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Width of Main Sequence – and Stellar Aging
•As star converts H to He you have more massive nuclei
–Pressure related to number of nuclei
–Gravity related to mass of nuclei
•Pressure would tend to drop unless something else happens
•Temperature must rise (slightly) to compensate
•Luminosity  must  rise (slightly) as heat leaks out faster
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From our text:  Horizons, by Sees