The stage where a star expands and cools after exhausting core hydrogen, often appearing red.

The stage where a star expands and cools after exhausting core hydrogen, often appearing red.

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

Answer: (B)

After a star exhausts hydrogen in its core, it leaves the main sequence. The core contracts and heats, while hydrogen continues fusing in a shell around the inert core. This extra energy pushes the outer layers outward, causing the star to swell dramatically. As the envelope expands, its surface temperature drops, so the star emits more red light and appears red. This combination of a large radius and cooler surface defines the red giant phase. Protostars are earlier pre-main-sequence objects, the main sequence is when hydrogen fusion happens in the core, and white dwarfs are compact remnants—none of these describe the expansion and cooling seen in a red giant.

After a star exhausts hydrogen in its core, it leaves the main sequence. The core contracts and heats, while hydrogen continues fusing in a shell around the inert core. This extra energy pushes the outer layers outward, causing the star to swell dramatically. As the envelope expands, its surface temperature drops, so the star emits more red light and appears red. This combination of a large radius and cooler surface defines the red giant phase. Protostars are earlier pre-main-sequence objects, the main sequence is when hydrogen fusion happens in the core, and white dwarfs are compact remnants—none of these describe the expansion and cooling seen in a red giant.