Correct Response: A. A main sequence star eventually uses up the hydrogen fuel in its core and switches to fusion of hydrogen in a shell surrounding the core. This initiates the transition to the red giant stage as the core contracts and its temperature increases, triggering fusion of helium. The energy produced by helium fusion in the core causes the star to expand outward to many times its original size. Although a main sequence star does expand and cool as it becomes a red giant, the core (B) actually contracts and heats up initially as hydrogen fusion comes to an end. The density and temperature are too low in the outer layers of main sequence stars to support nuclear fusion of helium (C). The hydrogen and helium in the visible surface layer of a main sequence star does not combust (D) or react in any way that could initiate red giant development.
