Variable star

Top 10 constellations by the number of variable stars
according to the OKPZ-4 catalog ^[1]

The first variable star was identified in 1638 when Johann Holvard noticed that the star Omicron Ceti , later called Mira, pulsates with a period of 11 months. Prior to this, the star was described by astronomer David Fabricius in 1596 and was mistakenly defined as a new one . This discovery, combined with observations of supernovae in 1572 and 1604, proved that the starry sky is not something forever unchanged, as Aristotle and other philosophers of antiquity have taught. The discovery of variable stars, thus, contributed to the revolution of astronomical views 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 Goodrayk . In 1686, the astronomer Gottfried Kirche 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 with his observations added to their number Delta Cephei (δ Cephei) and Sheliak (β Lyr) . Since 1850, the number of known variable stars has increased dramatically, especially since 1890, when the use of 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 galaxy, as well as 10,000 from other galaxies and another 10,000 possible variables.

The modern designation system for variable stars is a development of the system proposed by Friedrich Argelander in the mid-19th century. Argelander in 1850 proposed to name those variable stars that have not yet received their designation, the letters from R to Z in the order of detection in each constellation. For example, R Hydrae is the first variable star in the Hydra constellation, S Hydrae is the second and so on. Thus, 9 designations of variables for each constellation were reserved, that is, 792 stars. At the time of Argelander, such a reserve seemed quite sufficient. However, by 1881, the limit of 9 stars per constellation was exceeded, and E. Hartwig proposed to supplement the nomenclature with two-letter designations according to the following principle:

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

The letter J is excluded from two-letter combinations so as not to confuse it with I in handwriting. Only after the two-letter designation system had completely exhausted itself, it was decided to use simple numbering of stars indicating the constellation , starting from 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 classification of Argelander 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 the stars according to similar external morphological characteristics, such as the shape of the light curve, the amplitude and period of brightness, etc. Subsequently, along with an increase in the number of known variable stars, the number of groups with similar morphological values ​​also increased. By 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 by external, observable features, but also by physical processes leading to one or another kind of variability.

To indicate the types of variable stars use the so-called. prototypes are stars whose characteristics of variability are taken as standard for this type. For example, variable stars like RR Lyr .

Guzo system

The following division of variable stars into classes was proposed by Guzo ( fr. Jean-Charles Houzeau de Lehaie ) in the 19th century:

1. Stars whose brightness is continuously increasing or decreasing.
2. Stars with periodic shine.
3. * Stars like Worlds of the Whale - stars with large periods and significant changes in brightness.
4. * Stars with a fairly quick and correct change in brightness. Characteristic representatives of β Lyrae , δ Cephei , η Aquilae.
5. * Algol type stars (β Persei). Stars with a very short period (two to three days) and extreme correctness of the brightness change, which occupies only a small part of the period. The rest of the time, the star retains its greatest brilliance. Other stars of the Algol type: λ Tauri, R Canis majoris, Y Cygni, U Cephei, etc.
6. Stars with irregular brightness changes. Representative - η Argus
7. New stars .

OKPZ-3 classification system

In OKPZ-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 Variable Stars

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

1. 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
   • V-type stars (CW) - Cepheids of the spherical component of the Galaxy
2. Slow Incorrect Variables (L)
3. Stars of the Worlds of Whale type (M)
4. Semi-Variable Variables (SR)
5. Variables of type RR Lyrae (RR)
6. RV Taurus Type Variables (RV)
7. Variables of type β Cepheus or type β of Canis Majoris (βC)
8. Variables of type δ Shield (δ Sct)
9. ZZ type whale variables - pulsating white dwarfs
10. Magnetic Variables Type α² Hounds 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 the stars, in their vicinity or with the explosions of the stars themselves. This class of variable stars is divided into two subclasses: incorrect variables associated with diffuse nebulae, and fast incorrect ones, as well as a subclass of new and new-like stars.

