# Surface brightness

Also called: SB, mag/arcmin².

How bright an object is per patch of sky — what really decides whether a galaxy or nebula is easy to see.

Two galaxies of the same total [magnitude](https://astropilot.space/glossary/magnitude) can be easy or nearly invisible: if one packs its light into a small disk and the other spreads it over a big area, the big one has a low surface brightness and barely stands out from the sky. That's why M 33 and M 101, bright on paper, are notoriously hard from light-polluted places.

It's given in magnitudes per square arcminute (or arcsecond); smaller numbers are brighter. Magnification doesn't change it — but it darkens the sky background, which helps small objects.

**In AstroPilot:** AstroPilot's visibility ratings use surface brightness, not just total magnitude.

**Examples:** [M33](https://astropilot.space/object/M33.md), [M101](https://astropilot.space/object/M101.md), [M57](https://astropilot.space/object/M57.md)

**Related:** [Magnitude](https://astropilot.space/glossary/magnitude.md), [Light pollution](https://astropilot.space/glossary/light-pollution.md), [Exit pupil](https://astropilot.space/glossary/exit-pupil.md), [Contrast](https://astropilot.space/glossary/contrast.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/surface-brightness