The Blazhko effect, which is sometimes called long-period modulation, is a variation in period and amplitude in RR Lyrae type variable stars. It was first observed by Sergey Blazhko in 1907 in the star RW Draconis.
The physics behind the Blazhko effect is currently still a matter of debate, with there being three primary hypotheses. In the first, referred to as the resonance model, the cause of the modulation is a non-linear resonance among either the fundamental or the first overtone pulsation mode of the star and a higher mode.The second, known as the magnetic model, assumes the variation to be caused by the magnetic field being inclined to the rotational axis, deforming the main radial mode.  The third model assumes that cycles in the convection cause the alternations and the modulations.
Observational evidence based on Kepler (spacecraft) observations indicates much of the Blazhko effect's two-cycle light curve modulation is due to simple period-doubling. Many RR Lyrae stars have a variability period of approximately 12 hours and ground based astronomers typically make nightly observations about 24 hours apart; thus period-doubling results in brightness maximums during nightly observations that are significantly different than the daytime maximum.
- Horace A. Smith (2004). RR Lyrae Stars. Cambridge University Press. p. 103. ISBN 0-521-54817-9.
- Blazhko, S. (1907), "Mitteilung über veränderliche Sterne", Astronomische Nachrichten, 175: 325, Bibcode:1907AN....175..325B
- Kolláth, Z.; Molnár, L.; Szabó, R. (2011), "Period-doubling bifurcation and high-order resonances in RR Lyrae hydrodynamical models", MNRAS, 414: 1111, arXiv: , Bibcode:2011MNRAS.414.1111K, doi:10.1111/j.1365-2966.2011.18451.x
- Buchler, J. R.; Kolláth, Z. (2011), "On the Blazhko Effect in RR Lyrae Stars", ApJ, 731: 24, arXiv: , Bibcode:2011ApJ...731...24B, doi:10.1088/0004-637x/731/1/24
- Katrien Kolenberg (2008). "Explanations for the Blazhko effect in RR Lyrae stars". The Blazhko Project. Retrieved 2008-12-17.
- Stothers, R. B. (2010), "Observational Evidence of Convective Cycles as the Cause of the Blazhko Effect in RR Lyrae Stars", PASP, 122: 536, Bibcode:2010PASP..122..536S, doi:10.1086/652909
- Szabó, R.; Kolláth, Z.; Molnár, L.; et al. (2010), "Does Kepler unveil the mystery of the Blazhko effect? First detection of period doubling in Kepler Blazhko RR Lyrae stars", MNRAS, 409: 1244, arXiv: , Bibcode:2010MNRAS.409.1244S, doi:10.1111/j.1365-2966.2010.17386.x
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