When does a protostar become a main-sequence star?
A) When the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space
B) When a piece of a molecular cloud first begins to contract into a star
C) When it becomes luminous enough to emit thermal radiation
D) At the instant that the first hydrogen fusion reactions occur in the protostar's core

Question

Quizplus · Accepted Answer

A protostar becomes a main-sequence star when the rate of hydrogen fusion in the core becomes high enough to balance the rate at which the star radiates energy into space. This is the point at which the star achieves a stable balance between the inward pull of gravity and the outward pressure of the nuclear fusion reactions in its core. This marks the beginning of the star's main-sequence stage, which can last for billions of years.

When Does a Protostar Become a Main-Sequence Star