Classification

When a new variable is discovered, it is given a name according to the constellation in which it is located. The name of the first variable in a given constellation is R, followed by the name of the constellation (in the genitive case). The symbol for the second variable is S, and so on, to Z. After these, the two-letter symbols RR, RS,... to ZZ are used, and then AA to QZ (omitting I). This is only enough for 334 variables, a number that has long been exceeded in most constellations. The numbering therefore continues: V335, V336, etc. (V stands for variable). For some stars the established Greek letter symbol has been kept, although they have later been found to be variable (e. g. 8 Cephei).

The classification of variables is based on the shape of the lightcurve, and on the spectral class and observed radial motions. The spectrum may also contain dark absorption lines from material around the star. Ob servations can be made outside the optical region as well. Thus the radio emission of some variables (e. g. flare stars) increases strongly, simultaneously with their optical brightness. Examples of radio and X-ray variables are the radio and X-ray pulsars, and the X-ray bursters.

Variables are usually divided into three main types: pulsating, eruptive and eclipsing variables. The eclipsing variables are binary systems in which the components periodically pass in front of each other. In these variables the light variations do not correspond to any physical change in the stars. They have been treated in connection with the binary stars. In the other variables the brightness variations are intrinsic to the stars. In the pulsating variables the variations are due to the expansion and contraction of the outer layers. These variables are giants and supergiants that have reached an unstable stage in their evolution. The eruptive variables are usually faint stars ejecting mass.

Fig. 13.3. The variation of brightness, colour and size of a cepheid during its pulsation

They are mostly members of close binary systems in which mass is transferred from one component to the other.

In addition a few rotating variables are known, where the brightness variations are due to an uneven temperature distribution on the surface, starspots coming into sight when the star rotates. Such stars may be quite common - after all, our Sun is a weak rotating variable. The most prominent group of rotating variables are the magnetic A stars (e. g. the a2 Canum Venatico-rum stars). These stars have strong magnetic fields that may be giving rise to starspots. The periods of rotating variables range from about 1 day to 25 d, and the amplitudes are less than 0.1 mag.

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