First, careful job of calculating the time for the Sun to use up 10% of its Hydrogen fuel gives a main-sequence lifetime of 10 billion years.
|Stellar Mass||MS Lifetime|
|1 x 106 years
12 x 106 years
400 x 106 years
3 x 109 years
2 x 1012 years
The sequences in the H-R Diagram were known long before stellar evolution was understood and there were thoughts that stars might evolve along the main-sequence. But, with stellar models it is possible to let the models "evolve" and see what happens to stars as they begin to use up their H fuel in the regions where it is hot enough for fusion.
This contraction releases GPE which increases the temperature in the Hydrogen fusion shell and the total energy production goes up.
As a red giant the Sun will have these properties:
|Surface Temp||3200 K|