♞ Stefan-Boltzmann Law

If we sum up the contributions from all parts of the electromagnetic spectrum, we obtain the total energy emitted by a blackbody. What we usually measure from a large object like a star is the energy flux, the power emitted per square meter. The word flux means “flow” here: we are interested in the flow of power into an area (like the area of a telescope mirror). It turns out that the energy flux from a blackbody at temperature T is proportional to the fourth power of its absolute temperature. This relationship is known as the Stefan-Boltzmann law and can be written in the form of an equation as

F = σT⁴

where F stands for the energy flux (in units of watts per square meter),
T is given in Kelvins, and
σ (Greek letter sigma) is a constant number
σ = 5.67×10⁻⁸ (W/m²) / K⁴.