A star that has evolved beyond the main sequence but has not yet become a full giant star. Subgiants represent a transitional phase where the star's core hydrogen is depleted but helium fusion has not yet begun.
Formed with the prefix 'sub-' meaning 'below' or 'under' combined with 'giant' from Greek 'gigas'. The term was established in mid-20th-century stellar classification to describe this intermediate evolutionary stage between main sequence and giant phases.
Our Sun will spend about 2 billion years as a subgiant star, slowly expanding and brightening as it exhausts its core hydrogen fuel! During this phase, Earth's oceans will boil away as the Sun becomes 40% more luminous, marking the beginning of the end for life as we know it.
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