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Stellar pulsation
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<p>Stellar pulsations are caused by expansions and contractions in the outer layers as a star seeks to maintain <a href="/facts/Stellar_equilibrium/6bPfevnc">equilibrium</a>. These fluctuations in <a href="/facts/Stellar_radius/PFbd6ICx">stellar radius</a> cause corresponding changes in the <a href="/facts/Stellar_luminosity/1tdhpk9l">luminosity of the star</a>. Astronomers are able to deduce this mechanism by measuring the <a href="/facts/Stellar_spectrum/NDGQ7mmM">spectrum</a> and observing the <a href="/facts/Doppler_effect/bzrK1BKf">Doppler effect</a>. Many intrinsic <a href="/facts/Variable_stars/Ac8mp2gy">variable stars</a> that pulsate with large <a href="/facts/Amplitude/aEckuXJM">amplitudes</a>, such as the classical <a href="/facts/Cepheids/WQXaAFmW">Cepheids</a>, <a href="/facts/RR_Lyrae_variable/vjoHuKUe">RR Lyrae</a> stars and large-amplitude <a href="/facts/Delta_Scuti/bL3EAzDK">Delta Scuti</a> stars show regular <a href="/facts/Light_curves/zN0wdtaR">light curves</a>. </p><p>This regular behavior is in contrast with the variability of stars that lie parallel to and to the high-luminosity/low-temperature side of the classical variable stars in the <a href="/facts/Hertzsprung%E2%80%93Russell_diagram/53YWnCvR">Hertzsprung–Russell diagram</a>. These giant stars are observed to undergo pulsations ranging from weak irregularity, when one can still define an average cycling time or <a href="/facts/Periodic_function/Sm8lJQsF">period</a>, (as in most <a href="/facts/RV_Tauri_variable/AGdSsXfH">RV Tauri</a> and <a href="/facts/Semiregular_variables/RcJ9Qw9E">semiregular variables</a>) to the near absence of repetitiveness in the <a href="/facts/Irregular_variable/xWkauC0E">irregular</a> variables. The <a href="/facts/W_Virginis_variables/KDvLcEGo">W Virginis variables</a> are at the interface; the short period ones are regular and the longer period ones show first relatively regular alternations in the pulsations cycles, followed by the onset of mild irregularity as in the RV Tauri stars into which they gradually morph as their periods get longer. Stellar evolution and pulsation theories suggest that these irregular stars have a much higher luminosity to mass (L/M) ratios. </p><p>Many stars are non-radial pulsators, which have smaller fluctuations in brightness than those of regular variables used as standard candles. </p>