A graphical representation of the absolute magnitude of stars (usually along the y-axis) plotted against the spectral class or colour index (x-axis). The y-axis then represents the energy output of the star and the x-axis its surface temperature. The majority of stars on such a diagram fall on a band running from the top left to the bottom right of the graph. These are called main-sequence stars (the sun falls into this class). The few stars falling in the lower left portion are called white dwarfs. The giant stars fall in a cluster above the main sequence and the supergiants are above them. The diagram, which was first devised in 1911 by Ejnar Hertzsprung (1873–1969) and in 1913 by Henry Norris Russell (1897–1957), forms the basis of the theory of stellar evolution.
Subjects: Astronomy and Astrophysics — Physics.