Subgiant
A supergiant is a star with a higher absolute brightness ( luminosity class IV ) as a dwarf star ( luminosity class V ) of the same spectral type and a lower absolute magnitude than a giant star ( luminosity class III). In the Hertzsprung -Russell diagram, the subgiant lie above the main sequence, on which there are the dwarf stars.
It is believed that this is stars that end their fusion of hydrogen into helium inside straight or have already completed. In stars of similar mass as the sun, this causes a contraction of the core, causing temperatures to rise inside the star. The temperature increase leads to the fusion of hydrogen into a shell surrounding the core ( shell firing). This swells the star to become a giant. At the beginning of the subgiant stage increase in diameter and luminosity. At this stage a star is further cooled and changes its color accordingly. An example of a star at this stage is Procyon A.
Refined sub- giants, in a development phase through to giant stars, have larger diameters and lower temperatures than stars of comparable mass in the main sequence (main sequence stars, dwarf stars ). The overall brightness changed little during the subgiant stage, a clear mark in the Hertzsprung -Russell diagram of globular clusters.