in astronomy, the difference between the visual and radiometric magnitudes of a celestial body. Like the color index, the heat index characterizes the energy distribution in the spectrum of the body. The heat index of A0 type stars is taken as zero point. Radiometric observations are performed using detectors that register all the energy incident on them. Such detectors include bolometers, thermocouples, and radiometers. Heat indices are used to determine stellar temperatures. The concept of heat index was introduced in the 1920’s.