to the red or to the violet region. In the case of a single star, this means that its surface is either rapidly moving away from the observer, or rapidly approaching him - the star pulsates, growing and falling, and at the same time changing the color and temperature of the surface. Moreover, if at a minimum its diameter is equal to forty diameters of our Sun, then at a maximum it increases by four solar diameters at once.

What happens in the depths of Delta Cephei and similar stars?

Astrophysicists have managed to build a theoretical model of stars of this type. In the depths of Delta Cephei there is a layer of matter with special properties, which seems to accumulate energy released in the core of the star. When the amount of energy in it reaches its maximum, the layer instantly releases all the accumulated energy “upward”. From such an “energy shock,” the outer layers of the star either heat up or cool down, compressing or expanding accordingly. At the same time, at its minimum brightness, Delta Cephei belongs to the same spectral class as ours, and at its maximum it turns into a white star with a surface temperature above 10 thousand degrees.

Lighthouses of the Universe

At the beginning of the 20th century, American astronomer Henrietta Leavitt (1868-1921), who discovered about 2,400 variable stars, discovered the relationship between the period of change in the brightness of variable stars and their luminosity: the longer the period, the higher the luminosity. Having measured the period, it was now possible to determine the luminosity, and knowing it, to measure the distance to the star.

So stars like Delta Cephei - they were called Cepheids - became a kind of beacons for astronomers, by which researchers can determine the distances to those star systems in which the variables are located. And since most Cepheids belong to the class of yellow supergiants and emit a lot of energy, they can be seen at great distances and even in other galaxies.

There are also variable stars that change their brightness without any visible patterns - irregular variables, and even those stars that we habitually consider the most ordinary and stable are Cepheids. This, for example, is the North Star - it’s just that changes in its brightness are not as obvious as those of other Cepheids.

In 1922, the eminent American astronomer Edwin Powell Hubble discovered several Cepheids and, using variable stars as a luminosity standard, calculated their distance. Thus, for the first time in the history of astronomy, the existence of space objects outside our star system was proven - the Andromeda Nebula turned out to be a giant spiral galaxy, 2.5 million light years away from the Milky Way.

Variable stars

Although at first glance the stars that sparkle in the sky appear to be constant, it turns out that many of them change in apparent brightness over time. The star becomes brighter and fainter. Such stars are called variable stars. For some variable stars, the brightness changes strictly periodically. For others it changes more or less periodically, for others it changes in a completely chaotic manner. There are stars that flare up unexpectedly. Where a few days ago there was a barely noticeable star in photographs, today a star sparkles, visible to the naked eye. After a few months, the star's brightness drops again. Some stars have repeated flares. There are stars that have very fast flares. In a few minutes, the star becomes hundreds of times brighter, and after an hour it returns to its original state.

The amplitudes of brightness fluctuations of various variable stars range from several hundredths of a stellar magnitude. Stellar magnitude is a characteristic of the visible brightness of stars. The coefficient for determining the magnitudes of luminaries is 2.512. The zero point for the magnitude system was conventionally determined by a group of stars in the region of the North Star, called the northern polar series. Apparent magnitude has nothing to do with the size of the star. This term has historical origins and characterizes only the brilliance of a star. The brightest stars have zero or even negative magnitude. For example, stars such as Vega and Capella have approximately zero magnitude, and the brightest star in our sky, Sirius, has a magnitude of minus 1.5. The magnitude is indicated at the top by the small Latin letter m (from the word “magnitude” - magnitude). For stars not visible to the eye, the same magnitude scale is used. up to 15-17 magnitudes. With the development of technology and the improvement of receivers that record the brightness of stars, it has become possible to discover new variable stars with very small amplitudes and short periods. The total number of variable stars discovered in the Galactica Galaxy. Unlike other galaxies, its name is written with a capital letter. about 40,000, and in other galaxies the Galaxy is a huge rotating star system - more than 5000. To designate variable stars, Latin letters are used indicating the constellation in which the star is located. Within one constellation, variable stars are sequentially assigned one Latin letter, a combination of two letters, or the letter V with a number. For example: S Car, RT Per, V557 Sgr.

