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.
The image shows a red variable star called V838 Monocerotis.
A variable star is a star whose brightness changes over time as a result of physical processes occurring in its region. Strictly speaking, the brightness of any star changes over time to one degree or another. For example, the amount of energy released varies by 0.1% over the course of an eleven-year solar cycle, which corresponds to a change in absolute magnitude of one thousandth. A variable is a star whose brightness changes have been reliably detected at the current level of observational technology. To classify a star as variable, it is enough that the star’s brightness undergoes a change at least once.
Variable stars are very different from each other. Changes in gloss may be periodic. The main observational characteristics are the period, the amplitude of the brightness changes, the shape of the light curve and the radial velocity curve.
The reasons for changes in the brightness of stars can be: radial and non-radial pulsations, chromospheric activity, periodic eclipses of stars in a close binary system, processes associated with the flow of matter from one star to another in a binary system, catastrophic processes such as a supernova explosion, etc.
The variability of stars should not be confused with their flickering, which occurs due to fluctuations in the air of the earth's atmosphere. When observed from space, stars do not twinkle.
Top 10 constellations by number of variable stars according to the OKPZ-4 catalog
The first variable star was identified in 1638, when Johann Holvarda noticed that the star Omicron Ceti, later named Mira, pulsates with a period of 11 months. Previously, the star had been described as a nova by the astronomer David Fabricius in 1596. This discovery, combined with observations of supernovae in 1572 and 1604, proved that the starry sky was not something eternally unchanged, as Aristotle and others had taught philosophers of antiquity. The discovery of variable stars thus contributed to the revolution in astronomical views that occurred in the sixteenth and early seventeenth centuries.
The second variable star, which was described in 1669 by Geminiano Montanari, was the eclipsing variable Algol. The correct explanation of the reasons for its variability was given in 1784 by John Goodrike. In 1686, the astronomer Gottfried Kirchi discovered the star Chi Cygni, and in 1704, thanks to Giovanni Maraldi, R Hydrae became known. By 1786, 10 variable stars were already known. John Goodrike added Delta Cephei (δ Cephei) and Sheliak (β Lyr) to their number with his observations. Since 1850, the number of known variable stars has increased dramatically, especially since 1890, when photography became possible to detect them.
The latest edition of the General Catalog of Variable Stars (2008) lists more than 46,000 variable stars from our own, as well as 10,000 from other galaxies and another 10,000 possible variables.
The first catalog of variable stars was compiled by the English astronomer Edward Pigott in 1786. This catalog included 12 objects: two supernovae, one nova, 4 stars of the ο Cet type (Mirids), two Cepheids (δ Cep, η Aql), two eclipsing ones (β Per, β Lyr) and P Cyg. In the XIX - early XX centuries. German astronomers took the leading role in the study of variable stars. After the Second World War, by decision of the International Astronomical Union (IAU) in 1946, the work on creating catalogs of variables was entrusted to Soviet astronomers - the State Astronomical Institute. P.K. Sternberg (SAI) and the Astronomical Council of the USSR Academy of Sciences (now INASAN). Approximately once every 15 years, these organizations publish the General Catalog of Variable Stars (GCVS). The last 4th edition was published from 1985 to 1995. In the intervals between successive editions of the OKPZ, additions to it are published. In parallel with the creation of the GCVS, work is underway to create catalogs of stars suspected of brightness variability (CSV, English NSV).
The fourth edition of the GCP remains the last “paper” edition. In the 21st century, like many other astronomical catalogs, the GCVS is maintained in electronic form and is available in the VisieR system under the name General Catalog of Variable Stars. It consists of 3 parts: a catalog of variable stars, a catalog of stars suspected of variability, and a catalog of extragalactic variables.
The modern system of notation for variable stars is a development of the system proposed by Friedrich Argelander in the mid-19th century. Argelander in 1850 proposed naming those variable stars that had not yet received their designation with letters from R to Z in the order of discovery in each constellation. For example, R Hydrae is the first variable star in the Hydra constellation in terms of discovery time, S Hydrae is the second, etc. Thus, 9 variable designations were reserved for each constellation, that is, 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:
RR RS RT RU RV RW RX RY RZ
SS ST SU SV SW SX SY SZ
TT TU TV TW TX TY TZ
UU UV UW UX UY UZ
For example RR Lyr. However, this system soon exhausted all possible options in a number of constellations. Then astronomers introduced additional two-letter notations:
AA AB AC … AI AK … AZ BB BC … BI BK … BZ … II IK … IZ KK … KZ … QQ … QZ
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.
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, amplitude and period of brightness changes, etc. Subsequently, along with an increase in the number of known variable stars, the number of groups with similar morphological characteristics also increased. signs, 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 of the RR Lyr type.
