Luminosity in astronomy usually involves comparisons with known brightness references like our Sun. The luminosity of Sirius is almost twice that compared to our Sun.
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Not Surface Brightness
Luminosity is the total light emitted, not just how bright a star appears on the surface. Despite its size, the luminosity of Betelgeuse is due to its total energy output.
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Measurement Complexity
Astronomical luminosity is complex, taking into account distance and inherent brightness. Astronomers study a star's spectrum to accurately determine its luminosity.