Hertzsprung gap

This diagram is a plot of effective temperature versus luminosity for a population of stars.

The gap is named after Ejnar Hertzsprung, who first noticed the absence of stars in the region of the Hertzsprung–Russell diagram[1] between A5 and G0 spectral type and between +1 and −3 absolute magnitudes.

This gap lies between the top of the main sequence and the base of red giants for stars above roughly 1.5 solar mass.

When a star during its evolution crosses the Hertzsprung gap, it means that it has finished core hydrogen burning.

Full Hertzsprung–Russell diagrams of the 11,000 Hipparcos mission targets show a handful of stars in that region.

Hertzsprung–Russell diagram with the Hertzsprung Gap visible as an area containing few stars between the main sequence and red-giant branch