Hertzsprung gap

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The Hertzsprung gap is a feature of the HR diagram for a star cluster. It is named after Ejnar Hertzsprung who first noticed the absence of stars in the region of the HR diagram between A5 and G0 spectral type and between +1 and -3 absolute magnitudes (i.e. between the top of the main sequence and the 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 but has not yet started core helium burning.

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