Variable Stars

Classification

Each variable star is designated with a name and a number. Names are issued in the order of discovery in a particular constellation. For historical reasons the first variable discovered gets an R, the second an S down to Z. The 10th star is RR, followed by RS, RT ... to RZ; SS, ST ... to SZ etc. After that, the naming continues with AA, AB, AC and so on. The letter J is omitted. The constellation Sagittarius counts today far more than 4000 variables. After QZ (the 334th variable star) they become V335, V336 etc. which sets no limits to the number of stars. Although recent mass-discoveries of variables in globular clusters and other galaxies by satellites questions this naming system.


Typology

There are two major groups of variable stars: the intrinsic and the 'apparent' variables. The most important types are:

  Pulsating Variables:
  • Cepheids (yellow stars, periods between 1 and 70 days, amplitudes of 0.1 to 2 mag)

  • RR Lyrae Stars (white giant stars, periods of 0.05 to 1.2 days, amplitudes of 1 to 2 mag)

  • RV Tauri Stars (yellow supergiant stars, periods of 30 to 150 days, sometimes superimposed periods of 500 to 9000 days, amplitudes up to 3 mag).
  Eruptive Variables:
  • Supernovae (amplitudes up to 20 mag! 2 types in general)

  • Novae (close binary stars with mass transfer resluting in heavy outbursts)

  • Recurent Novae (like novae but several outbursts historically recorded)

  • Dwarf Novae (similar to Novae but periods of 10 to 1000 days, smaller amplitudes (3-6 mag))

  • Symbiotic Stars (similar to Novae but amplitudes only up to 3 mag)

  • Flare Stars (violent sudden outbursts with amplitudes of 1 to 6 mag, duration only minutes!)

  • R Corona Borealis Stars ('inverse Novae': most of the time on maximum brightness, then fading of up to 9 magnitudes, most probably due to condensating carbon).
  Longperiod Variables:
  • Mira Stars (amplitudes of 2 to 9 mag, Red Giants with quite regular periods of 80 to 1000 days).
  Semi-Regular Variables:
  • Quite regular periods of 30 to 1000 days, interrupted by phases of irregular brightness, amplitudes up to 2 mag.
  Irregular Variables:
  • Light curve shows no or very little periodicity.
  'Apparent' Variables:
  • Eclipsing Binaries (binary systems whose orbital plane lies in our line-of-sight, which causes the eclipses when on star is passing in front of the other one)

  • Rotating Variables (stars with spots (bright or dark)).

Type of Variability

(Detailed description)

Variability types are grouped according to the major astrophysical reasons for variability:

1. Eruptive Variable Stars
  Eruptive variables are stars varying in brightness because of violent processes and flares occurring in their chromospheres and coronae. The light changes are usually accompanied by shell events or mass outflow in the form of stellar winds of variable intensity and/or by interaction with the surrounding interstellar medium.This class includes the following types:
  BE, FU, GCAS, I, IA, IB, IN, INA, INB, INT, IT, IN(YY), IS, ISA, ISB, RCB, RS, SDOR, UV, UVN, WR

2. Pulsating Variable Stars
  Pulsating variables are stars showing periodic expansion and contraction of their surface layers. The pulsations may be radial or nonradial. A radially pulsating star remains spherical in shape, while in the case of nonradial pulsations the star's shape periodically deviates from a sphere, and even neighboring zones of its surface may have opposite pulsation phases. Depending on the period value, on the mass and evolutionary status of the star, and on the scale of pulsational phenomena, the following types of pulsating variables may be distinguished:
  ACYG, BCEP, BCEPS, BLBOO, CEP, CEP(B), CW, CWA, CWB, DCEP, DCEPS, DSCT, DSCTC, GDOR, L, LB, LC, LPB, M, PVTEL, RPHS, RR, RR(B), RRAB, RRC, RV, RVA, RVB, SR, SRA, SRB, SRC, SRD, SRS, SXPHE, ZZ, ZZA, ZZB, ZZO

3. Rotating Variable Stars
  Variable stars with nonuniform surface brightness and/or ellipsoidal shapes, whose variability is caused by axial rotation with respect to the observer. The nonuniformity of surface brightness distributions may be caused by the presence of spots or by some thermal or chemical inhomogeneity of the atmosphere caused by a magnetic field whose axis is not coincident with the rotation axis. These stars are subdivided into the following types:
  ACV, ACVO, BY, ELL, FKCOM, PSR, R, SXARI

4. Cataclysmic (Explosive and Nova-like) Variables
  These are variable stars showing outbursts caused by thermonuclear burst processes in their surface layers (novae) or deep in their interiors (supernovae). We use the term "novalike" for variables that show novalike outbursts caused by rapid energy release in the surrounding space (UG-type stars - see below) and also for objects not displaying outbursts but resembling explosive variables at minimum light by their spectral (or other) characteristics. The majority of explosive and novalike variables are close binary systems, their components having strong mutual influence on the evolution of each star. It is often observed that the hot dwarf component of the system is surrounded by an accretion disk formed by matter lost by the other, cooler, and more extended component. This category is subdivided into the following types:
  N, NA, NB, NC, NL, NR, SN, SNI, SNII, UG, UGSS, UGSU, UGZ, ZAND

5. Close Binary Eclipsing Systems
  We adopt a triple system of classifying eclipsing binary systems: according to the shape of the combined light curve, as well as to physical and evolutionary characteristics of their components. The classification based on light curves is simple, traditional, and suits the observers; the second and third classification methods take into account positions of the binary-system components in the (MV ,B-V) diagram and the degree of inner Roche lobe filling. Estimates are made by applying the simple criteria proposed by Svechnikov and Istomin (1979). The symbols for the types of eclipsing binary systems that we use are given below:
  a) Classification based on the shape of the light curve:
E, EA, EB, EP, EW
  b) Classification according to the components' physical characteristics:
GS, PN, RS, WD, WR
  c) Classification based on the degree of filling of inner Roche lobes:
AR, D, DM, DS, DW, K, KE, KW, SD

6. Optically Variable Close Binary Sources of Strong, Variable X-ray Radiation (X-ray Sources)
  AM, X, XB, XF, XI, XJ, XND, XNG, XP, XPR, XPRM, XM

7. Other Symbols
  In addition to the variable-star types described above, certain other symbols that need to be explained will be found in the Type data field:
  BLLAC, CST, GAL, L:, QSO, S, *, +, :

All of these classes include objects of a dissimilar nature that belong to different types of light variability. On the other hand, an object may be variable because of almost all of the possible reasons or because of any combination of them. If a variable belongs to several types of variability, the types are joined in the data field by a "+" sign, e.g., E+UG, UV+BY.

Despite considerable success in understanding stellar variability processes, the classification adopted in the Catalogue is far from perfect. This is especially the case for explosive, symbiotic and nova-like variables; X-ray sources and peculiar objects.

Source and credits to:
Samus N.N., Kazarovets E.V., Durlevich O.V., Kireeva N.N., Pastukhova E.N.
General Catalogue of Variable Stars: Version GCVS 5.1
Astronomy Reports, 2017, vol. 61, No. 1, pp. 80-88 {2017ARep...61...80S}