In stellar evolution, which phase is characterized by hydrogen fusion occurring in a shell around the core, with the outer layers expanding?

In stellar evolution, which phase is characterized by hydrogen fusion occurring in a shell around the core, with the outer layers expanding?

• A. Red Giant
• B. Main Sequence
• C. Protostar
• D. White Dwarf

Answer: (A)

When a star exhausts hydrogen in its core, the core contracts while hydrogen continues fusing in a shell around that shrinking, inert core. This shell-burning adds energy, driving the outer layers to expand greatly and cool, which makes the star appear much larger and redder: the red giant phase. This pattern—hydrogen fusion in a shell around a contracting core with an expanded envelope—is characteristic of becoming a red giant, not the other stages. On the main sequence, fusion happens in the core; a protostar hasn’t begun sustained hydrogen burning yet; a white dwarf is a compact remnant after the envelope is shed and fusion has ceased.

When a star exhausts hydrogen in its core, the core contracts while hydrogen continues fusing in a shell around that shrinking, inert core. This shell-burning adds energy, driving the outer layers to expand greatly and cool, which makes the star appear much larger and redder: the red giant phase. This pattern—hydrogen fusion in a shell around a contracting core with an expanded envelope—is characteristic of becoming a red giant, not the other stages. On the main sequence, fusion happens in the core; a protostar hasn’t begun sustained hydrogen burning yet; a white dwarf is a compact remnant after the envelope is shed and fusion has ceased.