The Hertzsprung gap is a sparsely populated region of the Hertzsprung-Russell diagram traversed rapidly by many stars evolving away from the main sequence toward the red giant region.
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The Hertzsprung gap, also known as the Hertzsprung-Russell gap, refers to a region in the Hertzsprung-Russell diagram (HR diagram) that is largely empty of stars. The HR diagram plots stars according to their absolute brightness (or luminosity) and their surface temperature (or spectral class). In this diagram, most stars fall along a diagonal band known as the main sequence, where they spend the majority of their lifetimes fusing hydrogen into helium.