The following division of variable stars into classes was proposed by Gouzeau (French: Jean-Charles Houzeau de Lehaie) in the 19th century:
Stars whose brightness continuously increases or decreases.
Stars with periodic changes in brightness.
Stars like Mira Ceti are stars with long periods and significant changes in brightness.
Stars with fairly rapid and regular changes in brightness. Characteristic representatives of β Lyrae, δ Cephei, η Aquilae.
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.
Stars with irregular brightness changes. Representative - η Argus
New stars.
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:
Long-period Cepheids (Cep) are high-luminosity stars with periods from 1 to ~70 days. Divided into two subtypes:
Classical Cepheids (Cδ) - Cepheids of the flat component of the Galaxy
W Virgo (CW) stars are Cepheids of the spherical component of the Galaxy.
Slow incorrect variables (L)
Mira Ceti (M) type stars
Semi-regular variables (SR)
Variables of type RR Lira (RR)
RV Tauri Variables (RV)
β Cephei or β Canis Majoris (βC) variables
Variables of type δ Shield (δ Sct)
Variable ZZ Ceti - pulsating white dwarfs
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.
UV Ceti (UV) type variables are stars of spectral type d Me that experience short-term flares of significant amplitude.
UVn type stars - a subtype of UV stars associated with diffuse nebulae
BY Draconis (BY) variables are emission stars of late spectral types that exhibit periodic changes in brightness with variable amplitude and changing light curve shape.
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 stars (N)
Fast new (Na)
Slow new (Nb)
Very slow new (Nc)
Repeated new (Nr)
Nova-like stars (Nl)
Symbiotic Type Z Andromeda Variables (ZAnd)
Northern Crown R type variables (RCB)
U Gemini (UG) type variables
Giraffe Z Type Variables (ZCam)
Supernovae (SN)
Doradus S Type Variables (SD)
γ Cassiopeia (γC) type variables
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.
Algol-type eclipse variables (EA) - light curves allow you to record the beginning and end of eclipses; In the intervals between eclipses, the brightness remains almost constant.
Eclipsing variables of type β Lyrae (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.
Eclipsing variables of type W Ursa Major (EW) are contact systems of stars of spectral classes F and later. They have periods of less than 1 day and amplitudes usually less than 0.8m.
Ellipsoidal variables (Ell) are 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.
During the time that elapsed between the publication of the third and fourth editions of the OKPZ, not only the quantity of observational 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.
Eruptive variable stars are stars that change their brightness due to violent processes and flares in their chromospheres and coronas. The change in luminosity usually occurs due to changes in the envelope or mass loss in the form of variable-intensity stellar wind and/or interaction with the interstellar medium. 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. Rotating variable stars 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 inhomogeneities caused by magnetic fields whose axes do not coincide with the star's rotation axis.
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 interiors (supernovae).
Eclipsing binaries
Optical variable binary systems with hard X-ray emission
Variables with other symbols
New types of variables are types of variability that were discovered during the publication of the catalog and therefore did not fall into the already published classes.
Classes 1 and 5 overlap - stars with RS and WR variability types belong to both of these classes.
Number of variable stars by type according to the OKPZ-4 catalog
As you know, our Sun also does not shine completely evenly, but slightly changes its activity. Every 11 years, the number of sunspots on the Sun increases and its activity increases. Of course, the pulsations of the Sun cannot be compared with the pulsations of Cepheids, much less novae and supernovae. Therefore, our Sun is a permanent star.
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 brightness 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.
Pulsating stars expand and contract, becoming larger and smaller, hotter and colder, brighter and dimmer. The physical properties of these stars are such that they simply move from one state to another and back again, as if they were oscillating or pulsating, just like hearts beating in the sky.
Cepheid variable stars
American astronomer Henrietta Leavitt discovered that Cepheids have a period-luminosity relation. This term means that the longer the brightness period (the interval between successive brightness peaks), the higher the average true brightness of the star. Therefore, if you measure the apparent magnitude of a Cepheid variable star as it changes over days and weeks, and then determine the period of brightness change, you can easily calculate the true brightness of the star.
Why is this necessary? And then, knowing the true brightness of a star, you can determine the distance to it. After all, the further away a star is, the dimmer it looks, but it is still the same star with the same true brilliance.
Distant dim stars obey the inverse square law. This means that if a star is 2 times further away, it will appear 4 times fainter. And if a star is 3 times further away, then it looks 9 times dimmer. If the star is 10 times further away, then it appears 100 times fainter.
Recently there were reports in the media that with the help of the Hubble Space Telescope it was possible to determine the scale and age of the Universe. In fact, this is the result of a study using the Hubble telescope of Cepheid variable stars. These Cepheids are located in distant galaxies. But by observing the change in their brightness and using the relationship between the period of brightness change and luminosity, astronomers determined the distance to these galaxies.
