Pulsations
Stars in the instability strip pulsate due to He III (doubly ionized helium). In normal A-F-G stars He is neutral in the stellar photosphere. Deeper below the photosphere, at about 25,000–30,000K, begins the He II layer (first He ionization). Second ionization (He III) starts at about 35,000–50,000K.
When the star contracts, the density and temperature of the He II layer increases. He II starts to transform into He III (second ionization). This causes the Opacity of the star to increase and the energy flux from the interior of the star is effectively absorbed. The temperature of the star rises and it begins to expand. After expansion, He III begins to recombine into He II and the opacity of the star drops. This lowers the surface temperature of the star. The outer layers contract and the cycle starts from the beginning.
The phase shift between a star's radial velocity pulsations and brightness variability depends on the distance of He II zone from the stellar surface in the stellar atmosphere.
Read more about this topic: Instability Strip
Famous quotes containing the word pulsations:
“Anyone informed that the universe is expanding and contracting in pulsations of eighty billion years has a right to ask, “What’s in it for me?””
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