When does a star become a main- sequence star?
A)the instant when hydrogen fusion first begins in the star's core
B)when the rate of hydrogen fusion in the star's core is high enough to sustain gravitational equilibrium
C)when the protostar assembles from its parent molecular cloud
D)when a star becomes luminous enough to emit thermal radiation
E)when hydrogen fusion is occurring throughout the star's interior

Question

Quizplus · Accepted Answer

A main-sequence star is one where the rate of hydrogen fusion in the core is high enough to sustain gravitational equilibrium. This marks the beginning of the star's stable phase, during which it will reside for the majority of its lifetime. It is not sufficient for fusion to simply begin in the core; it must occur at a sufficient rate to balance the inward force of gravity.

When Does a Star Become a Main- Sequence Star