A star may become a pulsating variable several times during its lifetime as evolution slowly changes its size, luminosity and internal structure. We can see this best by comparing theoretical evolutionary tracks with the positions of instability regions in the HR diagram (figure 4.6). We can determine which evolutionary track passes through the position of the star: hence we can deter-mine how the star reached its present position, and what its mass must be. A star may pass through the Cepheid instability strip even before it reaches the main sequence. (Two pulsating pre-main-sequence stars have been found in NGC 2264.) When it reaches the main sequence, it will not pulsate unless its effective surface temperature is about 9000 K, in which case it will be a ? Scuti star. It may pass through the Cepheid instability strip one or more times before becoming a red variable, and one or more times thereafter before becoming a white dwarf.

Evolution affects pulsation in other ways, as well. We may occasionally see a star enter or leave an instability region, and then begin or cease to pulsate. The number of evolutionary tracks which cross an instability region, and the rate at which they cross, both affect the number of pulsating variables found in that region. Finally, the period and range of pulsation may slowly change as the star evolves through the region, because both the period and the range depend on the luminosity and effective sur¬face temperature of the star. Evolution through a region usually takes millions of years, and the changes in the range are too small to detect. Changes in period can be detected, however, because they produce a cumulative discrepancy between the observed and expected times of maximum or minimum brightness. In the same way, small errors in the rate of a clock accumulate, and become readily apparent after several days. Period changes as small as one part in LO10 have been detected in this way, and provide a direct, observable confirmation of stellar evolution.

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