The luminosity of a star or a celestial body in general, is a measure of the emitted radiation.
In particular apparent luminosity is said to the luminosity with which a celestial source appears to us. This does not constitute a measure of the light energy effectively emitted by the source, because it is a function of distance. In other words, two stars that emit the same light energy can appear to us with different apparent luminosity, because they are at different distances from our observation point.
Absolute luminosity is instead a measure of the light energy actually emitted by the source, regardless of distance. The stars have been subdivided into classes of luminosity or size or, as they say more often, of magnitude.
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