Cepheid instability strip

PN nucleus, horizontal-branch and white-dwarf regions. The dotted line shows a schematic main sequence and evolutionary track for Population II, while various dashed lines show roughly the Cepheid instability strip, the transition to surface convection zones and the helium-shell flashing locus for Population I. After Pagel (1977). Copyright by the IAU. Reproduced with kind permission from Kluwer Academic Publishers. 

Stars less massive than about 8 M0 will avoid the supernova fate. In the 3-8 Mq range, helium ignition occurs in a non-degenerate core, so is not explosive. Core helium burning is associated with a blueward loop through the Cepheid instability strip, after which the star develops a double shell structure and becomes an asymptotic giant branch star. 

Early models suggest at least two phases of mass transfer necessary to explain the very low surface hydrogen abundances e.g. [J27]. The current best model for the evolution of v Sgr [9] is that it began as a f 0+3 Mq, f 50 d binary in which the envelope was blown to infinity, with little change of orbit, as the more massive star approached both Roche Lobe overflow and the Cepheid instability strip simultaneously. 

As a star undergoes post-main sequence evolution, it may make several passes through the instability strip in the H-R diagram. There the star becomes variable, i.e. a Cepheid variable which alternately blows up and shrinks again causing an observable brightness variation. 

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