Radiated power
A source may emit substantial energy each second while a distant detector receives only a small portion. Luminosity describes the emitted power, with one watt equal to one joule per second, rather than the brightness of an image pixel.
Values can also be expressed relative to solar luminosity. Such comparisons still require identifying whether all wavelengths or a selected band are included.
Distance and received flux
For isotropic emission at nearby distances with negligible absorption, F=L/(4πd²). Doubling distance reduces received flux to one quarter for the same luminosity.
A distant luminous star can therefore look fainter than a nearby weak one. Inferring luminosity requires distance and suitable propagation assumptions, particularly when emission is directional or absorption matters.
Magnitudes and spectral distribution
Absolute magnitude makes a common-distance comparison but can still refer to a band such as V. Converting a band-limited measurement into total radiated power requires the spectral distribution.
Surveys such as Gaia combine photometry, distances and spectra to study intrinsic properties. Distance addresses geometry, while colour and spectra supply other information; one apparent brightness does not replace those measurements.