stellar middle age and maturity

During its main-sequence life, the star changes only slightly, typically a small increase in luminosity as its

nuclear reactions speed up as a result of the changing core composition.

when a star is in its main phase, it stays the same except for shining brighter over time

Since the luminosity can be predicted from the mass, we can also predict the main sequence lifetime from the ratio of mass (the fuel supply) to the luminosity (the rate of use). It turns out that more massive stars, even though they have more fuel, burn faster and have shorter lives.

more massive stars burn through their fuel(mass) faster (higher luminosity)

Core fusion turns off when the fuel is exhausted; core contraction follows, releasing gravitational energy which heats the core and ignites shell fusion. These changes make the surface of the star expand rapidly and cool, making the star a red giant (or supergiant).

when a stars core runs out of fuel, it shrinks and releases energy, igniting a new fire outside that makes the star expand and cool down, turning it into a supergiant

If the core gets hot enough, new core fusion begins, the triple-alpha process, for example. In this case, three helium nuclei can be fused into a carbon if the temperature reaches 100 million K. The core contraction is temporarily halted at this stage and the exterior expansion and cooling is briefly reversed.

when the core of a star gets really hot and stars fusing, it briefly stops shrinking and expanding, and warms up a bit

When the new fusion runs out of fuel, the star repeats the two steps above. The greater the star's mass, the more times it can cycle through these steps, fusing still heavier nuclei. Ultimately the core collapses and the outer layers of the star are expelled.

when the burning in the core runs out of fuel again, the star goes back to shrinking and expanding. bigger stars repeat the cycle more times