Pulsating stars expand and contract, getting bigger 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 making some kind of oscillation or pulsing, just like hearts beating in the sky.
Cepheid variable stars
American astronomer Henrietta Leavitt discovered that Cepheids have a relationship between the period-luminosity relation and the luminosity. This term means that the longer the period of change in brightness (the interval between successive peaks of brightness), the higher the average true brightness of the star. Therefore, if one measures the apparent magnitude of a Cepheid variable as it changes over days and weeks, and then determines the period of change in brightness, then one can easily calculate the true brightness of the star.
Why is this needed? And then, that, knowing the true brightness of a star, you can determine the distance to it. After all, the farther away the star, 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, then it looks 4 times dimmer. And if the star is 3 times further away, then it looks 9 times dimmer. If the star is 10 times further away, then it looks 100 times dimmer.
Recently, there have been reports in the media that the Hubble Space Telescope has been able to determine the size 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 found in distant galaxies. But by observing the change in their brightness and using the relationship between the period of change in brightness and luminosity, astronomers have determined the distance to these galaxies.
Stars like RR Lyrae
RR Lyrae stars are similar to Cepheids, but they are not as big and 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 change in brightness 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 with a width of only 60-100 light years, in which from several hundred thousand to a million stars are "packed". By observing the change in brightness of RR Lyrae stars, astronomers can estimate the distance to such stars. And if these stars are in globular clusters, then you can determine the distance to these globular clusters.
Why is it important to know the distance to a star cluster? Here's why. All the stars located in the same cluster formed simultaneously from a common cloud. And they are all located at about the same distance from the Earth, because they are in the same cluster. Therefore, when scientists build an H-R diagram for the stars in a cluster, there will be no errors caused by the difference in distances between the various stars. And if we know the distance to the star cluster, then all the values of stellar magnitudes plotted on the diagram can be converted into luminosity, that is, into the intensity of the energy emitted by the star per second. And these values can be directly compared with theoretical data. That is what astrophysicists do.
Long-period variable stars
While astrophysicists are processing information from Cepheids and RR Lyrae variable stars, amateur astronomers are enjoying observing long-period variable stars, the so-called Mira Ceti type variable stars. Mira is another name for the star Omicron Ki.
Variable stars like Mira Ceti pulsate like Cepheids, but they have much longer periods of brightness change, 10 months or more on average, and, in addition, they have a larger amplitude of brightness change. 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 change in brightness of long-period stars is also much more irregular than that of Cepheids. The maximum magnitude that a star reaches can vary greatly from one period to another. Observations of such stars, which are not difficult to make, allow scientists to obtain important scientific information. And you, too, can contribute to the study of variable stars (I will discuss this in more detail in the last section of this chapter).
The image shows a red variable star called V838 Monocerotis.
Variable star -, the brightness of which changes with time as a result of physical processes occurring in its area. Strictly speaking, the brightness of any star changes with time to one degree or another. For example, the amount of energy released changes by 0.1% during an eleven-year solar cycle, which corresponds to a change in absolute magnitude by 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 a variable, it is sufficient that the brightness of the star undergo a change at least once.
Variable stars are very different from each other. Changes in brightness 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 the change 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 twinkling, which occurs due to fluctuations in the air of the earth's atmosphere. Stars do not twinkle when viewed from space.
Top-10 constellations by the number of variable stars according to the OKPS-4 catalog
The first variable star was identified in 1638, when Johann Holvarda noticed that the star Omicron Ceti, later called Mira, pulsates with a period of 11 months. Prior to this, the star had been described as a nova by the astronomer David Fabricius in 1596. This discovery, combined with supernova observations in 1572 and 1604, proved that the starry sky was not something eternally fixed, as Aristotle and others had taught. ancient philosophers. The discovery of variable stars thus contributed to the revolution in astronomical thought that took place 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 Goodryke. In 1686, the astronomer Gottfried Kirkhi discovered the star Chi Cygni (χ Cygni), and in 1704, thanks to Giovanni Maraldi, R Hydra (R Hydrae) became known. By 1786, 10 variable stars were already known. John Goodryk, with his observations, added Delta Cephei (δ Cephei) and Sheliak (β Lyr) to their number. 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 over 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 creation of catalogs of variables was entrusted to Soviet astronomers - the State Astronomical Institute. P.K. Sternberg (GAISh) and the Astrosoviet of the Academy of Sciences of the USSR (now INASAN). Approximately once every 15 years, these organizations publish the General Catalog of Variable Stars (GCVS). The latest 4th edition was published from 1985 to 1995. In the intervals between the next editions of the OKPZ, supplements 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, eng. NSV).
