Question

What point defines the end of the pre-main-sequence phase of a star's life and the start of the main-sequence phase?

A) The star begins to expand and become a red giant.
B) Convection begins in its interior.
C) The star stops accreting mass from the interstellar cloud.
D) Nuclear reactions begin in its core.

Answer 1

The transition from pre-main-sequence to main-sequence phase occurs when hydrogen fusion starts in the star's core, initiating a long period of stellar equilibrium and energy production.

Step-by-step explanation:

The end of the pre-main-sequence phase and the start of the main-sequence phase of a star's life is defined by the point at which nuclear reactions begin in its core, specifically when the central temperature becomes high enough (around 10 million Kelvin) for hydrogen to fuse into helium. This transition signifies that the star has reached a state of equilibrium and will remain relatively stable for most of its life, a period which some astronomers analogize as the star's 'prolonged adolescence' or 'adulthood'. Thus, the correct answer to the question is D) Nuclear reactions begin in its core.