A logarithmic measure of the brightness of an astronomical object.Magnitude is a logarithmic scale used to measure the brightness of astronomical objects. It originated with the ancient Greek astronomer Hipparchus, who classified stars into six groups by apparent brightness, with first magnitude being the brightest and sixth the faintest visible to the naked eye. The system was formalized in the 19th century by Norman Pogson, who defined a difference of five magnitudes as exactly a factor of 100 in brightness, making each magnitude step a factor of about 2.512.
The system was formalized by Norman Pogson in 1856, who established the logarithmic relationship that a five-magnitude difference equals a brightness ratio of 100.
The apparent magnitude of an object depends on its intrinsic luminosity and its distance from Earth. A lower magnitude number means a brighter object; for example, the Sun has an apparent magnitude of about -26.7, while the faintest objects detected by the Hubble Space Telescope are around magnitude 31. The scale is logarithmic, so a difference of one magnitude corresponds to a brightness ratio of the fifth root of 100, approximately 2.512. Astronomers also use absolute magnitude, which normalizes brightness to a standard distance of 10 parsecs, to compare the true luminosities of objects.
Magnitude is used universally in astronomy to catalog stars, galaxies, and other celestial objects, and to measure their variability, such as the changing brightness of stars like Betelgeuse or the unpredictable brightness evolution of Starlink satellites.
The brightest object in the night sky, Venus, can reach an apparent magnitude of about -4.9, which is bright enough to cast shadows on Earth.
| m | variable | apparent magnitude — magnitude |
| F | variable | flux — flux watt per square metre |
| ZP | variable | zero point — zero point |
| log10 | operator | base-10 logarithm — logarithm base 10 |
