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RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and are used as standard candles to measure (extra)galactic distances. This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra. RR Lyraes are pulsating horizontal branch stars of spectral class A (and sometimes F), with a mass of around half the Sun's. They are thought to have previously shed mass and consequently, they were once stars with similar or slightly less mass than the Sun, around 0.8 solar masses. RR Lyrae stars pulse in a manner similar to Cepheid variables, so the mechanism for the pulsation is thought to be similar, but the nature and histories of these stars is thought to be rather different. In contrast to classical Cepheids, RR Lyraes are old, relatively low mass, metal-poor "Population II" stars. They are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae is about 0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, sometimes ranging down to seven hours. The RR Lyrae subtype RRab (see Subtypes section below) sometimes exhibit the Blazhko effect in which there is a conspicuous phase and amplitude modulation. The prototype RR Lyrae itself exhibits this behavior. The period of pulsation and absolute magnitude of RR Lyraes makes them good standard candles for relatively nearby targets, especially within the Milky Way and Local Group. Beyond the Milky Way they are difficult to detect due to their low luminosity. They are extensively used in globular cluster studies, and also used to study chemical properties of older stars. ==Abundances and distribution== RR Lyrae stars were formerly called "cluster variables" because of their strong (but not exclusive) association with globular clusters; conversely, about 90% of all variables known in globular clusters are RR Lyraes. RR Lyrae stars are found at all galactic latitudes, as opposed to classical Cepheids, which are strongly associated with the galactic plane. Several times as many RR Lyraes are known as all Cepheids combined; in the 1980s, about 1900 were known in globular clusters. Some estimates have about 85000 in the Milky Way.〔Smith, Horace A., ''(RR Lyrae Stars )'', Cambridge (2004)〕 Though binary star systems are common for typical stars, RR Lyrae are very rarely observed in pairs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「RR Lyrae variable」の詳細全文を読む スポンサード リンク
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