Variable stars are divided into three large classes: pulsating, eruptive (explosive) and eclipsing. Pulsating stars have a smooth change in brightness. It is caused by periodic changes in the radius and surface temperature. As stars contract, the temperature increases. An increase in temperature leads to an increase in luminosity. Luminosity is the total energy emitted by a star per unit time, despite the fact that the radius decreases. The periods of pulsating stars vary from fractions of a day (RR Lyrae type stars) to tens (Cepheids) and hundreds of days (Mirids - Mira Ceti type stars). In Cepheids and RR Lyrae stars, periodicity is maintained with amazing accuracy. In variable stars with semi-regular or chaotic changes in brightness, pulsations, although more powerful, occur irregularly. All Cepheids are giants, stars of great luminosity, many of them are supergiants, these include stars with the highest luminosity. Mirids are called long-period variable stars. Changes in their brightness are accompanied by changes in their temperature. Mira Ceti at its greatest is almost as bright as the North Star. Variable stars of this type are also supergiant stars. About 14 thousand pulsating stars have been discovered.

The second class of variable stars is explosive, or, as they are also called, eruptive stars. These include, firstly, supernovae. Supernovae are the brightest stars that appear in the sky as a result of stellar flares. New novae are stars whose brilliance suddenly increases by hundreds, thousands, and sometimes millions. times, repeated novae, U Gemini stars, nova-like and symbiotic stars. All these stars are characterized by single or repeated bursts of an explosive nature with a sudden increase in brightness. Many of these stars are components of close binary systems, and violent processes arise when the components in such systems interact. variable star satellite

Previously, it was thought that new stars were truly re-emerged. But these stars existed before - they show up as faint stars in photographs of the starry sky taken earlier.

Some (and perhaps all) of the new stars flare up repeatedly. So, very hot stars that have a special, unstable state can suddenly flare up and increase in size at a speed equal to hundreds of kilometers per second. During a flash, their outer gas layers are torn off and rush into space at great speed. Over time, these gases dissipate.

On rare occasions, supernova explosions are observed. They differ in that their luminosity during a flare is tens and hundreds of millions of times greater than the luminosity of the Sun. Currently, astronomers and physicists are working hard to solve the question of what physical causes cause such a grandiose phenomenon as supernova explosions.

Secondly, eruptive stars include young fast irregular variable stars, UV Ceti type stars and a number of related objects. The number of open eruptions exceeds 2000.

Pulsating and eruptive stars are called physical variable stars, since changes in their apparent brightness are associated with physical processes occurring on them. This changes the temperature, color, and sometimes the size of the star.

The third class of variable stars includes eclipsing variables. These are binary systems whose orbital plane is parallel to the line of sight. As stars move around a common center of gravity, they alternately eclipse each other, which causes fluctuations in their brightness.

Light curve of the Algol star. Horizontal shows time in hours

Algol satellite motion diagram

In close systems, changes in the total brightness can be caused by distortions in the shape of stars. The periods of change in the brightness of eclipsing binaries range from several hours to tens of years. More than 4,000 such stars are known in the Galaxy.

There is also a small separate class of variable stars - magnetic stars. In addition to a large magnetic field, they have strong inhomogeneities in surface characteristics. Such inhomogeneities during the rotation of the star lead to a change in brightness.

For approximately 20,000 stars the variability class has not been determined.

Variable stars are studied very carefully by astronomers. Observed changes in brightness, spectrum and other quantities make it possible to determine the main characteristics of a star, such as luminosity, radius, temperature, density, mass, as well as to study the structure of atmospheres and the characteristics of various gas flows. From observations of variable stars in various stellar systems, it is possible to determine the age of these systems and the type of their stellar population. The remarkable “period-luminosity” relationship discovered for Cepheids makes it possible to calculate the true brightness of the star, and therefore the distance to it, from the established period. If a Cepheid is discovered in some very distant cluster of stars, then the period of change in its brightness, and hence its luminosity, is measured from observations. And after this it is easy to calculate at what distance this Cepheid is located, if at a given luminosity it appears to us in its brightness as a star of such and such magnitude. The dimensions of the cluster, no matter how large they are, are insignificant compared to the distance to it, which means that all the stars included in it are at approximately the same distances from us. In this way, distances to distant parts of our Galaxy, as well as to other galaxies, were measured. Modern observations have shown that some variable double stars are cosmic sources of X-ray radiation.

I continue the series of articles “astronomical reference book”. And today I will consider another important topic that will be useful to you when reading articles from the section - variable stars. Over time, stars can change their brightness (brilliance); such stars are called variable. Variable stars change their brightness due to physical changes in the state of the star itself, as well as due to eclipses, if we are talking about binary (multiple) systems - these are eclipsing variable stars.