The fourth edition of the OKPZ 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 variable star designation system is a development of the system proposed by Friedrich Argelander in the middle of the 19th century. Argelander in 1850 proposed to name those variable stars that have not yet received their designation by letters from R to Z in the order of discovery in each constellation. For example, R Hydrae is the first variable star in the constellation Hydra, S Hydrae is the second, and so on. Thus, 9 variable designations were reserved for each constellation, that is, 792 stars. In Argelander's time, such a supply seemed quite sufficient. However, by 1881, the limit of 9 stars per constellation was surpassed, and E. Hartwig proposed supplementing the nomenclature with two-letter designations 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 designations:
AA AB AC … AI AK … AZ BB BC … BI BK … BZ … II IK … IZ KK … KZ … QQ … QZ
The letter J was excluded from two-letter combinations so as not to be confused with I in handwritten writing. Only after the two-letter notation had completely exhausted itself, it was decided to use a simple numbering of stars indicating the constellation, starting with number 335, for example V335 Sgr. This system is still in use today. Most variable stars are found in the constellation Sagittarius. It is noteworthy that the last place in the Argelander classification was taken in 1989 by the star Z Cutter.
Throughout the history of the study of variable stars, attempts have been repeatedly made 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 change in light, etc. Subsequently, along with an increase in the number of known variable stars, the number of groups with similar morphological signs, some large ones were divided into a number of smaller ones. At the same time, thanks to the development of theoretical methods, it became possible to classify 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 taken as standard for a given type. For example, variable stars like RR Lyr.
The following division of variable stars into classes was proposed by Guzo (French Jean-Charles Houzeau de Lehaie) in the 19th century:
Stars that continuously increase or decrease in brightness.
Stars with a periodic change in brightness.
Stars of the Mira Ceti type are stars with long periods and significant variations in brightness.
Stars with a fairly fast and regular change in brightness. Characteristic representatives of β Lyrae, δ Cephei, η Aquilae.
Stars of the Algol type (β Persei). Stars with a very short period (two or three days) and an extremely correct brightness measurement, which occupies only a small part of the period. The rest of the time the star retains 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 subdivided 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 a periodic change 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. They are divided into two subtypes:
Classical Cepheids (Cδ) - Cepheids of the flat component of the Galaxy
Virgo W type stars (CW) - Cepheids of the spherical component of the Galaxy
Slow wrong variables (L)
Stars like Mira Ceti (M)
Semi-Regular Variables (SR)
Variables of type RR Lyrae (RR)
Variables of type RV Taurus (RV)
β Cephei or β Canis Major (βC) variables
Variables of type δ Shield (δ Sct)
Variables like ZZ Kita - pulsating white dwarfs
Magnetic variables like α² Hounds of the Dogs (αCV)
Eruptive variable stars. This class includes stars that change their brightness irregularly or once during the observation period. All changes in the brightness of eruptive stars are associated with explosive processes occurring on 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 new and nova-like stars.
Variables such as UV Ceti (UV) are stars of spectral type d Me that experience short-term bursts of significant amplitude.
UVn stars - a subtype of UV stars associated with diffuse nebulae
Variables like BY Draconis (BY) are emission stars of late spectral types, showing periodic brightness changes with variable amplitude and changing shape of the light curve.
Wrong variables (I). Characterized by indices a, b, n, T, s. Index a indicates that the star belongs to the spectral type O-A, index b denotes the spectral type F-M, n symbolizes the connection with diffuse nebulae, s is fast variability, T describes the emission spectrum characteristic of the T Tauri star. So the designation Isa is assigned to a fast irregular variable of an early spectral type.
New stars (N)
Fast new (Na)
Slow new (Nb)
Very slow novae (Nc)
Repeated new (Nr)
Nova-like stars (Nl)
Z Andromeda Symbiotic Variables (ZAnd)
Northern Corona R Type Variables (RCB)
Variables of type U Gemini (UG)
Giraffe Z Type Variables (ZCam)
Supernovae (SN)
Doradus S Type Variables (SD)
Variables of type γ Cassiopeia (γC)
Eclipsing variable stars include systems of two stars, the total brightness of which periodically changes over time. The reason for the change in brightness can be eclipses of stars by each other, or a change in their shape by mutual gravity in close systems, that is, variability is associated with a change in geometric factors, and not with physical variability.
Algol-type eclipsing variables (EA) - light curves allow fixing the beginning and end of eclipses; in the intervals between eclipses, the brightness remains almost constant.
Eclipsing variables like β Lyrae (EB) - Binary stars with ellipsoidal components that continuously change brightness, including in the interval between eclipses. A secondary minimum is obligatory observed. Periods are usually more than 1 day.
Eclipsing variables of the Ursa Major W type (EW) are contact systems of stars of spectral classes F and later. They have periods less than 1 day and amplitudes are usually less than 0.8m.
Ellipsoidal variables (Ell) are binary systems that do not show eclipses. Their brightness changes due to a change in the area of the radiating surface of the star facing the observer.
During the time that has elapsed between the third and fourth editions of the OKPS, not only the amount of observational material has increased, but also its quality. This made it possible to introduce a more detailed classification, introducing into it the idea of the physical processes that cause the variability of stars. 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 is usually due to changes in the shell or loss of mass in the form of a stellar wind of varying intensity 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 and 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 adjacent zones of the star may be in opposite phases. Rotating variable stars are stars in which the distribution of brightness 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 fixes their variability. The inhomogeneity of surface brightness may be due to the presence of spots or temperature or chemical inhomogeneities caused by magnetic fields whose axes do not coincide with the axis of rotation of the star.
