Measure if the brightness of a star or other celestial body. The birghter the object the lower the number assigned as a magnitude.One magnitude is defined as a ratio of of brightness of 2.512 timese.g., a star of magnitude 5.0 is 2.512 timess as bright as one of magnitude 6.0. Thus, a difference of five magnitudes corresponds to a brightness ratio of 100 to 1. After standardization and assignment of the zero point, the brightest class was found to contain too great a range of luminosities, and negative magnitudes were introduced to spread the range.