Surface brightness

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 . ${\displaystyle B}$

observation

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 .

calculation

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

${\displaystyle B={\frac {H}{F}}}$

or logarithmic in magnitudes:

${\displaystyle B_{\text{mag}}=-2{,}5\cdot \log {\frac {H}{F}}}$

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

${\displaystyle B_{\text{mag}}=m+2{,}5\cdot \log F}$

units

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 .

Conversions:

• ${\displaystyle B=1{,}084\cdot 10^{5}\cdot 10^{-0{,}4\cdot X}}$

With

• ${\displaystyle B}$ in cd / m²
• ${\displaystyle X}$ 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

• Surface brightness fluctuation
• Low Surface Brightness Galaxy

Individual evidence

1. ↑ Other notation: ${\displaystyle B=1{,}084\cdot 10^{5}\cdot e^{-0{,}92104\cdot X}}$
2. ↑ Converter cd / m² - mag / arcsec²