How Does a Main Sequence Star Evolve?
Now, we’ve said that the hydrogen fusion reactions produce helium, and so over a period of time the concentration of helium in any given star will increase and physics dictates that this helium will migrate to the core. As more and more helium is produced it becomes compressed in the core and becomes hotter and hotter, until eventually a point is reached when the helium itself is able to begin burning. This burning takes place via another nuclear reaction which requires these higher temperatures in order to proceed, and this reaction produces carbon and oxygen. All the time this process is going on, hydrogen from the star also continues to burn in a thin shell which surrounds the helium core. The rate of energy generation is now much greater than it was when the star was only burning hydrogen, and so the star is correspondingly brighter. More energy is also flowing from the core to the surface, and this increased pressure causes the outer layers of the star to swell, sometimes to an en