There are the following types of physical variable stars:

• pulsating- characterized by continuous and smooth changes in brightness: Cepheids, Miras, RR Lyrae type, irregular, semi-regular;
• eruptive- characterized by irregular, rapid and strong changes in brightness caused by processes of an explosive (eruptive) nature: new stars, supernovae.

Variable stars have special designations. These stars in each constellation are designated by a sequence of letters of the Latin alphabet: R, S, T, ..., Z, RR, RS, ..., RZ, SS, ST, .... ZZ, AA, …, AZ, QQ, …, QZ with the addition of the name of the corresponding constellation (RR Lyr). In this way we can designate 334 variable stars in each constellation. If the number exceeds 334, then the next ones are designated V 335, V 336, etc.

Eclipsing variable stars

Eclipsing variable stars- close pairs of stars that cannot be separated even in the most powerful telescopes; the apparent magnitude changes due to periodic eclipses of one component of the system by the other for an observer from Earth. The star with greater luminosity is the main one, and the star with less luminosity is the satellite. The most popular examples are: β Perseus (Algol) and β Lyrae.

Due to the overlap of one star by another, the total magnitude changes periodically.

Light curve- a graph that depicts the change in the radiation flux of a star depending on time. When a star is at its maximum brightness, it is maximum era, minimum (or maximum) - minimum epoch. The difference between the maximum and minimum stellar magnitudes is called amplitude, and the time interval between two maximums (minimums) is period of variability.

Graph of changes in the star's radiation flux over time

Based on the graph data, you can determine the relative sizes of the components and get a general idea of ​​their shape. The minimum values ​​(valleys) on the graph may differ in magnitude depending on which of the stars overlapped its component: the main satellite or the main satellite.

Today, about 4,000 eclipsing stars of various types are known. The minimum period of revolution of stars known to astronomers is just under an hour, the maximum is 57 years.

Physical variable stars

Cepheids

Cepheids - pulsating giants F and G, which are named after the star δ (delta) Cephei. The pulsation period ranges from 1.5 to 50 days. The amplitude (the difference between the maximum and minimum) of the Cepheid brightness can reach 1.5 m. A typical representative of Cepheids is the North Star.

When the brightness changes, the temperature of the photosphere, color indices, and radius of the photosphere change. A star's pulsation occurs when the opacity of the star's outer layers blocks some of the radiation from the inner layers. This is due to the substance helium, which first ionizes and then cools and recombines.

Graph of brightness changes η Aql (eta Aquila) and δ Cep (delta Cephei)

In our Milky Way galaxy today there are more than 700 Cepheids.

In turn, Cepheids are divided into 3 more groups:

1. Delta Cepheids (Cδ) are classical Cepheids.
2. W Virgo (CW) Cepheids are not located in the galactic plane. Typically found in . Interestingly, they reach their maximum temperature in the intervals between maximum and minimum luminosity.
3. Zeta Cepheids (Cζ) are low-amplitude Cepheids. They have symmetrical light curves.

RR Lyrae stars

A separate type includes stars of the type RR Lyra. These are giants of spectral class A. The variability period for these stars is 0.2 - 1.2 days. They change brightness very quickly, with the amplitude reaching one magnitude. As the brightness changes, the color index changes, which is associated with a change in the temperature of the photosphere. At maximum, the star brightens (turns white), i.e. It's getting hotter. The radius of the star (radial velocities) also changes.

The vast majority of stars of this type are concentrated in globular star clusters. Below (spectrum-luminosity) shows the approximate location of Cepheids and RR Lyrae stars:

Image taken from Wikipedia

Mirids

The Mirids are called differently long-period variable stars. These are ω (omega) Ceti type stars. The amplitude of the brightness change reaches the 10th (!) magnitude. The period of variability varies greatly and lies in the range of 90 - 730 days.

The Miras include spectral class M (and additional S and N - even colder).

Brightness variability occurs due to temperature fluctuations. Miras include stars in which emission lines appear in their spectra.

Incorrect variables

These are stars that exhibit unpredictable changes in brightness. They are difficult to observe and require more time to determine their characteristics. A representative of this type of star is μ (mu) Cephei.

The amplitude of the brightness change does not exceed one magnitude. The moments of maximums or minimums cannot be determined by formulas, or their frequency can be calculated. The light curve can have a period of up to 4500 days. In an astronomy book I found a graph of the star μ Cephei, the brightness of which was calculated from 1916 to 1928:

If it is possible to determine the average value of the cycle and some periodicity is observed, they are called semi-regular, otherwise - wrong.

