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A subgiant star is brighter than a normal main sequence star of the same spectral type would be, but not as bright as such a giant star. The stellar luminosity class IV indicates a subgiant star. Some somewhat-low mass stars (e.g., the Sun) have a phase in their stellar evolution when they fit this category. The phase, termed the subgiant branch (SGB), is their time transitioning from the main sequence to the red-giant branch (RGB). On the H-R diagram (HRD), they fall in a relatively empty region (due to the short lifetime of this phase) termed the Hertzsprung gap (HG) and the stars are sometimes referred to as HG stars.