The stable phase where hydrogen fuses into helium in the core.

The stable phase where hydrogen fuses into helium in the core.

• A. Death of a star
• B. Nebula
• C. Protostar
• D. Main Sequence

Answer: (D)

Hydrogen fusion in the star’s core during a long, stable period is what defines the main sequence. In this phase, the energy produced by fusing hydrogen into helium creates outward pressure that balances gravity, keeping the star’s structure and brightness relatively steady for billions of years. The specific fusion processes vary by mass—proton-proton chains dominate in smaller stars, while the CNO cycle is more important in larger ones—but the key idea is steady hydrogen burning that sustains hydrostatic equilibrium. A nebula is just the cloud of gas from which a star forms, not a persistent energy-producing phase. A protostar is an early, pre-fusion stage where the object is still contracting and hasn’t started sustained hydrogen burning. The death of a star refers to the evolutionary stages after hydrogen is exhausted, when the star leaves the main sequence and undergoes later changes.

Hydrogen fusion in the star’s core during a long, stable period is what defines the main sequence. In this phase, the energy produced by fusing hydrogen into helium creates outward pressure that balances gravity, keeping the star’s structure and brightness relatively steady for billions of years. The specific fusion processes vary by mass—proton-proton chains dominate in smaller stars, while the CNO cycle is more important in larger ones—but the key idea is steady hydrogen burning that sustains hydrostatic equilibrium.

A nebula is just the cloud of gas from which a star forms, not a persistent energy-producing phase. A protostar is an early, pre-fusion stage where the object is still contracting and hasn’t started sustained hydrogen burning. The death of a star refers to the evolutionary stages after hydrogen is exhausted, when the star leaves the main sequence and undergoes later changes.