Wrong Variables Associated with Diffuse Nebulae and Fast Wrong Variables

1. Variables of the UV Ceti type (UV) are stars of the spectral class d Me, experiencing short-term flares of significant amplitude.
   • UVn stars are a subtype of UV stars associated with diffuse nebulae.
2. Variables like Dragon BY (BY) are emission stars of late spectral classes, showing periodic changes in brightness with variable amplitude and changing shape of the light curve.
3. Incorrect variables (I). They are characterized by indices a, b, n, T, s. The subscript a indicates that the star belongs to the O – A spectral class, the subscript b denotes the F – M spectral class, n symbolizes the connection with diffuse nebulae, s the fast variability, T describes the emission spectrum characteristic of the Taurus T star. So the designation Isa is assigned to the fast wrong variable of the early spectral class.

New and New Stars

1. New Stars (N)
   • Fast new (Na)
   • Slow New (Nb)
   • Very Slow New (Nc)
   • Repeated New (Nr)
2. New Star (Nl)
3. Symbiotic variables of type Z Andromeda (ZAnd)
4. Type R Northern Crown Variables (RCB)
5. U Type Twins (UG)
6. Giraffe Type Z Variables (ZCam)
7. Supernovae (SN)
8. Type S Golden Fish Variables (SD)
9. Variables of type γ Cassiopeia (γC)

Eclipsing Variable Stars

Eclipsing-variable stars include systems of two stars, the total brightness of which periodically changes over time. The reason for the brightness change can be the eclipse 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.

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

1. Eclipsing variables of type β Lyrae (EB) - Binary stars with ellipsoidal components that continuously change brightness, including in the interval between eclipses. Be sure to observe a secondary minimum. Periods, usually more than 1 day.
2. Eclipsing variables of type W Ursa Major (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 0.8 ^m .
3. Ellipsoidal variables (Ell) are binary systems that do not show eclipses. Their brightness changes due to a change in the area of ​​the star's radiating surface facing the observer.

OKPZ-4 classification system

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

1. Eruptive variable stars are stars that change their brightness due to violent processes and flashes in their chromospheres and coronas. The change in luminosity usually occurs due to changes in the shell or mass loss in the form of a stellar wind of variable intensity and / or interaction with the interstellar medium.
2. Pulsating variable stars are stars that show periodic expansions and contractions of their surface layers. Ripples can be radial and non-radial. The radial pulsations of a star leave its shape spherical, while non-radial pulsations cause the star to deviate from the spherical shape, and the neighboring zones of the star can be in opposite phases.
3. Rotating variable stars are stars in which the brightness distribution over the surface is heterogeneous 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 can be caused by 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.
4. Cataclysmic (explosive and like-like) variable stars . The variability of these stars is caused by explosions, the cause of which are explosive processes in their surface layers (new) or deep in their bowels (supernovae).
5. Eclipsing binary systems are stellar systems in which a periodic change in brightness is observed due to eclipses of one star by another.
6. Hard x-ray optical variable binary systems are binary systems that are a source of hard variable x-ray radiation and do not belong to other types of variable stars.
7. Variables with other characters
8. New types of variables are types of variability that were discovered in the process of publishing a catalog and therefore did not fall into classes that were already published .

Classes 1 and 5 intersect - stars with variability types RS and WR belong to both of these classes.

A detailed list of types of variable stars in JCHP

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 can not be compared with the pulsations of Cepheids, and even more so of new and supernova stars. Therefore, our Sun belongs to the constant stars.

• Algol
• δ Cepheus
• Deneb
• Of the world
• polar Star
• Betelgeuse

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• Ponyatov, Alexey. Flashing // Science and life . - 2018. - No. 3. - S. 2-8.
• Kholopov PN On the classification of variable stars // Variable stars. - 1981. - T. 21 . - S. 465-484 .

• Pulsating stars
• N. N. Samus . Variable stars. Study guide for the course "Astronomy" on the site "HERITAGE - astronomy, astronomical education with the preservation of traditions"
• How To Decipher Those Classification Codes (Variable Star Nomenclature)
• American Association of Variable Star Observers (AAVSO)
• OKPZ-4
• Variable stars - classification, instructions, articles, news on www.variablestars.ru
• Pulsating stars. Problems and stereotypes
• Lecture "Variable Stars: Yesterday, Today, Tomorrow" 02/27/2013 Samus N. N. ( video , lecture at the Moscow Planetarium)
• Soviet popular science film for students "Variable stars" (video on Youtube)