Everything about Instability Strip totally explained
Instability strip is a nearly vertical region in
HR diagram which is occupied by pulsating
variable stars (including
RR Lyrae variable,
Cepheid variable,
W Virginis variable,
ZZ Ceti variable,
RV Tauri variable,
Delta Scuti variable,
SX Phoenix variable and rapidly oscillating Ap stars).
The instability strip intersects the
main sequence in the region of A and F stars (1-2
solar mass) and extends upwards almost vertically (slightly inclined to the right) to the highest
luminosities. The lower part of instability strip appears as the
Hertzsprung gap on the
HR diagram.
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 increase. He II starts to transform to He III (second
ionization).
Opacity increases and the
energy flux from the interior of the star is effectively absorbed. The temperature of the layer increases and it starts to expand. After expansion, density and temperature decrease and He III begins to
recombine into He II. 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.
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