The measured total of all radiation at all wavelengths from a star is called a bolometric magnitude. The corrections required to reduce visual magnitudes to bolometric magnitudes are large for very cool stars and for very hot ones but are relatively small for stars such as the Sun. A determination of the true total luminosity of a star affords a measure of its actual energy output. When the energy radiated by a star is observed at the Earth, only that portion to which the energy detector is sensitive and that can be transmitted through the atmosphere is recorded. Most of the energy of stars like the Sun is emitted in spectral regions that can be observed from the Earth's surface; but a cool dwarf star with a surface temperature of 3,000 K has an energy maximum on a wavelength scale at 10,000 Angstroms in the far-infrared, and most of its energy cannot therefore be measured as light. Bright, cool stars can be observed at infrared wavelengths, however, with special instruments that measure the amount of heat radiated by the star. Corrections for the heavy absorption of the infrared waves by water and other molecules in the Earth's air must be made unless an infrared payload has been lofted by balloon or rocket above the atmosphere.
The hotter stars pose more difficult problems, since the Earth's atmosphere extinguishes all radiation at wavelengths shorter than 2900 Angstroms. A star whose surface temperature is 20,000 K or above radiates most of its energy in the inaccessible ultraviolet part of the electromagnetic spectrum. Measurements made with detectors flown in rockets or orbited in satellites extend the observable wavelength region down to 1000 Angstroms or less, though most radiation of distant stars is extinguished below 912 Angstroms --a region in which absorption by neutral hydrogen atoms in intervening space becomes effective.
To compare the true luminosities of two stars, the appropriate bolometric corrections must first be added to each of their absolute magnitudes. The ratio of the luminosities can then be calculated.
Excerpt from the Encyclopedia Britannica without permission.