Surface brightness

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The surface brightness (Engl. Surface Brightness ) is a luminance . It is used in astronomy to record the brightness of astronomical objects with large areas such as comets , nebulae or galaxies .


During visual observations, the surface brightness in the telescope cannot be greater than that of the free-eyed . It is maximum when the exit pupil AP of the eyepiece corresponds to the pupil diameter of the dark-adapted eye .

Dark adaptation is hardly possible near the city because of the light pollution . Therefore, the observation of astronomical objects is only possible up to a certain surface brightness. This mainly affects faint galaxies and gas nebulae .


The surface brightness is the ratio of the apparent brightness  H to the surface  F :

or logarithmic in magnitudes:

If the apparent brightness of the object is replaced by the apparent brightness  m in magnitudes, it follows:


The surface brightness is measured in  cd / m². The unit mag / arcsec² is common in astronomy  ; The unit S 10 describes the brightness as the number of stars of brightness 10 mag within one square degree .


  • in cd / m²
  • in mag / arcsec²
  • 1 S 10 = 0.69 · 10 −6  cd / m².

Example: Lowest brightness of the night sky under optimal conditions: 21.6 mag / arcsec² = 2.5 · 10 −4  cd / m² = 370 S 10 .

See also

Individual evidence

  1. Other notation:
  2. Converter cd / m² - mag / arcsec²