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.
Related terms
- Magnitude
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.
- Galaxy
A vast system of billions of stars, gas and dust held together by gravity. The Andromeda Galaxy (M31), 2.5 million light-years away, is visible to the naked eye as an elongated smudge. In a telescope most look like faint grey glows: their light set off millions of years ago.
- SQM
The Sky Quality Meter, a small device that measures the brightness of the sky background in magnitudes per square arcsecond. The higher the number, the darker the sky: about 21.7 to 22 under a perfect sky, 19 in the suburbs, 17 or less in a city. It allows sites to be compared objectively.
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