Cataclysmic (explosive and nova-like) variable stars. The variability of these stars is caused by explosions caused by explosive processes in their surface layers (novae) or deep in their interiors (supernovae).
eclipsing binaries
Optical variable binary systems with hard X-rays
Variables with other symbols
New types of variables - types of variability discovered during the publication of the catalog and therefore not included in the already published classes.
Classes 1 and 5 intersect - stars with RS and WR variability types belong to both of these classes.
The 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 spots on the Sun increases and its activity increases. Of course, the pulsations of the Sun cannot be compared with the pulsations of Cepheids, and even more so of new and supernovae stars. Therefore, our Sun is a permanent star.
A variable star is one whose brightness (brightness) changes over time due to physical processes in or around the star. This true variability of the stars must be distinguished from their twinkling and other variability caused by the inconsistency of the earth's atmosphere.
But when observing from the Earth, it is not so easy to separate the natural fluctuations in the brightness of a star from those caused by the influence of the atmosphere. Therefore, the accuracy of photometry, i.e., measurements of the radiation flux from stars, was not high until the 1990s: no better than 0.1 m (magnitude). And the number of variable stars did not exceed 30,000.
Space telescopes, and above all the Hipparcos telescope, revolutionized the study of stellar variability by the end of the 20th century: photometry of millions of stars with an accuracy better than 0.01 "showed that almost all stars are variable to one degree or another. For example, our Sun changes brightness by about 0.001m during the 11-year solar cycle.But we, like professional astronomers, for convenience, will consider as variables only stars with a significant amplitude of variability.Information about them is collected and systematized in the General Catalog of Variable Stars (GCVS) by the State Astronomical Institute named after P. K. Sternberg (GAISh) in Moscow.
Variable stars have long been denoted by one or two large Latin letters.
before the name of the constellation, for example, BW Cam is a variable in the constellation Giraffe. And when such combinations of letters were exhausted, they began to be denoted by a capital letter V (from the word variable - “variable”) followed by a number, for example, V838 Mon - a variable in the constellation Unicorn.
All variable stars with a noticeable amplitude of brightness fluctuations can be divided into four broad categories. Here, the reason for the variability of the radiation flux observed by us is partial or total eclipses of one star in a pair by another star. The second category is pulsating variable stars. By the way, most of the currently known variable stars with significant amplitudes belong to them. Here, the reason for the variability is the pulsations of the star, i.e., changes in its size, density, brightness, color, temperature, spectrum, and other characteristics. The causes of pulsations are different, but they all follow from the physical properties of the matter of the star. The third category is eruptive, i.e. exploding, or flaring, variable stars. These are unstable stars, usually on the verge of transition from one stage of evolution to another. The fourth category is rotating variable stars with unequal surface brightness. We can say that these are stars with spots or stripes of different brightness. The Sun also belongs to them, but its spots are insignificant in comparison with the giant spots of some stars.
eclipsing variable stars
The fading of the star Algol (Vetta Perseus) was noticed in antiquity and explained in 1783 by John Goodryke. Approximately every 69 hours, the star fades for 10 hours - this is visible to the naked eye. Therefore, Algol is in the table of variable stars in Workshop No. 40. Behind the “wink” of the star lies a close pair of “waltzing” Algol, in which one periodically obscures the other. Of course, we observe eclipses in this pair only because both stars and the Earth are approximately on the same straight line (the deviation is less than 8°). And this means that, in general, the eclipses in the Algol pair are not total: just as the Moon in our sky sometimes partially obscures the Sun, so here one star partially obscures the other - partial eclipses. In this case, the total light of the two stars of the pair goes out for 1.3 m. If the plane of the orbit of the stars was inclined to the line "star-Earth" by 27 °, then we would not observe eclipses, and Algol would not be considered a variable star. And if the angle were reduced to 3 °, the eclipses would become total, and then we would see much deeper extinctions of Algol - by more than 3 m (i.e., Algol would become invisible to the eye for half an hour). According to ancient chronicles, astronomers found out what happened. Just as the axis of a rapidly rotating top slowly sways from side to side, so does the plane of Algol's orbit rotate with a period of about 20,000 years. At the beginning of our era, Algol was not a variable star. That is why his “winks”, clearly visible to the eye, are not mentioned by the ancient astronomers Hipparchus and Ptolemy, although they studied the sky when compiling their star catalogs. From 161 to 1482 AD, the eclipses were, as they are now, partial. And in 1482-1768 - complete. Which attracted the attention of John Goodryke and other astronomers of the 18th century. Partial eclipses will continue until 3044.
Pulsating variable stars
The star of b Cephei and the like pulsate: either they swell and, accordingly, cool and dim, then they shrink, heat up and become brighter. By the way, this is reminiscent of the work of a car engine: the bowels of the star act as fuel, and the shell acts as a piston. The fuel turns into gas, the pressure of which pushes the piston. As in the engine, the process has several stages. In the general case, the energy of a star, rushing to the surface from the depths, in a certain layer at an intermediate depth is spent on the decay of molecules into atoms or on the ionization of matter - that is, it accumulates in this layer and does not reach the surface. When all the matter in the mentioned layer turns into atoms or ionizes, the energy of the depths no longer lingers in it, breaks through to the outer layers of the star and goes to its expansion. The expansion of the shell also cools a special layer where energy was stored. In fact, for a short time, while the star has its maximum size and brightness, it releases into outer space the energy stored in this special layer. It cools: atoms combine into molecules, or ions into atoms. The cooled star shrinks under the influence of the attraction of its own particles, and the cycle repeats. Remember that any star is in balance of two forces: the mutual attraction of its own particles and the pressure of hot matter from the depths. Pulsations - in fact, the struggle of these forces, going with varying degrees of success.
