The Blazhko effect is a phenomenal astronomical effect which refers to the change in period and amplitude in “RR Lyrae” type variable stars. The Blazhko effect was first noticed by Sergey Blazhko in 1907 on observing a star named RW Draconis. It is sometimes also called a long-period modulation effect.
EARLY HYPOTHESIS – BLAZHKO EFFECT
The physics behind the Blazhko effect is still considered a matter of debate and revolves primarily on below 3 hypotheses:
- The first hypothesis referred to it as a Resonance model. The cause of the change is non-linear resonance. The non-linear resonance is either among the first overtone pulse mode or the fundamental tone of the star and a higher mode.
- The second hypothesis is known as the Magnetic model. It assumes the variation to be caused by the magnetic field instead. The magnetic model was ruled out in the year 2004 with the rise of high-resolution Spectro-polarimetric observations.
- The third hypothesis model assumes that cycles of the convection cause the alternations and such modulations.
In the first two hypotheses (Resonance model & Magnetic model), both models generally predicted the existence of non-radial oscillations. Many theorists have found it difficult to predict which non-radial modes are exactly seen in the process.
Some observational evidence was based on Kepler observations. His observations indicated that much of the Blazhko effect’s light curve modulation is due to simple period-doubling. Many RR Lyrae stars are seen to have a variability period of approximately 12 hours. The ground-based astronomers make nightly observations of about 24 hours apart. Thus, this period-doubling results in brightness maximums during night observations. The night maximums are significantly different from the daytime maximum.
RR LYRAE CATEGORIZATION – BLAZHKO EFFECT
Around 1900, Solon Bailey divided the RR Lyrae stars into three main groups. They were RRa, RRb, and RRc. The groups were based upon the amplitude and skewness (a kind of deviation) of the light curves. It was later noticed that RRa and RRb stars pulsate in the same mode. They both pulsate in the fundamental mode. Thus, they were merged into one ‘RRab’ type.
The pulsation periods of the fundamental-mode of RR Lyrae stars range from about 0.3 to 1.0 days. A large amount of them had periods longer than 0.45 days. The distribution of periods is related to the metallicity of RR Lyrae stars. It means more metal-poor variables have on average larger periods of pulsation.
WHAT EXACTLY IS RR LYRAE?
RR Lyrae variables are periodic variable stars. A variable star is a star whose brightness fluctuates as seen from Earth. A variable star is commonly found in globular clusters; which is a group of bright shining stars. Stars in a globular cluster are so closely associated that they look globular in shape.
Globular clusters are used as standard candles to measure distances in the galaxies. These are assisted with the cosmic distance ladder. The first star similar to the RR Lyrae type found outside a cluster was U Leporis discovered by J. Kapteyn in 1890.
The prototype star RR Lyrae was discovered before 1899 by Williamina Fleming. A prototype star is the first star (basic star) from which other stars were derived. The RR Lyrae have been difficult to observe in external galaxies because of their internal faintness.
A researcher named Walter Baade failed to find the same in the Andromeda Galaxy. This failure led him to suspect that the galaxy was much far away than it was predicted to be. By using the Canada-France-Hawaii Telescope in the 1980s, Pritchet & Van den Bergh found RR Lyraes in Andromeda’s galactic halo; and that too in their globular clusters.
POST RESEARCH – BLAZHKO EFFECT
More than a decade passed before Prager and Shapley in 1916 reported their observations that the light curve of RR Lyraes was modulated with a longer secondary period. Around 20%–30% of RR Lyrae variables of Bailey type ab show secondary modulations. This was known as the Blazhko effect. The period of secondary modulations of the Bailey type ab stars of the Blazhko effect ranged from 11 to 533 days.
In the past few years, some secondary modulations similar to the Blazhko effect have also been discovered. These modulations were for a small percentage of the RR Lyrae stars of Bailey type “c”. The recent reviews of the phenomenon have been given by Szeidl, Smith, and Kovacs. According to them in the case of RR Lyrae, the length of the Blazhko cycle is about 41 days. Although many more models have been proposed, there is still no accurate explanation for the Blazhko phenomenon.
RR Lyrae stars that exhibit multiple periodism come in several types. Some are double‐mode stars, for which the light curve can be described as a combination of radial fundamental (n=0) and first‐overtone modes (n=1). Others may be multiperiodic RR Lyrae stars which show secondary periods. These secondary periods could be much longer than the primary.
Fourier analyzed and identified two characteristic patterns among the RR Lyrae. These secondary patterns were of longer secondary periods. The Blazhko effect stars tend to show a triplet structure in their frequency spectrum.
There are even stars that did not show the Blazhko effect, these were called non-Blazhko stars. The non-Blazhko stars do not show modulations as well. The light curves of non-Blazhko stars can be replicated with a good degree of accuracy. But this reproduction is not perfect and small departures can be seen when the data is analyzed in more detail.
There are several observations that may in the future help test theoretical models of the Blazhko effect. It is still difficult to identify non-radial pulsation modes in RR Lyrae stars. This difficulty is for the observation that was based only on photometric observations.