The brighter the object, the lower the number assigned as a magnitude. In ancient times, stars were ranked in six magnitude classes, the first magnitude class containing the brightest stars. In 1850 the English astronomer Norman Robert Pogson proposed the system presently in use. 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.
Apparent magnitude is the brightness of an object as it appears to an observer on Earth.
The Sun's absolute magnitude is 4.8. Bolometric magnitude is that measured by including a star's entire radiation, not just the portion visible as light. Monochromatic magnitude is that measured only in some very narrow segment of the spectrum. Because ordinary photographic plates are more sensitive to blue light than is the eye, photographic magnitude is sometimes called blue magnitude.