In this equation, λ represents the wavelength (color) of light, and T the surface temperature of a star. Since wavelength is a size, it is measured in meters, or billionths of a meter (nm) in the equation, and surface temperature is in Kelvins, the temperature scale used in astronomy. To make the equation an equality, we need the constant,
2,900,000 nm • K that tells us how the two units relate to each other in this relationship.