RR Lyrae stars
RR Lyrae stars are similar to Cepheids, but they are not as large or bright. Some of them are located in a globular star cluster in our Milky Way galaxy, and they also have a relationship between the period of brightness change and luminosity.
Globular clusters are huge spherical formations filled with old stars born during the formation of the Milky Way. These are areas of space only 60-100 light years wide, in which from several hundred thousand to a million stars are “packed”. By observing changes in the brightness of RR Lyrae stars, astronomers can estimate the distance to such stars. And if these stars are in globular clusters, then the distance to these globular clusters can be determined.
Why is it so important to know the distance to a star cluster? Here's why. All stars located in one cluster were formed simultaneously from a common cloud. And they are all located at approximately the same distance from the Earth, since they are in the same cluster. Therefore, when scientists construct an H-R diagram for stars in a cluster, there will be no errors caused by differences in distances to different stars. And if we know the distance to the star cluster, then all the magnitudes plotted on the diagram can be converted into luminosity, that is, into the intensity of energy emitted by the star per second. And these values can be directly compared with theoretical data. This is exactly what astrophysicists do.
Long-period variable stars
While astrophysicists process information from Cepheid and RR Lyrae variable stars, amateur astronomers enjoy observing long-period variable stars called Mira Ceti variables. Mira is another name for the star Omicron Ki
Variable stars such as Mira Ceti pulsate like Cepheids, but they have much longer periods of brightness variation, on average 10 months or more, and, in addition, they have a larger amplitude of brightness variation. When the brightness of Mira Ceti reaches its maximum value, it can be seen with the naked eye, and when the brightness is minimal, a telescope is needed. The brightness changes of long-period stars also occur much more irregularly than for Cepheids. The maximum magnitude that a star reaches can vary greatly from one period to the next. Observations of such stars, which are not at all difficult to carry out, allow scientists to obtain important scientific information. And you, too, can contribute to the study of variable stars (I will talk about this in more detail in the last section of this chapter).
Variable stars are stars whose brightness varies. Some variable stars change their brightness periodically, while others experience a random change in brightness. Periodic variable stars include, for example, eclipsing variable stars, which, as you know, are binary systems. However, in contrast to them, tens of thousands of single stars are known, the brightness of which changes due to the physical processes occurring on them. Such stars are called physical variables. Their discovery and research showed that the diversity of stars is manifested not only in the fact that stars differ from each other in mass, size, temperature, luminosity and spectra, but also in the fact that some of these physical characteristics do not remain unchanged for the same stars
Cepheids
Cepheids are a very common and very important type of physical variable star.
A study of the spectra of Cepheids shows that near the maximum brightness, the photospheres of these stars approach us with the greatest speed, and near the minimum, they move away from us with the greatest speed. This follows from the analysis of line shifts in the spectra of Cepheids based on the Doppler effect.
This is the first time we are encountering the movement of the star’s photosphere, and therefore the change in its size. In fact, the size of the Sun and other stars similar to it practically does not change. Therefore, unlike such stationary stars, Cepheids are non-stationary stars. Cepheids are pulsating stars that periodically inflate and contract. As the Cepheid pulsates, the temperature of its photosphere also changes. The star has the highest temperature at its maximum brightness.
There is a relationship between the pulsation period of long-period Cepheids and the luminosity of these stars, called “period-luminosity.” If the period of change in the Cepheid’s brightness is known from observations, then using the “period-luminosity” relationship, one can determine its absolute magnitude, and then using the formula it is easy calculate the distance to the Cepheid, knowing its apparent magnitude from observations. Since Cepheids belong to giant and supergiant stars (i.e. those that have enormous sizes and luminosities), they are visible from great distances. By detecting Cepheids in distant star systems, the distance to these systems can be determined.
Cepheids are not among the rarest stars. Many stars are likely to be Cepheids for some time during their lives. Therefore, studying Cepheids is important for understanding the evolution of stars.
Other physical variable stars
Cepheids are just one of many types of physical variable stars. The first variable star was discovered in 1596 in the constellation Cetus (Mira Ceti, or the Amazing Cetus). This is not a Cepheid. Its brightness fluctuations occur with a period of about 350 d, and the brightness at maximum reaches 3 m, and at minimum 9 m. Subsequently, many other long-period stars such as Mira Ceti were discovered.
These are mainly “cold” stars - giants of the spectral class M. The change in the brightness of such stars is apparently associated with pulsation and periodic eruptions of hot gases from the interior of the star into higher layers of the atmosphere.
Not all physical variable stars exhibit periodic changes. There are many known stars that belong to semi-regular or even irregular variables. For such stars, it is difficult or even impossible to notice a pattern in the change in brightness.
