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Stothers, R., and N.R. Simon, 1969: An explanation for the blue sequence of variable stars. Astrophys. J., 157, 673-681.
Nuclear energy of the stellar core is found to be the direct source of energy fed into radial pulsations in two classes of blue variable stars. The μ-mechanism (reversal of the gradient of mean molecular weight, which reduces the central condensation) or the β-mechanism (high radiation pressure) can explain the basic variability of β Cephei stars if they are burning hydrogen in their cores, have accreted a helium-rich envelope, and are the luminous secondaries of close binary systems. If the composition reversal does not lead to significant mixing in the envelope, the μ-mechanism will operate in inhomogeneous stars of 6-95 M☉. If rapid mixing takes place, the β-mechanism will operate in completely homogeneous stars of 12-60 M☉. The β-mechanism can also explain the variability of many Wolf-Rayet stars if they are burning helium in their cores, have masses in excess of ~8 M☉ with a thin or absent hydrogen envelope, and are the luminous primaries of close binary systems.
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