The closest Cepheid to Earth is the Cepheus-type star, the Polaris. In addition, it is a triple system. A close companion star flies around the central star with a period of about 30 years. But apart from one Hubble observation, Polaris and its companion star have always been observed together, and orbital characteristics calculated from changes in their combined brightness. However, everything is complicated by the fact that Polyarnaya changes brightness due to pulsations, and even has some strange long-term changes in brightness: over the 20th century, the amplitude of its variability decreased from 8% to almost zero (in the 21st century, Polar almost does not pulsate!) that on average over the last century it has become brighter by 15%. It turns out that the main discoveries in the physics of the North Star and all Cepheids are yet to come. And although Polyarnaya is not marked in Workshop No. 40, but look at it - suddenly it will clearly flare up or go out before your eyes. By the way, like Polaris, many pulsating stars with giant shells pulsate incorrectly. Hence - a large variety of non-periodic and semi-periodic giants.
Stars produce diamonds. And you can already think about their extraction, because these jewels are intensively scattered by stars into space along with the rest of the dust. Dust, gas, including molecules and organic matter, are especially intensively lost by heavily swollen giant stars and supergiants. At the periphery of their cool shells, the attraction of the star is so small that particles of matter easily leave the star. We remind you that such a star should eventually shed its shell in the form of a planetary nebula and become a white dwarf. Therefore, stars on the verge of such a transformation are extremely interesting: they pulsate especially strongly and change brightness with a large amplitude; are the reddest, even incredibly red-burgundy due to the strong absorption of light by the dusty shell; the spectrum demonstrates amazing shell substances, for example, fullerenes, crystals of 60 or more carbon atoms; and are doomed to remain in this state for so short that we can wait for radical changes before our eyes. For a dozen of these stars, astronomers are waiting for the outburst and shedding of the shell already this century!
The star Omicron Ceti every 332 days appears in the sky among the brightest stars (magnitude 2 m), and then disappears for the eye (10 m, visible at the limit in the Galileo-200 telescope). Astronomer David Fabricius in 1596 called it Mira, which in Latin means "amazing". Astronomers marveled at it until the 21st century! To explain the variability of Mira and similar stars (they are called Mirids), both mechanisms seemed to be unsuitable: an eclipsing satellite was not observed in it, and to explain such unprecedented brightness differences, pulsations are needed hundreds of times. Imagine that the Sun every year would either expand by half the solar system, or shrink to its current size. A star simply has nowhere to get so much energy from, and it is unlikely that it would survive such pulsations!
The situation began to clear up when a very dim satellite of Mira, a white dwarf, was discovered. But it is located so far from the main star that it cannot directly affect it. In 2007, the GALEX ultraviolet telescope discovered that Mira was flying through space at a tremendous speed of over 100 km/s, leaving behind a gigantic 13 light-year tail of gas and dust. This tail reaches not only to the satellite of the star, but also to neighboring stars. The loss of matter also had to be revised: every year Mira loses a mass equal to the mass of the Moon. There is a lot of black soot in this stream - carbon and its compounds. Well, exactly - a smoking steam locomotive at full speed! And Mira's satellite star, the "locomotive trailer", collects some of this soot for itself. So much so that the layer of soot on the “trailer” is many times greater than the weight of the trailer itself and, by the way, makes it even less noticeable: they have been looking for it for 200 years. As a result, Mira's satellite, flying around it, controls the flow of its substance: it passes or delays and, thus, manifests or obscures Mira. When it manifests, its magnitude soars to 2m. By the way, soot, graphite and diamond are all the same carbon. Diamonds crystallizing in Mira's core can be searched for in the smoke of this "space locomotive". A similar role is played by the so far invisible satellite of the star R Sculptor (Fig. 5): it turns the substance lost by the star into a spiral visible to us.
light echo
RS Puppies (RS Pup) - a Cepheid that changes brightness 5 times with a period of 41.4 days. When looking at its surroundings, it seems that clouds of gas are flying away from it (Fig. 6). In fact, in different phases of the pulsation of a star, it differently illuminates the motionless clouds of dust surrounding it. They consist of several layers and therefore look like luminous rings around the star. The essence of the light echo effect arising here is that the observer sees the light of the star, which came to him in different ways: directly and reflected from different parts of the dust cloud. For a large cloud (as in the case of RS Korma), the speed of light plays a role: the light reflected by the part of the cloud close to the star arrives at us noticeably later than directly. And the light reflected by the distant part of the cloud comes even later. Because of this, parts of the cloud far from the star "light up" for us later, and thus, there is the appearance of spreading bright rings. Particularly impressive is the light echo of the star V838 Monocerotis.
