Limb darkening

When limb darkening is called in astronomy, the phenomenon that the apparent disk of a star near the edge emits a significantly lower radiation flux in the direction of the observer. Based on the solar disk, the effect can be directly observed around other stars, the limb darkening is available to demonstrate the spectrum.

How could indicate the beginning of the 20th century, Karl Schwarzschild, it comes to limb darkening, because stars are made of gas whose temperature decreases towards the surface. At the disk center deeper, hotter regions can be seen that radiate stronger. At the edge of the view, however, falls only on higher, less hot and therefore even less bright layers. Had stars Solid, no limb darkening would be to notice. The exact course of the limb darkening gives information on the pressure and temperature profile in the upper regions of the stellar atmosphere.

Geometric and physical principles

Is propagated light in a medium continues, it comes to extinction: Only a portion of the light particles manage to pass through the medium; the others are absorbed or scattered. For a homogeneous medium, the number of remaining as it passes through a medium of thickness L photons exponentially depends on L and the characteristics of the material, and is proportional to.

The characteristic length, called the penetration depth, specific to the material. It is a measure of how deep an external observer can see into the medium. From a layer, which has the straight distance L from the surface, the observer still achieve only the emitted photons.

The figure at right shows in cross section a star ball, which consists of onion skin- like superimposed layers of different gas density and temperature. Purely due to the geometry, an observer located at a great distance beyond the right edge of the image, look in deeper into the star, namely up to layer A, when he looks directly to the center O. For the observer, this corresponds to the center of the Scheibchens, as he perceives the star in the sky. In the peripheral regions of the star Scheibchens, as an example of the point B is located, corresponds to the same characteristic length of a more outwardly located Stern layer. Accordingly, the observer can look less deep in the stellar atmosphere.

The effective temperature of those layers of the stellar gas that produce the familiar to us, visible light, continues to grow toward deeper layers. The respective layers behave approximately like a black body radiator whose radiation intensity, following the Stefan- Boltzmann law, increases with increasing temperature. The visible slices in mid deeper layers are therefore lighter than the visible slices on edge higher layers: It depends on the limb darkening of the star Scheibchens.

Limb darkening for planetary

A limb darkening is also seen for example in observations of Uranus.