Extinction (astronomy)

As extinction is known in astronomy, the attenuation of the light of heavenly bodies when passing through the Earth's atmosphere or interstellar matter.

The term extinction weakening by actual absorption in the medium and crossed by scattering is summarized, see absorbance ( optical ). This attenuation depends on the wavelength, that is connected to a stain. It also depends on the volume of the irradiated atmosphere and of their specific levels of water vapor, carbon dioxide, ozone, clouds, and aerosols.

Extinction in the Earth's atmosphere

The extinction in the Earth's atmosphere depends on the location, the wavelength and the zenith distance. If a star or other celestial bodies near the horizon, the path is longer through the atmosphere and the light attenuation greater. Thus, the atmospheric extinction increases with the zenith distance. At the height she makes at sea level about 0.28 mag ( 23% ) and is mainly caused one half of the Rayleigh scattering by air molecules and scattering by aerosol particles. Since blue light is scattered in the atmosphere than red light, the stars appear bluish reddish near the horizon and the sky.

Interstellar extinction

The extinction in the interstellar medium is mainly caused by interstellar dust. The absorbance values ​​are very different due to the irregular structure of the interstellar medium for different lines of sight in the Milky Way. The extinction parameter is defined by the distance module:

In this case, call:

• The apparent brightness of the wavelength,
• The absolute magnitude at the wavelength,
• The distance of the source in parsecs.
• The absorbance at the wavelength,

The absorbance as a function of wavelength is shown as the optical density curve (see Figure ). In the optical domain (see UBV system) varies little, the absorbance, however, is not constant. This means that initially white light is not only diluted by the interstellar extinction, but also discoloration. The blue component is more strongly absorbed / scattered as the red component, which results in a flushing of the light. In the field of infrared and radio waves, the interstellar extinction is very small, that is, in these spectral regions can be (eg the galactic center ) watching almost undisturbed. In the high-energy spectrum of the absorbance on the other hand is growing very strongly.

The interstellar extinction is dominated by dust. UV bump at 220 nm is likely to be caused by the graphite. A detailed explanation of the course of extinction curve is not possible at present, since there is still no consistent model for the composition of interstellar dust.