Luminosity and Intensity

Two different quantities describe how bright a star is, and it is important not to mix them up.Luminosity (L) is the total power output of a star as electromagnetic radiation, measured in watts (W). It is a property of the star itself and does not depend on where it is observed from. The Sun's luminosity is about 3.9 × 10²⁶ W.Intensity (I), also called radiation flux, is the power received per unit area at right angles to the direction of the radiation, measured in W m⁻²:I = P ÷ AIntensity is what we actually measure on Earth, and it determines how bright a star appears. It depends on the star's luminosity and also on its distance: the further away the star, the more its power has spread out and the smaller the intensity. (The next lesson gives the equation for this.)So two stars can look equally bright but have very different luminosities, if one is much further away than the other. A faint-looking star could be a very luminous star a long way off.Worked example: a telescope mirror of area 0.50 m² collects 2.0 × 10⁻⁹ W of light from a star. Find the intensity.I = 2.0 × 10⁻⁹ ÷ 0.50 = 4.0 × 10⁻⁹ W m⁻²Rearranged, P = IA gives the power collected by a detector of known area.