Everything about Surface Brightness totally explained
Surface brightness is a concept used in
astronomy when describing extended
astronomical objects such as
galaxies and
nebulae.
General description
The
apparent magnitude of an astronomical object is generally given as an integrated value—if a
galaxy is quoted as having a magnitude of 12.5, it means we see the same total amount of light from the galaxy as we'd from a star with magnitude 12.5. However, while a
star is so small it's effectively a in most observations, the galaxy may extend over several
arcseconds or
arcminutes. Therefore, the galaxy will be harder to see than the star against the
airglow 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 can't 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
Surface brightness is constant with
luminosity distance. For nearby objects, the
luminosity distance is equal to the physical distance of the object. For a nearby object emitting a given amount of light, radiative
flux decreases with the square of the distance to the object, but the physical area corresponding to a given
solid angle (e. g. 1 square arcsecond) increases in the same fashion, resulting in the same surface brightness.
Relationship to physical units
The surface brightness in magnitude units is related to the surface brightness in physical units of
solar luminosities per square
parsec by
is the absolute magnitude of the sun in
V band.
Further Information
Get more info on 'Surface Brightness'.
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