This is a list of galaxies sorted by surface brightness. Surface brightness is a measure of how bright a diffuse object like a galaxy or nebula appears over its extended surface. The brightness over the entire galaxy is called apparent magnitude.
Table The surface brightness is calculated via . Where is surface brightness, is total magnitude, and is the total area in square arcseconds. Area is calculated using the formula for an ellipse; , where is the semi-major axis and is the semi-minor axis. Each axis is half of the dimension, because each dimension is the entire length/height but the axis is only the length/height to the centre, this combined with the symmetry of an ellipse means that half the dimension is the axis.
Combining this with the original formula we get: , which simplifies to . +Galaxy ImageSurface Brightness(mag/arcsecond2)Apparent Magnitude DimensionsDistance(Mly)Type Notes CitationsMessier 8217.40(Rc) 19.10(r) 20.87(B)8.49x4 arcminutes(540x240 arcseconds)12SpiralHas rapid star formation due to interactions with M81 making red streaks.Messier 3220.938.18x6 arcminutes(480x360 arcseconds)2.5EllipticalSatellite galaxy of the Andromeda GalaxyNGC 363221.1710.63x2 arcminutes(180x120 arcseconds)70SpiralMessier 8121.29(B) 19.30(r)6.921x10 arcminutes(1260x600 arcseconds)11.6SpiralFaintest galaxy visible to the naked eye, despite the naked eye limiting magnitude of a star being fainter, surface brightness limits this for galaxies.Whirlpool Galaxy21.748.411x7 arcminutes(660x420 arcseconds)31SpiralHas a nearby galaxy in a merger.Messier 5821.919.85.5x4.5 arcminutes(330x270 arcseconds)62SpiralAndromeda Galaxy22.193.44178x63 arcminutes(10680x3780 arcseconds)2.54Spiral/ringIts core is significantly brighter than its outer disk.Messier 8722.459.67 arcminutes(138544.23 square arcseconds)54EllipticalRoughly round, so is used along with angular diameter.Messier 4922.69.49x7.5 arcminutes(540x450 arcseconds)60EllipticalTriangulum Galaxy235.770.8x41.7 arcminutes(4248x2502 arcseconds)3Spiral
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