Question

Once a star has evolved onto the Main Sequence in the HR diagram, what happens?

1) It remains on the Main Sequence for the rest of its life
2) It expands and becomes a red giant
3) It collapses and becomes a white dwarf
4) It explodes in a supernova

Answer 1

The answer is 2) It expands and becomes a red giant.

How to explain

The HR diagram is a graph that shows the relationship between the luminosity and surface temperature of stars. Stars that are fusing hydrogen in their cores lie on the Main Sequence.

Once a star has evolved onto the Main Sequence, it will remain there for the majority of its life.

However, eventually, the hydrogen in the core will run out, and the star will begin to fuse helium in its core. This will cause the star to expand and cool, becoming a red giant.