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 becomes high enough to balance the rate at which the star radiates energy into space. This leads to a stable equilibrium and the star settles into the main sequence phase of stellar evolution.

When Does a Protostar Become a Main-Sequence Star