Eruptive Variables

A variable dwarf star, which manifests its variability in the form of repeated flares explained by various types of ejections of matter (eruptions), is called eruptive variable. Eruptive stars can be either young or old.

Young stars

Stars that have not completed the process of gravitational compression are called young. For example, T Taurus. Young stars include dwarfs of spectral classes F and G with emission lines in the spectrum. Many young stars can be found in the Orion Nebula (in the constellation Orion), where active star formation is taking place. It is impossible to establish a pattern of changes in such stars. The amplitude of the brightness change can reach 3 m.

The chaotic variability is explained by the fact that small bright nebulae are observed around young stars, which indicates the existence of extensive gaseous envelopes.

Separately allocate UV Ceti type flare stars. These are dwarfs of spectral classes K and M. They are distinguished by a very rapid increase in luminosity during flares. In less than one minute, the radiation flux can increase several times. However, there is a large group of flare stars whose flares last for a long time, exceeding several minutes. In the Pleiades cluster, all the stars belong to such stars.

To date, only about 80 flare stars have been discovered that have low luminosity and can be observed at a short distance from the Sun.

In general, everything you need to know and understand about variable stars. And now, when you come across incomprehensible names or designations of a variable star type, you can always refer to this article to find out what is what.

Thank you for taking your time to read this important topic. If you have questions, don’t hesitate to write in the comments, we’ll figure it out together.

Catalogs of variable stars

Variable star designation system

The modern system of notation for variable stars is a development of the system proposed by F. Argelander in the mid-19th century. Argelander suggested naming those variable stars that have not yet received their designation with letters from R to Z in the order of discovery in each constellation. For example, for example, R Hydrae is the first star in the constellation Hydra (constellation) in terms of discovery time, S Hydrae is the second, etc. Thus, 9 variable designations were reserved for each constellation, i.e. 792 stars. In Argelander's time, such a reserve seemed quite sufficient. However, by 1881 the limit of 9 stars per constellation had been exceeded and E. Hartwig proposed adding two-letter designations to the nomenclature according to the following principle:

For example RR Lyr. However, this system soon exhausted all possible options in a number of constellations. Then astronomers introduced additional two-letter notations:

The letter J has been excluded from two-letter combinations so as not to confuse it with I in handwriting. Only after the two-letter notation system had completely exhausted itself was it decided to use a simple numbering of stars indicating the constellation, starting with number 335, for example V335 Sgr. This system is still used today. Most variable stars were discovered in the constellation Sagittarius. It is noteworthy that the last place in Argelander's classification was occupied in 1989 by the star Z Incisor.

Classification of variable stars

Throughout the history of the study of variable stars, attempts have been made repeatedly to create their adequate classification. The first classifications, based on a small amount of observational material, mainly grouped stars according to similar external morphological features, such as the shape of the light curve, the amplitude and period of brightness changes, etc. Subsequently, along with the increase in the number of known variable stars, the number of groups with similar morphological characteristics, some large ones were divided into a number of smaller ones. At the same time, thanks to the development of theoretical methods, it has become possible to carry out classification not only according to external, observable signs, but also according to physical processes leading to one or another type of variability.

To designate the types of variable stars, the so-called. prototypes are stars whose variability characteristics are accepted as standard for a given type. For example, variable stars like R.R. Lyr.

Guzo system

The following division of variable stars into classes was proposed by Houzeau in the 19th century:

1. Stars whose brightness continuously increases or decreases.
2. Stars with periodic changes in brightness.
3. * Stars like Mira Ceti- stars with long periods and significant changes in brightness.
4. * Stars with fairly rapid and regular changes in brightness. Characteristic representatives of β Lyrae, δ Cephei, η Aquilae.
5. * Algol type stars (β Persei). Stars with a very short period (two to three days) and extremely accurate brightness measurements, which occupy only a small part of the period. The rest of the time the star maintains its greatest brilliance. Other Algol-type stars: λ Tauri, R Canis majoris, Y Cygni, U Cephei, etc.
6. Stars with irregular brightness changes. Representative - η Argus

Classification system adopted in OKPZ-3

In GCVS-3, all variable stars are divided into three large classes: pulsating variables, eruptive variables and eclipsing variables. Classes are divided into types, some types into subtypes.

