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4.2: Luminosity

The most important differences between "standard" and "astrophysical" usages are in the meanings of flux and intensity. What we have hitherto called the "radiant flux" \(\Phi\) of a source of radiation, expressed in watts, is generally called by astronomers, when describing the radiant flux from an entire star, the luminosity of the star, and the symbol used is \(L\). While this can be expressed in watts, it is commonly expressed in units of the luminosity of the Sun, which is \(3.85 \times 10^{26} \text{watts}\).