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

Question

Quizplus · Accepted Answer

A protostar becomes a main-sequence star when the rate of hydrogen fusion in its core becomes high enough to balance the rate at which the star radiates energy into space. This is known as hydrostatic equilibrium and marks the beginning of a star's stable, long-lived existence as a main-sequence star.

When Does a Protostar Become a Main- Sequence Star