Pulsating variables include those stars whose variability is caused by processes occurring in their interiors. These processes lead to periodic changes in the brightness of the star, and with it other characteristics of the star - surface temperature, photosphere radius, etc. The class of pulsating variables is divided into the following types:

Light curve of the star δ Cephei

1. Long-period Cepheids(Cep) - high luminosity stars with periods from 1 to ~70 days. Divided into two subtypes:
2. * Classic Cepheids(Cδ) - Cepheids of the flat component of the Galaxy
3. * Virgo W stars(CW) - Cepheids of the spherical component of the Galaxy
4. Slow Invalid Variables(L)
5. Variables of type RR Lyra(RR)
6. RV Taurus Variables(RV)
7. β Cephei or β Canis Majoris type variables(βC)
8. Variables of type δ Shield(δ Sct)
9. China ZZ Type Variables- pulsating white dwarfs
10. Magnetic variables α² Canes Venatici (αCV)

Eruptive variable stars

This class includes stars that change their brightness irregularly or once during observations. All changes in the brightness of eruptive stars are associated with explosive processes occurring on the stars, in their vicinity, or with explosions of the stars themselves. This class of variable stars is divided into two subclasses: irregular variables associated with diffuse nebulae and fast irregular ones, as well as a subclass of novae and nova-like stars.

Irregular variables associated with diffuse nebulae and fast irregular ones

1. UV Type Variables(UV) - stars of spectral class d Me, experiencing short-term flares of significant amplitude.
2. * UVn type stars- a subtype of UV stars associated with diffuse nebulae
3. BY Dragon Variables(BY) - emission stars of late spectral types, showing periodic changes in brightness with variable amplitude and a changing shape of the light curve.
4. Incorrect variables(I). Characterized by indices a, b, n, T, s. The index a indicates that the star belongs to the spectral class O-A, the index b denotes the spectral class F-M, n symbolizes the connection with diffuse nebulae, s - rapid variability, T describes the emission spectrum characteristic of a T Tauri star. Thus, the designation Isa is assigned to a fast irregular variable of early spectral type.

New and nova-like stars

1. * Quick new(Na)
2. * Slow new ones(Nb)
3. * Very slow new ones(Nc)
4. * Repeated new(Nr)
5. Nova-like stars(Nl)
6. Symbiotic Type Z Andromeda Variables(ZAnd)
7. Northern Crown Type R Variables(RCB)
8. Gemini U Type Variables(UG)
9. Giraffe Z Type Variables(ZCam)
10. Variables of type S Dorado(SD)
11. γ Cassiopeia type variables(γC)

Eclipsing variable stars

Eclipsing variable stars include systems of two stars whose total brightness changes periodically over time. The reason for the change in brightness may be the eclipsing of stars by each other, or a change in their shape by mutual gravity in close systems, that is, the variability is associated with changes in geometric factors and not with physical variability.

1. Algol-type eclipsing variables(EA) - light curves allow you to record the beginning and end of eclipses; In the intervals between eclipses, the brightness remains almost constant.

Light curve of the star β Lyrae

1. β Lyrae eclipsing variables(EB) - Double stars with ellipsoidal components that continuously change brightness, including between eclipses. A secondary minimum is definitely observed. Periods are usually longer than 1 day.
2. W-type eclipsing variables of Ursa Major(EW) - contact systems of stars of spectral classes F and later. They have periods of less than 1 day and amplitudes usually less than 0.8 m.
3. Ellipsoidal variables(Ell) - binary systems that do not show eclipses. Their brightness changes due to changes in the area of ​​the emitting surface of the star facing the observer.

Classification system adopted in OKPZ-4

During the time that elapsed between the publication of the third and fourth editions of the OKPZ, not only the quantity of observed material increased, but also its quality. This made it possible to introduce a more detailed classification, introducing into it an idea of ​​the physical processes that cause stellar variability. The new classification contains 8 different classes of variable stars.

1. Eruptive variable stars- these are stars that change their brightness due to violent processes and flares in their chromospheres and coronas. Changes in luminosity usually occur as a consequence of changes in the envelope or mass loss in the form of stellar wind of variable intensity and/or interaction with the interstellar medium.
2. Pulsating Variable Stars are stars that exhibit periodic expansion and contraction of their surface layers. Pulsations can be radial or non-radial. Radial pulsations of a star leave its shape spherical, while non-radial pulsations cause the star's shape to deviate from spherical, and neighboring zones of the star may be in opposite phases.
3. Rotating variable stars- these are stars whose brightness distribution over the surface is non-uniform and/or they have a non-ellipsoidal shape, as a result of which, when the stars rotate, the observer records their variability. Inhomogeneities in surface brightness can be caused by spots or temperature or chemical irregularities caused by magnetic fields whose axes are not aligned with the star's rotation axis.
4. Cataclysmic (explosive and nova-like) variable stars. The variability of these stars is caused by explosions, which are caused by explosive processes in their surface layers (novae) or deep in their depths (supernovae).
5. Eclipsing binaries
6. Optical variable binary systems with hard X-ray emission
7. New Variable Types- types of variability discovered during the publication of the catalog and therefore not included in the already published classes.

