The brightness of an extended astronomical object, such as a planet, nebula, galaxy, or the sky background, expressed as magnitudes per unit area. Surface brightness is calculated by dividing the object’s magnitude by its dimensions. For example, the average surface brightness of the planetary nebula M57 is given as magnitude 17.6 per square arc second, compared with 5.2 for Jupiter and 23.0 for the darkest night sky. Contours of surface brightness are termed isophotes.