Semiregular variable stars are giants or supergiants of intermediate and late spectral type showing considerable periodicity in their light changes, accompanied or sometimes interrupted by various irregularities. Periods lie in the range from 20 to more than 2000 days, while the shapes of the light curves may be rather different and variable with each cycle. The amplitudes may be from several hundredths to several magnitudes (usually 1-2 magnitudes in the V filter).
Semiregular variables are classified in several subtypes:
- SRA: Spectral-type (M, C, S or Me, Ce, Se) giants displaying persistent periodicity and usually small amplitude, less than 2.5 magnitudes in V. Z Aquarii is an example of this class. Amplitudes and light-curve shapes generally vary and periods are in the range of 35–1200 days. Many of these stars differ from Mira variables only by showing smaller light amplitudes.
- SRB: Spectral-type (M, C, S or Me, Ce, Se) giants with poorly defined periodicity (mean cycles in the range of 20 to 2300 days) or with alternating intervals of periodic and slow irregular changes. Some may occasionally cease varying at all for a time. RR Coronae Borealis and AF Cygni are examples of this behavior. Every star of this type may usually be assigned a certain mean period. In a number of cases, the simultaneous presence of two or more periods of light variation is observed.
- SRC: Spectral-type (M, C, S or Me, Ce, Se) supergiants with amplitudes of about 1 mag and periods of light variation from 30 days to several thousand days. Mu Cephei and Betelgeuse are bright examples of this class.
- SRD: Giants and supergiants of F, G, or K spectral types, sometimes with emission lines in their spectra. Amplitudes of light variation are in the range from 0.1 to 4 mag, and the range of periods is from 30 to 1100 days. SX Herculis and SV Ursae Majoris are examples of this class. The globular cluster M13 contains a dozen red variable stars from 11.95 to 12.25 visual magnitude, and with period of 43 days (V24) to 97 days (V43).
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