variable star

variable star Any star that varies in brightness. Two broad categories are recognized: extrinsic variables, which vary for a mechanical reason (e.g. rotation); and intrinsic variables, which undergo a real change in luminosity of either an individual star or some element in a binary system. Certain stars may combine both forms of variation. The standard reference for the classification of types of variable stars and their nomenclature is the General Catalogue of Variable Stars (GCVS). The current total of designated variables is over 40 000.

The classification of variable stars was originally based upon the form of their light-curve, its amplitude (1), and periodicity (or lack of it). Increasingly, however, the physical mechanisms that underlie the different forms of variation, or the physical structure of the stars (and binaries) are used to define six groups of variables, each with specific features. The groups are subdivided into individual types of variation, often named after specific stars, and commonly referred to by capital-letter abbreviations, as listed in Table 8, Appendix. Some stars exhibit more than one form of variability, in which case a combined abbreviation is used, such as E+UG, BY+UV, or EA+UV+BY. Eclipsing binaries are classified in three ways: by their light curve, their physical characteristics, and the evolutionary state of their components. For these, combined abbreviations of the form E/DM, EA/DS/RS, EB/AR, EW/KW are used.

Eruptive variables exhibit unpredictable changes in the form of flares or fades, most of which originate in chromospheric or coronal activity. The group includes flare stars, Gamma Cassiopeiae stars, Orion variables, R Coronae Borealis stars, and T Tauri stars.

Pulsating variables expand and contract or experience wave-like motion of the surface because of fluctuations in the flow of energy from their interiors. Notable examples are Cepheid variables, Mira stars, RR Lyrae stars, RV Tauri stars, semiregular variables, and ZZ Ceti stars.

Rotating variables are a small group whose variations arise from non-uniform surface brightness or ellipsoidal shape.

Cataclysmic variables generally exhibit powerful outbursts with a sudden release of energy. The group includes novae, dwarf novae, and supernovae, and should not be confused with eruptive variables.

Eclipsing variables are binary stars which exhibit partial or total eclipses of one or both components. They are subdivided, on the basis of the shape of the light-curve, into Algol stars, Beta Lyrae stars, and W Ursae Majoris stars. This group also includes some systems that do not eclipse, but where the distorted shape of the components produce fluctuations in the light-curve.

Optically variable X-ray sources bear many similarities to cataclysmic variables (specifically to cataclysmic binaries). The optical variations are often induced by the X-ray variability. Two examples are the AM Herculis stars (or polars) and the HZ Herculis stars (or X-ray pulsars).