Magnitude, in astronomy, a unit of measurement of the brightness of stars. The scale of magnitude extends from negative numbers (for example, the minus first magnitude) for very bright stars to positive numbers (for example, the fourth magnitude) for dimmer ones. The dimmest stars visible without a telescope are of the sixth magnitude (+6).

Apparent magnitude describes the brightness of stars as seen from earth. Absolute magnitude describes the brightness of stars as they would appear if they were all 10 parsecs (32.6 light-years) away. The sun has the greatest apparent magnitude (-26.7), but an absolute magnitude of only +4.8. The sun appears to be the brightest star in the sky because it is the closest.

The original scale of magnitudes was based on how bright stars appeared to the eye. In the 19th century, the scale was modified and cataloged with observations made with telescopes. This scale, in use today, is a geometric progression with a factor of 2.512. An increase of one magnitude means that the brightness increases 2.512 times. Thus, a first-magnitude star is 100 times as bright as a star of the sixth magnitude because 2.5125 = 100.