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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 $$3.85 \times 10^{26} \text{watts}$$.