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# 1.2: Radiant Flux or Radiant Power, $$\phi$$ or P
It is particularly unfortunate that, even with this most fundamental of concepts, astronomical usage is often different. When describing the radiant power of stars, it is customary for astronomers to use the word luminosity, and the symbol $$L.$$ In standard usage, the symbol $$L$$ is generally used for the quantity known as radiance, while in astronomical custom, the word "flux" has yet a different meaning. Particle physicists use the word “luminosity” in yet another quite different sense.
The radiant power ("luminosity") of the Sun is $$3.85 \times 10^{26} \text{W}$$.