Effective radius
Encyclopedia
The effective radius of a galaxy
is the radius
at which one half of the total light
of the system is emitted interior to this radius. This assumes the galaxy is circularly symmetric
. This is an important length scale in de Vaucouleurs
law which is given as:
where
is the surface brightness
at
. Note that at 
,
Thus the central surface brightness is approximately
.
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
is the radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...
at which one half of the total light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
of the system is emitted interior to this radius. This assumes the galaxy is circularly symmetric
Circular symmetry
Circular symmetry in mathematical physics applies to a 2-dimensional field which can be expressed as a function of distance from a central point only. This means that all points on each circle take the same value....
. This is an important length scale in de Vaucouleurs
Gérard de Vaucouleurs
-External links:* * http://nedwww.ipac.caltech.edu/level5/normal_galaxies.html-Other resources:...
law which is given as:where
is the surface brightnessSurface brightness
The overall brightness of an extended astronomical object such as a galaxy, star cluster, or nebula, can be measured by its total magnitude, integrated magnitude or integrated visual magnitude; a related concept is surface brightness, which specifies the brightness of a standard-sized piece of an...
at
. Note that at ,Thus the central surface brightness is approximately
.