A star first reaches the main sequence when it
A) starts to collapse.
B) becomes a protostar.
C) becomes a T Tauri star.
D) attains hydrostatic equilibrium.

Question

Quizplus · Accepted Answer

A star first reaches the main sequence when it has attained hydrostatic equilibrium, which means that the opposing forces of gravity and radiation pressure are balanced. This occurs when the core of the star reaches a sufficiently high temperature and pressure to ignite hydrogen fusion reactions, thereby releasing energy in the form of light and heat. Once a star is on the main sequence, it will remain there for most of its lifetime, gradually converting hydrogen into helium in its core.

A Star First Reaches the Main Sequence When It