Question

if a horizontal branch star burns helium at a higher temperature than a red giant star burns hydrogen, why is a red giant star more luminous? red giant stars are younger, and therefore brighter. stars with a lower temperature emit more energy. red giant stars are bigger and give off more total light. red giant stars burn hydrogen in a shell around the core, so the photons escape more quickly than in a horizontal branch star which is burning helium at its core.

Explanation:

Luminosity depends on both temperature and surface - area. Red giant stars are much larger in size compared to horizontal - branch stars. Even though horizontal - branch stars may have a higher temperature, the large surface - area of red giants allows them to emit more total light.

Answer:

Red giant stars are bigger and give off more total light.