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Apparent magnitude or visual magnitude is how bright a star, planet, etc looks – how bright it appears to us on Earth.

Absolute magnitude is how bright a star etc actually is, if viewed from a standard distance of 10 parsecs.

The magnitude scale is logarithmic – each step/number is a multiple of the next step/number, in this case about 2.5x. The difference between 5 magnitude steps (eg, between 1 and 6) is 100 times difference in brightness (2.5125 = 100). Also, the higher the magnitude number the dimmer the object, so a magnitude 1 star is about 2.5x brighter than a magnitude 2 star. Particularly bright objects have negative magnitude values.

Scale of apparent magnitudes
Apparent Magnitude Celestial object
-26.74 Sun.
-12.74 Mean full Moon (or about -12.9 when it's full at perigee – a supermoon).
-9.5 Maximum brightness of an Iridium Flare (normally not over -8.0).
-5.9 Maximum brightness of International Space Station.
-4.4 Maximum brightness of Venus.
-4.0 Faintest objects observable during the day with naked eye.
-2.8 Maximum brightness of Mars.
-1.5 Brightest star (except for the sun) at visible wavelengths: Sirius.
-0.7 Second brightest star: Canopus.
-0.27 Third brightest star: Alpha Centauri (binary system).
0 The zero point by definition: This used to be Vega (which is now 0.03).
0.5 Sun as seen from Alpha Centauri.
about 3 Faintest stars visible in an urban neighbourhood.
about 6 Faintest stars observable with naked eye.
12.6 Brightest quasar.
18.7 Current opposition brightness of Eris.
27 Faintest objects observable in visible light with 8m ground-based telescopes.
30 Faintest objects observable in visible light with Hubble Space Telescope.
38 Faintest objects observable in visible light with planned OWL (2020).

Most stars are very dim. 78% of stars are red dwarfs which don't give off much light (comparatively speaking). Within 5 parsecs (16.3 light years) of the Sun there are 62 stars. Most of these are so dim they can't be seen with the naked eye. Only 9 of the 62 stars (14.5%) can be seen with the naked eye.

Other uses

The word magnitude is also used to describe the maximum amount of an eclipse. See measuring an eclipse for more information about the magnitude of an eclipse.