semiregular variable

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A type of giant or supergiant pulsating variable, with intermediate or late spectra, whose variations show a greater or lesser degree of regularity; abbr. SR. The amplitudes (average 1–2 mag.) are less than those of the Mira stars, and the mean periods range from 20 days to more than 2000 days. Four subtypes are defined. SRA are giants with reasonably persistent periodicity, although amplitudes (less than 2.5 mag.) and the form of the light-curves vary. SRB are giants with occasional periodicity interspersed with intervals of slow, irregular variations (sometimes with larger amplitude) or even constant brightness. SRC are supergiants with moderate periodicity and low amplitudes (about 1 mag.). These three types have M, C, S, or Me, Ce, Se spectra. SRD are giants and supergiants with F, G, or K spectra, sometimes with emission lines, and amplitudes of 0.1–4 mag.

Subjects: Astronomy and Astrophysics.

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