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 Dra.[1] The recent Kepler (spacecraft) observations caught the community by surprise. [2] In addition to the modulations the observations showed cycle to cycle alternations in the light curves.
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. [3] [4] 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. [5] The third model assumes that cycles in the convection cause the alternations and the modulations. [6]