Recently, astronomers have taken advantage of light echoes to literally see the distant past. Supernova SN1572 was seen in 1572 - this light came in a straight line. And in 2008, a very faint reflection of that flash was seen as a light echo on the clouds of the Milky Way. The explosion of the supernova Cassiopeia A around 1660 was not noticed at all on Earth because of the cosmic clouds that obscured it. But the light echo, the reflection of that flash on other cosmic clouds, was seen in 2010.
Eruptive variable stars
Rare strong flares are inherent in different stars. For example, the flow of matter from an ordinary star to a white dwarf can cause repeated powerful explosions, which are traditionally called new stars. Young T Tauri stars flare. Flashes are also possible during the destruction of a planet near a young star.
Rotating variable stars
In 1984, the IRAS space telescope discovered a disk of dust around the star Vega. Such are typical for very young stars, less than 100 million years old, around which planets form from a gas and dust disk. Vega is older - about 450 Ma. In search of a clue, scientists discovered that Vega rotates very quickly: at its equator, the speed is 280 km / s. For comparison, the speed of rotation of the Sun is 140 times less - only 2 km / s. At this speed, Vega is not a ball at all, but a strongly flattened ellipsoid, so Vega's equator is noticeably farther from its center and therefore colder than the poles. Temperature is related to brightness. Therefore, the equator of Vega is a dark band, and the poles are light caps.
We saw one of the poles all the time and did not suspect that the top was striped. If one day Vega turns to us so that it is alternately observed either poles or sides, it will become a variable star.
Light echo - an effect that occurs in astronomy, when the light from the flash of a luminary comes to the observer, reflected from the "screens" away from the luminary, later than the light that came in a straight line. In this case, in some cases, there is an appearance of removal of the reflecting light "screen" from the source luminary at a speed higher than the speed of light.
In addition, the speed of rotation of Vega at the equator is equal to the speed of separation of matter from the star by centrifugal forces. Sometimes clumps of matter really break away from Vega and join the disk surrounding it. Therefore, although the stellar wind blows the disk matter into space, the disk is constantly replenished with new matter from the star. Of course, the disk around the star must rotate, otherwise it will fall on the star. Due to the rotation, different parts of the disk slightly obscure Vega itself at different times. So there are small fluctuations in its brightness, discovered recently.
Gas and dust disks around stars sometimes play such an important role that it is not clear to which category some variable stars should be assigned.
Please enable JavaScript to view the
Stars whose luminosity changes over relatively short periods of time are called physical variable stars. Changes in the luminosity of this type of stars are caused by physical processes that occur in their interiors. According to the nature of variability, pulsating variables and eruptive variables are distinguished. New and supernovae stars, which are a special case of eruptive variables, are also distinguished into a separate species. All variable stars have special designations, except for those that were previously designated by the letter of the Greek alphabet. The first 334 variable stars of each constellation are designated by a sequence of letters of the Latin alphabet (for example, R, S, T, RR, RS, ZZ, AA, QZ) with the addition of the name of the corresponding constellation (for example, RR Lyr). The following variables are designated V 335, V 336, etc. (for example, V 335 Cyg).
Physical variable stars
Stars that are characterized by a special shape of the light curve, which displays a smooth periodic change in the apparent magnitude and a change in the luminosity of the star by several times (usually from 2 to 6), are called physical variable stars or Cepheids. This class of stars was named after one of its typical representatives - the star δ (delta) Cepheus. Cepheids can be attributed to giants and supergiants of spectral classes F and G. Due to this circumstance, it is possible to observe them from great distances, including far beyond our star system - the Galaxy. One of the most important characteristics of Cepheids is the period. For each individual star, it is constant with a high degree of accuracy, but the periods are different for different Cepheids (from a day to several tens of days). In Cepheids, the spectrum changes simultaneously with the apparent magnitude. This means that along with the change in the luminosity of the Cepheids, the temperature of their atmospheres also changes by an average of 1500°. The shift of spectral lines in the spectra of Cepheids revealed a periodic change in their radial velocities. In addition, the radius of the star also changes periodically. Stars such as δ Cephei are young objects that are located mainly near the main plane of our star system - the Galaxy. Cepheids are also found in, but they are older and somewhat less luminous. These stars, which have reached the Cepheid stage, are less massive and therefore evolve more slowly. They are called Virgo W stars. Such observed features of Cepheids indicate that the atmospheres of these stars experience regular pulsations. Thus, they have conditions for maintaining a special oscillatory process at a constant level for a long time.
Rice. cepheid
Long before it was possible to find out the nature of pulsations cepheid, the existence of a relationship between their period and luminosity was established. When observing Cepheids in the Small Magellanic Cloud - one of the star systems closest to us - it was noticed that the smaller the apparent magnitude of the Cepheid (i.e., the brighter it seems), the longer the period of change of its brightness. This relationship turned out to be linear. From the fact that they all belonged to the same system, it followed that the distances to them were practically the same. Consequently, the discovered dependence simultaneously turned out to be a dependence between the period P and the absolute magnitude M (or luminosity L) for Cepheids. The existence of a relationship between the period and absolute magnitude of Cepheids plays a significant role in astronomy: thanks to it, distances to very distant objects are determined when other methods cannot be applied.
