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brightnessThe intensity of light or other radiation emitted from – absolute or intrinsic brightness – or received from – apparent brightness – a celestial body, the latter decreasing as the distance from the body increases. Intrinsic brightness is directly related to the luminosity of a body in a given spectral region. Apparent brightness is considered in terms of apparent magnitude: a star one magnitude less than another is about 2.5 times brighter. If two stars belong to the main sequence then the brighter star is the hotter of the two. See also radio brightness.
(also surface brightness), in astronomy, a characteristic of the emittance or reflectance of the surface of a celestial body. The brightness of faint celestial sources is expressed in terms of the number of stars of a given stellar magnitude in an area measuring 1 square second of arc (arcsec2), 1 square minute of arc (arcmin2), or 1 square degree (deg2). In other words, the illuminance from such an area is compared with the illuminance produced by a star of known stellar magnitude.
The brightness of the moonless night sky in clear weather, which is equal to 2 × 10–8 stilbs (sb), is characterized by one star of stellar magnitude 22.4 per arcsec2, or one star of stellar magnitude 4.61 per deg2. The brightness of an average nebula is equal to one star of stellar magnitude 19–20 per arcsec2. The brightness of Venus is equal to about one star of stellar magnitude 3 per arcsec2. The brightness of an area of 1 arcsec2 over which the light of a zero-magnitude star is distributed is equal to 9.25 sb. The brightness of the center of the solar disk is equal to 150,000 sb; that of the full moon, to 0.25 sb.
A surface for which the brightness does not depend on the angle of inclination of the area to the line of sight is said to be orthotropic. The flux emitted by such a surface behaves in accordance with Lambert’s law and is called the luminance. The unit of luminance is the lambert, which corresponds to a total flux of 1 lumen/cm2.
D. IA. MARTYNOV