The Hertzsprung–Russell diagram is a scatter graph of star

Star

A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...

s showing the relationship between the stars' 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 or luminosities versus their spectral types or classifications

Stellar classification

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 and effective temperature

Effective temperature

The effective temperature of a body such as a star or planet is the temperature of a black body that would emit the same total amount of electromagnetic radiation...

s. Hertzsprung–Russell diagrams are not pictures or maps of the locations of the stars. Rather, they plot each star on a graph measuring the star's absolute magnitude or brightness against its temperature and color.

Hertzsprung–Russell diagrams are also referred to by the abbreviation H–R diagram or HRD.