In addition to Cepheids, there are also other types pulsating variable stars. The best known of these are RR Lyrae stars, which were previously called short period Cepheids because of their similarity to regular Cepheids. RR Lyrae stars are giants of spectral class A, whose luminosity exceeds that of the Sun by more than 100 times. The periods of RR Lyrae stars range from 0.2 to 1.2 days, and the amplitude of brightness changes reaches one magnitude. Another interesting type of pulsating variables is a small group of β Cephei (or β Canis Major) type stars, belonging mainly to the giants of the early spectral subclasses B. By the nature of the variability and the shape of the light curve, these stars resemble RR Lyrae stars, differing from them in an exceptionally small amplitude magnitude changes. The periods are in the range from 3 to 6 hours, and, like in Cepheids, there is a dependence of the period on the luminosity.
In addition to pulsating stars with a regular change in luminosity, there are also several types of stars whose light curves change. Among them are RV-type stars Taurus, whose luminosity changes are characterized by an alternation of deep and shallow minima, occurring with a period of 30 to 150 days and with an amplitude of 0.8 to 3.5 magnitudes. RV Tauri stars belong to the spectral types F, G, or K. Stars of type m Cephei belong to the spectral class M and are called red semiregular variables. They are sometimes distinguished by very strong irregularities in the change in luminosity, occurring over a period of several tens to several hundreds of days. Next to the semi-regular variables in the spectrum–luminosity diagram, there are class M stars in which it is not possible to detect repeatability of luminosity changes (irregular variables). Below them are stars with emission lines in the spectrum that smoothly change their luminosity over very long time intervals (from 70 to 1300 days) and within very large limits. A remarkable representative of this type of stars is o (omicron) Kita, or, as otherwise called Mira. This class of stars is called long-period variables like Mira Kita. The period length of long-period variable stars fluctuates around the average value ranging from 10% in both directions.
Among dwarf stars with lower luminosity, there are also variables of various types, the total number of which is about 10 times less than the number of pulsating giants. These stars manifest their variability in the form of periodically repeating outbursts, the nature of which is explained by various kinds of ejections of matter, or eruptions. Therefore, this entire group of stars, together with new stars, is called eruptive variables. It is worth noting that among them there are stars of a very different nature, both in the early stages of their evolution and completing their life path. The youngest stars, apparently, which have not yet completed the process of gravitational contraction, should be considered variables of type τ (tau) Taurus. These are dwarfs of spectral classes, most often F - G, found in large numbers, for example, in the Orion Nebula. Stars of the RW Aurigae type, belonging to spectral classes from B to M, are very similar to them. For all these stars, the change in luminosity occurs so incorrectly that no regularity can be established.
Eruptive variable stars of a special type, in which an outburst (a sudden sharp increase in luminosity) of at least 7-8 magnitudes was observed at least once, are called new. Usually, during the outburst of a new star, the apparent stellar magnitude decreases by 10m-13m, which corresponds to an increase in luminosity by tens and hundreds of thousands of times. After the outburst, new stars are very hot dwarfs. In the maximum phase of the outburst, they resemble supergiants of classes A - F. If the outburst of the same new star was observed at least twice, then such a new one is called repeated. The increase in luminosity in repeated novae is somewhat less than in typical novae. In total, about 300 new stars are currently known, of which about 150 appeared in our Galaxy and over 100 - in the Andromeda Nebula. In the known seven repeated novae, about 20 outbreaks were observed in total. Many (perhaps even all) novae and repeated novae are close binaries. After an outburst, novae often exhibit weak variability. The change in the luminosity of the new star shows that during the outburst there is a sudden explosion caused by the instability that has arisen in the star. According to various hypotheses, this instability can arise in some hot stars as a result of internal processes that determine the release of energy in the star, or due to the influence of some external factors.
supernovae
Supernovae are stars that flare up in the same way as new ones and reach absolute magnitudes from -18m to -19m and even -21m at maximum. Supernovae have an increase in luminosity by more than tens of millions of times. The total energy emitted by a supernova during a flash is thousands of times greater than for novae. About 60 supernova outbursts in other galaxies have been photographically recorded, and often their luminosity turned out to be comparable with the integrated luminosity of the entire galaxy in which the outburst occurred. According to descriptions of earlier observations made with the naked eye, several cases of supernova explosions in our Galaxy have been established. The most interesting of them is the Supernova of 1054, which erupted in the constellation of Taurus and was observed by Chinese and Japanese astronomers as a "guest star" that suddenly appeared, which seemed brighter than Venus and was visible even during the day. Although this phenomenon is similar to the outburst of an ordinary nova, it differs from it in its scale, smooth and slowly changing light curve and spectrum. Two types of supernovae are distinguished by the character of the spectrum near the epoch of maximum. Of great interest are the rapidly expanding ones, which in several cases were found at the site of type I supernovae. The most remarkable of these is the famous Crab Nebula in the constellation Taurus. The shape of the emission lines of this nebula indicates its expansion at a speed of about 1000 km/sec. The current dimensions of the nebula are such that expansion at this rate could begin no more than 900 years ago, i.e. just in time for the supernova explosion of 1054.
