What is the stable, hydrogen-fusing stage that comprises about 90% of a star's life?

What is the stable, hydrogen-fusing stage that comprises about 90% of a star's life?

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

Answer: (C)

Most of a star’s life is spent in a stable hydrogen-fusing phase called the main sequence. During this stage, hydrogen burning in the core provides the energy needed to counterbalance gravity, so the star stays in balance and maintains a fairly steady size and brightness. This fusion process turns hydrogen into helium, and for stars like the Sun it lasts about 10 billion years—roughly 90% of the star’s total lifetime. When the core hydrogen is exhausted, fusion changes character, the core contracts while the outer layers expand, and the star evolves into a red giant. After more stages, it becomes a white dwarf as it cools and fades. The other stages noted—protostar, red giant, and white dwarf—represent earlier or later phases rather than the long, stable hydrogen-fusing period.

Most of a star’s life is spent in a stable hydrogen-fusing phase called the main sequence. During this stage, hydrogen burning in the core provides the energy needed to counterbalance gravity, so the star stays in balance and maintains a fairly steady size and brightness. This fusion process turns hydrogen into helium, and for stars like the Sun it lasts about 10 billion years—roughly 90% of the star’s total lifetime. When the core hydrogen is exhausted, fusion changes character, the core contracts while the outer layers expand, and the star evolves into a red giant. After more stages, it becomes a white dwarf as it cools and fades. The other stages noted—protostar, red giant, and white dwarf—represent earlier or later phases rather than the long, stable hydrogen-fusing period.