Luminosity is a measure of how much energy in the form of light (or other electromagnetic radiation) is _emitted_ by a star or galaxy in a given time interval. It is an intrinsic property of the star or galaxy itself and may be measured in the SI unit of power, which is watts (W) or equivalently joules per second (J s−1). Luminosity can be related to absolute magnitude. L = 4$\pi R^2 \sigma T^4$ [[../Chapters/Part 2 Photometry measuring brightness]] [[Docent/S284 Astronomy]] [[../Chapters/Part 2 Spectral lines]]