Pulsars
In August 1967, in the English city of Cambridge, cosmic radio emission was recorded, which came from point sources in the form of clear pulses following one after another. The duration of an individual pulse for such sources can range from a few milliseconds to several tenths of a second. The sharpness of the pulses and the correctness of their repetitions make it possible to determine with great accuracy the periods of pulsations of these objects, which are named pulsars. The period of one of the pulsars is approximately 1.34 seconds, while the others have periods ranging from 0.03 to 4 seconds. Currently, about 200 pulsars are known. All of them produce highly polarized radio emission over a wide range of wavelengths, the intensity of which increases steeply with increasing wavelength. This means that the radiation has a non-thermal nature. It was possible to determine the distances to many pulsars, which turned out to be in the range from hundreds to thousands of parsecs, which indicates the relative proximity of objects that obviously belong to our Galaxy.
The most famous pulsar, which is usually designated by the number NP 0531, exactly coincides with one of the stars in the center of the Crab Nebula. Observations have shown that the optical radiation of this star also changes with the same period. In an impulse, the star reaches 13m, and between impulses it is not visible. The same pulsations from this source are also experienced by X-ray radiation, the power of which is 100 times higher than the power of optical radiation. The coincidence of one of the pulsars with the center of such an unusual formation as the Crab Nebula suggests that they are just the objects into which supernovae turn after flares. If supernova outbursts really end in the formation of such objects, then it is quite possible that pulsars are neutron stars. In this case, with a mass of about 2 solar masses, they should have radii of about 10 km. When compressed to such dimensions, the density of matter becomes higher than nuclear, and the rotation of the star accelerates to several tens of revolutions per second. Apparently, the time interval between successive pulses is equal to the rotation period of the neutron star. Then the pulsation is explained by the presence of irregularities, peculiar hot spots, on the surface of these stars. Here it is appropriate to speak of a "surface", since at such high densities the substance is closer in its properties to a solid body. Neutron stars can serve as sources of energetic particles that are constantly entering their associated nebulae like the Crab Nebula.
photo: Radio emission from the Crab Nebula
Variable stars are one of the most curious phenomena in the sky, accessible to observation with the naked eye. Moreover, there is scope for the scientific activity of a simple astronomy lover, and there is even an opportunity to make a discovery. There are a lot of variable stars today, and it is quite interesting to observe them.
Variable stars are stars that change their brightness over time. Of course, this process takes some time, and does not happen literally before our eyes. However, if you periodically observe such a star, changes in its brightness will become clearly visible.
The reasons for the change in brightness can be different reasons, and depending on them, all variable stars are divided into different types, which we will consider below.
How variable stars were discovered
It has always been believed that the brightness of the stars is something constant and unshakable. A flash or just the appearance of a star has been attributed to something supernatural since ancient times, and this clearly had some kind of sign from above. All this can be easily seen in the text of the same Bible.
However, many centuries ago, people knew that some stars can still change their brightness. For example, Beta Perseus is not in vain called El Ghoul (now it is called Algol), which in translation means nothing more than "the star of the devil." It is named so because of its unusual property to change the brightness with a period of a little less than 3 days. This star was discovered as a variable in 1669 by the Italian astronomer Montanari, and at the end of the 18th century, the English amateur astronomer John Goodryke studied, and in 1784 he discovered the second variable of the same type - β Lyrae.
In 1893, Henrietta Lewitt came to work at the Harvard Observatory. Her task was to measure the brightness and catalog the stars on the photographic plates accumulated in this observatory. As a result, Henrietta discovered more than a thousand variable stars in 20 years. She was especially good at investigating pulsating variable stars, the Cepheids, and made some important discoveries. In particular, she discovered the dependence of the Cepheid period on its brightness, which allows you to accurately determine the distance to the star.
Henrietta Lewitt. After that, with the rapid development of astronomy, thousands of new variables were discovered.
Classification of variable stars
All variable stars change their brightness for various reasons, so a classification was developed on this basis. At first it was quite simple, but as data accumulated, it became more and more complicated.
Now in the classification of variable stars, several large groups are distinguished, each of which contains subgroups, which include stars with the same causes of variability. There are a lot of such subgroups, so we will briefly consider the main groups.
eclipsing variable stars
Eclipsing variables, or simply eclipsing variable stars, change their brightness for a very simple reason. In fact, they are not one star, but a binary system, moreover, quite close. The plane of their orbits is located in such a way that the observer sees how one star closes the other - there is, as it were, an eclipse.
If we were a little away, we would not be able to see anything like this. It is also possible that there are many such stars, but we do not see them as variables, because the plane of their orbits does not coincide with the plane of our view.
Many types of eclipsing variable stars are also known. One of the most famous examples is Algol, or β Perseus. This star was discovered by the Italian mathematician Montanari in 1669, and its properties were studied by John Goodrick, an English amateur astronomer, at the end of the 18th century. The stars that form this binary system cannot be seen individually - they are located so closely that their period of revolution is only 2 days and 20 hours.
If you look at the Algol brightness curve, you can see a small dip in the middle - a secondary minimum. The fact is that one of the components is brighter (and smaller), and the second is weaker (and larger). When the weak component covers the bright one, we see a strong drop in brightness, and when the bright one covers the weak one, the drop in brightness is not very pronounced.
