The stellar output
 
If a star is considered simply as a spherical source radiating as a black body, its total energy output can be determined by equation (1), according to its surface temperature and its surface area.
E_btot = σ T ⁴ Wm⁻² (1)
This total output is referred to as the stellar luminosity, L, and may be expressed as
L = 4π R²σ T ⁴ W (2)
where R is the radius of the star.
A typical value of stellar luminosity may be of the order of 10²⁷ W, that of the Sun being 3·85 × 10²⁶ W. The power received per unit area at the Earth depends on the stellar luminosity and on the inverse square of the stellar distance. If the latter is known, the flux provided by the source may be readily calculated and expressed in terms of watts per square metre (W m⁻²). More usually, the flux density from a point source such as a star is defined as the power received per square metre per unit bandwidth within the spectrum, i.e.Wm⁻² Hz⁻¹, with the bandpass expressed in terms of a frequency interval, or W m⁻² Δλ⁻¹, with the selected spectral interval expressed in terms of wavelength (m). If an extended source is considered, then this pair of expressions would be rewritten asWm−2 Hz⁻¹ sr⁻¹ and W m⁻² Δλ⁻¹ sr⁻¹, respectively.