The Hertzsprung gap is a feature of the Hertzsprung–Russell diagram

Hertzsprung–Russell diagram

The Hertzsprung–Russell diagram is a scatter graph of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their spectral types or classifications and effective temperatures. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars...

 for a star cluster. It is named after Ejnar Hertzsprung

Ejnar Hertzsprung

Ejnar Hertzsprung was a Danish chemist and astronomer.Hertzsprung was born in Copenhagen. In the period 1911–1913, together with Henry Norris Russell, he developed the Hertzsprung–Russell diagram....

 who first noticed the absence of stars in the region of the Hertzsprung–Russell diagram between A5 and G0 spectral type and between +1 and -3 absolute magnitude

Absolute magnitude

Absolute magnitude is the measure of a celestial object's intrinsic brightness. it is also the apparent magnitude a star would have if it were 32.6 light years away from Earth...

s (i.e. between the top of the main sequence

Main sequence

The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...

 and the red giant

Red giant

A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...

s for stars above roughly 1.5 solar mass). When a star during its evolution

Stellar evolution

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Depending on the mass of the star, this lifetime ranges from only a few million years to trillions of years .Stellar evolution is not studied by observing the life of a single...

 crosses the Hertzsprung gap, it means that it has finished core hydrogen burning

Proton-proton chain reaction

The proton–proton chain reaction is one of several fusion reactions by which stars convert hydrogen to helium, the primary alternative being the CNO cycle. The proton–proton chain dominates in stars the size of the Sun or smaller....

 but has not yet started core helium burning

Helium fusion

Helium fusion is a kind of nuclear fusion, with the nuclei involved being helium.The fusion of helium-4 nuclei is known as the triple-alpha process, because fusion of just two helium nuclei only produces beryllium-8, which is unstable and breaks back down to two helium nuclei with a half-life of...

.

Stars do exist in the Hertzsprung gap region, but because they move through this section of the Hertzsprung–Russell diagram very quickly in comparison to the lifetime of the star (thousands of years, compared to tens of millions of years for the lifetime of the star), that portion of the diagram is less densely populated. Full Hertzsprung–Russell diagrams of the 11,000 Hipparcos

Hipparcos

Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...

mission targets show a handful of stars in that region.