An Astronomer Studying a Globular Cluster Plots Its Stars on a Hertzsprung-Russell

Multiple Choice

An astronomer studying a globular cluster plots its stars on a Hertzsprung-Russell diagram and finds that certain stars in the cluster lie on the horizontal branch. What does this astronomer immediately know about these stars?

A) They are still contracting toward the main sequence.
B) They are burning hydrogen into helium in their cores.
C) They are burning helium into carbon and oxygen in their cores.
D) They are burning helium into carbon and oxygen in a shell around their cores.