In 1784, Goodryk discovered another eclipsing variable, Lyrae's β. Its period is 12 days 21 hours and 56 minutes. In contrast to Algol, the graph of the change in brightness for this variable is smoother. The fact is that here the binary system is very close, the stars are so close to each other that they have an elongated, elliptical shape. Therefore, we see not only eclipses of the components, but also changes in brightness when elliptical stars rotate wide or narrow.
Graph of the change in the brightness of β Lyra. defense. Because of this, the change in gloss here is smoother.
Another typical eclipsing variable is Ursa Major W, discovered in 1903. Here, the chart shows a secondary low of almost the same depth as the main one, and the chart itself is smooth, like that of β Lyra. The fact is that here the components are almost the same in size, also elongated, and so closely spaced that their surfaces almost touch.
There are other types of eclipsing variable stars, but they are less common. This also includes ellipsoidal stars, which, during rotation, turn to us with either a wide or a narrow side, due to which their brightness changes.
Pulsating Variable Stars
Pulsating variable stars are a large class of objects of this kind. Changes in brightness occur due to changes in the volume of the star - it either expands or contracts again. This happens due to the instability of the balance between the main forces - gravity and internal pressure.
With such pulsations, an increase in the photosphere of the star and an increase in the area of the radiating surface occur. At the same time, the surface temperature and color of the star change. Gloss, respectively, also changes. Some types of fluctuating variables change their brightness periodically, and some do not have any stability - they are called irregular.
The first pulsating star was Mira Kita, discovered in 1596. When its brilliance reaches its maximum, it can be clearly seen with the naked eye. At a minimum, good binoculars or a telescope are required. Mira's shine period is 331.6 days, and such stars are called Mirids or ο Ceti-type stars - several thousand of them are known.
Another widely known type of pulsating variable is the Cepheid, named after a star of this type, Ϭ Cephei. These are giants with periods from 1.5 to 50 days, sometimes more. Even the North Star belongs to the Cepheids with a period of almost 4 days and with brightness fluctuations from 2.50 to 2.64 stars. quantities. Cepheids are also divided into subclasses, and their observations have played a significant role in the development of astronomy in general.
Pulsating variables of the RR Lyrae type are distinguished by a rapid change in brightness - their periods are less than a day, and the fluctuations on average reach one magnitude, which makes it easy to observe them visually. This type of variables is also divided into 3 groups, depending on the asymmetry of their light curves.
Even shorter periods in dwarf Cepheids are another kind of pulsating variable. For example, the CY of Aquarius has a period of 88 minutes, while the SX of Phoenix has a period of 79 minutes. The graph of their brightness is similar to the graph of ordinary Cepheids. They are of great interest for observation.
There are many more types of pulsating variable stars, although they are not as common or very convenient for amateur observations. For example, stars of the RV Taurus type have periods from 30 to 150 days, and there are some deviations in the brightness graph, which is why stars of this type are referred to as semi-regular.
Wrong variable stars
Irregular variable stars are also pulsating, but this is a large class that includes many objects. Changes in their brightness are very complex and often impossible to predict in advance.
However, for some irregular stars, periodicity can be detected in the long run. When observing over several years, for example, one can notice that irregular fluctuations add up to a certain average curve that repeats. Such stars, for example, include Betelgeuse - α Orion, whose surface is covered with light and dark spots, which explains the fluctuations in brightness.
Irregular variable stars are not well understood and are of great interest. There are still many discoveries to be made in this field.
How to observe variable stars
Various methods are used to detect changes in the brightness of a star. The most accessible is visual, when an observer compares the brightness of a variable star with the brightness of neighboring stars. Then, based on the comparison, the brightness of the variable is calculated and, as this data is accumulated, a graph is built on which brightness fluctuations are clearly visible. Despite the apparent simplicity, the determination of brightness by eye can be done quite accurately, and such experience is acquired quite quickly.
There are several methods for visually determining the brightness of a variable star. The most common of these are the Argelander method and the Neuland-Blazhko method. There are others, but these are fairly easy to learn and give sufficient accuracy. We will tell you more about them in a separate article.
Advantages of the visual method:
- No equipment required. You may need binoculars or a telescope to observe faint stars. Stars with a minimum brightness of up to 5-6 stars. quantities can be observed with the naked eye, there are also quite a lot of them.
- In the process of observation, there is a real "communication" with the starry sky. This gives a pleasant feeling of unity with nature. In addition, it is quite a scientific work that brings satisfaction.
The disadvantages include, nevertheless, non-ideal accuracy, which causes errors in individual observations.
Another method for estimating the brightness of a star is with the use of equipment. Usually a picture of a variable star with its surroundings is taken, and then the brightness of the variable can be accurately determined from the picture.
Is it worth it for an amateur astronomer to observe variable stars? Definitely worth it! After all, these are not only one of the simplest and most accessible objects for study. These observations also have scientific value. Professional astronomers are simply not able to cover such a mass of stars with regular observations, and for an amateur there is even an opportunity to contribute to science, and such cases have happened.