The apparent brightness of which varies. These changes can have a period of several years or thousandths of a second, and the magnitude of the changes varies from a thousandth of the average brightness to a 20-fold increase. More than 100,000 variable stars have been cataloged, including the Sun. The energy flux density of our star varies by about 0.1 percent, or part per thousand, during the 11-year solar cycle.

History of Variable Stars

The first variable star identified was Omicron Ceti, later named Mira. In 1596 it was classified as a nova, and in 1638 Johann Hallwards observed changes in the star's brightness during an 11-month cycle. The distance to the star is 200-400 light years. This is a binary system consisting of a red giant variable star. The period of brightness fluctuations is 332 days, and the brightness in the visible range changes hundreds of times during one cycle, while in the infrared part of the spectrum the brightness fluctuates only twice. The second star is also variable, but without an exact period. Its speed fluctuations are caused by the influx of matter from the first star. This was an important discovery because, together with supernovae, it showed that stars were not permanent entities, as had been believed since the days of Ancient Greece.

Properties of Variable Stars

There are many reasons for changes in the apparent brightness of stars. Let us emphasize that it is visible, that is, the star itself should not change at all; the observation conditions usually change - as, for example, in the case of Algol. However, some stars blink due to changes in their properties - pulsating variables have variable radius or mass. Some variable stars are binary systems, in which fellow stars are so close that material is constantly flowing from one to the other and back again. In general, the classification of variable stars is very rich, but they are primarily divided due to variability - internal (in Russian astronomy it is customary to consider eruptive variables separately) or external.

Internal reasons

Cepheids are very bright stars, with a brightness of 500-300,000 solar, and with a very short pulsation period - from 1 to 100 days. These stars expand and contract in a clear pattern. These stars are especially valuable to astronomers, since measuring changes in their brightness makes it possible to very accurately determine their distances, turning Cepheids into the roadblocks of the Universe. Other types of variable stars with internal causes of brightness fluctuations: RR Lyrae, short-period, old stars smaller than Cepheids; RV Taurus, supergiants with huge fluctuations in brightness; Mira type (named after the first variable star), cool red supergiants; irregular, red giants or supergiants with long periods ranging from 30 to 1000 days, Betelgeuse belongs to this type and are mainly red supergiants.

Eruptive variables are also associated with internal processes; they sharply increase their brightness due to thermonuclear explosions inside or on the surface of the star. These include nearby double stars that exchange mass. Supernovae, novae, repeat novae, dwarf novae and others are a group of stars that experience large abrupt changes in brightness, usually due to an explosion. The most famous of them are supernovae, which can outshine an entire galaxy and increase their brightness a hundred million times. Novae and repeated novae are close double stars on the surfaces of which explosions occur, but, unlike supernovae, the stars are not destroyed. Dwarf novae are binary systems of white dwarfs that exchange mass, causing them to periodically explode. They are similar to symbiotic variables, consisting of a red giant and a hot blue star, enclosed in a common shell of dust and gas.

External reasons

Eclipsing variables are stars that pass in front of each other, blocking out some of the light. It can also be caused by the star's planets. Rotating stars have variable brightness due to the presence of dark or, conversely, bright spots on their surface and the rotation of the star. Similar changes are observed in the case of a star whose shape is noticeably different from a sphere (usually in a binary system). In this case, rotation of the ellipsoid leads to changes in the area of ​​the radiating surface. Pulsars also belong to this type.

Future Research

Studies of variable stars provide astronomers with data on the masses, radii, temperatures and other properties of stars. Information about the structure and evolution of the star is indirectly obtained. However, studying long-period variable stars takes a long time—usually decades. Amateur astronomers play a major role in the ongoing observation of variable stars. Some variables are especially important to science, such as Cepheids, which provide information about the age of the Universe. The study of Mira type variables provides information about the Sun and stars similar to it, type Ia supernovae are used to measure the expansion rate of the Universe, eruptive variables - in the study of active galactic nuclei and supermassive