When a star leaves the main sequence and expands toward the red-giant region,what is happening inside the star?
A) Hydrogen burning is taking place in a spherical shell just outside the core;the core itself is almost pure helium.
B) Hydrogen burning is taking place in a spherical shell just outside the core;the core has not yet started thermonuclear reactions and is still mostly hydrogen.
C) Helium burning is taking place in a spherical shell just outside the core;the core itself is almost pure carbon and oxygen.
D) Helium is being converted into carbon and oxygen in the core.

Question

Quizplus · Accepted Answer

When a star leaves the main sequence and expands toward the red-giant phase, its core is composed almost entirely of helium, having exhausted its hydrogen fuel. The hydrogen burning then continues in a shell surrounding the helium core, which leads to the expansion and cooling of the star's outer layers, characteristic of the red giant phase.

When a Star Leaves the Main Sequence and Expands Toward