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The Blazhko Effect mystery
by Ivan Adamin in July-August 2013 (#1)
The Blazhko effect is a long-term, irregular modulation of the amplitude and period that occurs in some RR Lyrae stars and related pulsating variables The physical origin of the effect remains unknown for over a century.
RR Lyrae variables are periodic pulsating variable stars with periods of 0.2-1.1 day. They commonly found in globular clusters. The relationship between pulsation period and absolute magnitude of RR Lyraes allows to use them as "standard candles" for distance measurements to relatively near objects, particularly within the Milky Way. Having similar light curve properties to Cepheids, the RR Lyraes became distinct from them in the first half of the XX century due to shorter periods and differing locations within the galaxy. Moreover, their chemical differences and metal-poor nature means they are old Population II stars.
Sergey Blazhko (1870 - 1956), a Russian and Soviet astronomer, was the first to report this phenomenon in 1907 for RW Dra.
In 2010, interesting news came from the data analysis of the Kepler spacecraft mission. For V808 Cyg and V355 Lyr stars (which are RR Lyrae stars), the was the first detection of the period doubling phenomenon is reported. Remarkably, both of these stars exhibit Blazhko modulation as well.
The most plausible theories to explain the phenomenon mostly focus on two types of models, both involving non-radial pulsation components: the resonance models and the magnetic models. There is also the third model which assumes that cycles in the convection cause the alternations and the modulations.
More on the subject can be read in the Doug Welch's article - "The (Now) Less Mysterious Blazhko Effect in RR Lyrae Variables" and "Does Kepler unveil the mystery of the Blazhko effect?" (Szabo et al., 2010).
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