From here on out, we will be considering the post-Main Sequence evolution for different types of stars.
When any star has used up the majority of the hydrogen in its core, it is ready to leave the Main Sequence and begin its subsequent evolution. By the end of its Main Sequence lifetime, it will be approximately twice as luminous as it is now! For example, we expect our Sun to brighten and its color to vary slowly over its roughly 10 billion year lifetime on the Main Sequence. Over a star's Main Sequence lifetime, as it fuses hydrogen into helium, its outer envelope will respond to slow internal changes, so its position in the HR diagram is not completely fixed. When the star initially begins fusing hydrogen it is said to be on the Zero Age Main Sequence (ZAMS). The star's color (a measurement of its surface temperature) and luminosity only change slightly over the course of its Main Sequence lifetime as the rate of nuclear fusion changes as the star slowly converts hydrogen to helium. During this time, the star is fusing hydrogen in its core. The first stage of the evolution of a star is the Main Sequence stage, and this accounts for approximately 80% of the star's total lifetime. When nuclear fusion is going on in a star's core, the pressure created by this process pushes outward and balances exactly the inward pull of gravity. The most important concept to recall when studying stars is the concept of hydrostatic equilibrium.
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