Magnitude is the scale used by astronomers to measure the brightness of objects in space. The brighter a star or planet, the lower its magnitude number. The magnitude system is based on the work of the ancient Greek astronomer Hipparchus. About 125 B.C., Hipparchus classified the stars according to brightness. He called the brightest stars first magnitude; the next brightest, second magnitude; and so on down to the faintest stars visible with the unaided eye. He called such stars sixth magnitude.
Later astronomers found that first magnitude stars were about 100 times as bright as sixth magnitude stars. They adopted a system that made a star of any magnitude about 21/2 times as bright as a star of the next brightest magnitude. This scale has been extended to zero and negative magnitudes because some stars and planets are brighter than first magnitude ones. For example, the sun has a magnitude of –27.
The word magnitude generally refers to apparent magnitude, or the brightness of a star as seen from the earth. To compare actual brightness, astronomers use absolute magnitude, which shows how bright stars would appear if they all were the same distance—32.6 light-years—from the earth. At that distance, the sun would be a fifth magnitude star.
