The brightness scale of objects, running backwards: the smaller the number, the brighter the object. Five magnitudes make a factor of 100, one magnitude a factor of about 2.5. Vega is 0, Sirius −1.5, Venus up to −4.6; the eye reaches 6 under a dark sky.
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Related terms
- Absolute magnitude
How bright an object would look if it were placed 10 parsecs (32.6 light-years) away. It measures its true luminosity, regardless of distance. The Sun, dazzling at −26.7, has an absolute magnitude of only +4.8; Rigel reaches about −7.
- Limiting magnitude
The brightness of the faintest stars you can see. With the naked eye it ranges from 3 or 4 in a city to 6.5 or more under a very dark sky. A telescope of aperture D (in mm) pushes it further by about 5 log₁₀(D ÷ 7) magnitudes: a 200 mm telescope reaches roughly magnitude 14.
- Surface brightness
The brightness of an extended object spread over its area, in magnitudes per square arcsecond. A large magnitude 9 galaxy can be harder to see than a small magnitude 11 nebula. It is what matters when judging how visible a deep-sky object will be.
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