- Surface brightness
Surface brightness is a concept used in
astronomy when describing extendedastronomical object s such as galaxies andnebula e.General description
The
apparent magnitude of an astronomical object is generally given as an integrated value—if agalaxy is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we would from a star with magnitude 12.5. However, while astar is so small it is effectively a in most observations, the galaxy may extend over severalarcsecond s orarcminute s. Therefore, the galaxy will be harder to see than the star against theairglow background light. Quoting an object's surface brightness gives an indication of how easily observable it is.Calculating surface brightness
Surface brightnesses are usually quoted in magnitudes per square arcsecond. Because the magnitude is logarithmic, calculating surface brightness cannot be done by simple division of magnitude by area. Instead, for a source with magnitude "m" extending over an area of "A" square arcseconds, the surface brightness "S" is given by
S = m + 2.5 cdot log A.
Surface brightness is constant with
luminosity distance . For nearby objects, theluminosity distance is equal to the physical distance of the object. For a nearby object emitting a given amount of light, radiativeflux decreases with the square of the distance to the object, but the physical area corresponding to a givensolid angle (e. g. 1 square arcsecond) increases in the same fashion, resulting in the same surface brightness. [harvtxt|Sparke|Gallagher|2000|loc=§ 5.1.2]Relationship to physical units
The surface brightness in magnitude units is related to the surface brightness in physical units of
solar luminosities per squareparsec byS(mag/arcsec^2)=M_{V,odot}+21.57-2.5log S (L_{odot}/pc^2),
where M_{V,odot} is the absolute magnitude of the sun in V band. [Other absolute magnitudes of the sun can be obtained from harvtxt|Binney|Merrifield|1998 or [http://www.ucolick.org/~cnaw/sun.html Absolute Magnitude of the Sun in Several Bands] ]
References
General references
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*ee also
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Low surface brightness galaxy
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