Photosphere

The photosphere is the lowest layer of a stellar atmosphere. From it are both the continuous spectrum of visible light as well as the absorption line spectrum of a star. In solar-type and late main-sequence and giant stars is also closes to the chromosphere, directly in the early stars follows the stellar wind.

Low lying layers of a star can not be directly observed because the then -derived photons are scattered by the free electrons in the plasma star. The number of such variations, which must bring a photon statistical average behind to leave the star is called the optical depth. As a convention in astrophysics the photosphere starts at an optical depth of 2 /3, and the function associated with the optical depth radius is considered stellar radius. If followed by a chromosphere, the photosphere ends at the point at which reverses the normal, decreasing outward temperature stratification and the chromospheric heating begins. If the stellar wind, connecting directly with the photosphere ends at the point at which the velocity of the stellar wind exceeds the local speed of sound.

The continuous spectrum of the photo- sphere is first and the approximation of a black body by the so-called effective temperature of the star, which is however modified by both continuous absorption, for example, the neutral hydrogen atom, as well as by the absorption of the spectral line.

Photosphere of the Sun

The photosphere of the sun was a few years ago the only one that could be spatially resolved. The solar photosphere is about 300 km thick (0.063 % of the radius ) and has a mean gas density of 10-7 g/cm3 ( corresponding to the density of the atmosphere at about 70 km altitude) at an effective temperature of about 5778 K (about 5504 ° C). The strongest absorption lines of the solar atmosphere are named after their discoverer Fraunhofer lines. Above the photosphere of the Sun is the chromosphere.

Continuous absorption in the solar photosphere

The absorption of visible light occurs at relatively low temperatures. However, at 5000 to 6000 K can free-free transitions only infrared light to be triggered. Visible light can not emerge significantly by transitions on neutral hydrogen, because it exists only to 0.01%.

Here, the German - American astronomer Rupert Wildt in 1938 was an important statement with the help of the negative hydrogen ion. They are formed by addition of a free electron to a neutral H atom and are weakly stable. The necessary free electrons resulting from the slight ionization of sodium atoms. The negative H- ion has only one bound state.

When photons with an energy higher than 0.75 eV, ie a wavelength of less than 1650 nm, a negative H- ion hit, strike out an electron and is left again a neutral H atom. Conversely, if a neutral H atom captures one electron, light is emitted at this wavelength. This process is the most important for the transport of energy in the photosphere.

The stable gaseous negative H atom was in 1930 by Hans Bethe and Egil Hylleraas ( 1898-1965 ) predicted and demonstrated in 1950 by Herbert Massey in the laboratory.

Center-limb darkening of the sun

The photosphere appears largely uniformly bright, only interrupted by sunspots and flares. At higher resolution, however, it shows the granulation, which leaves the surface of the sun appear grainy. The granular structures are convection cells caused by upwardly directed tubular currents and corresponding downward currents in the gaps and pass away within a few minutes of heat.

The apparent surface brightness of the photosphere, as reflected in the telescope, taking the center of the projected sun ( " sun disc " ) to the outer edge. This center - to-limb variation is stronger for short wavelengths ( blue, violet, ultraviolet ) than for long-wavelength light (red, infrared). It is approximately given by

Where ρ the geometric distance from the center of the solar disk is in units of the solar disk radius. The coefficient β varies between 0.9 in the visible at the border in the IR (800 nm) 1.2 (red, 680 nm), 1.6 (yellow, 580 nm ), 2.0 ( green, 540 nm ), 3.0 (blue, 480 nm ), 5.0 (Violet, 425 nm ) and 10 ( UV, 380 nm). The center- to-limb variation is caused by the temperature stratification of the photosphere., The temperature increases with increasing depth. In shallow exit angle, corresponding to the peripheral areas of the projected sun, a larger portion of the light from the deeper layers is absorbed by the overlying again as with vertical outlet in the center of the solar disk, so that the light from the cooler layers of the larger share of the total light has.

Limits of the concept of a photosphere

The definition of the star radius as the radius at which the optical depth of τ = 2/ 3, is problematic in certain stars, since the optical depth is a function of the wavelength of light. In the infrared region is τ = 2/3 only achieved at lower densities, as in the visual light. The fact that this definition still used frequently in practice, is that the density in the outer regions of normal stars relatively sharp drops, and thus vary the radius values ​​for τ = 2/ 3 in main-sequence stars by only a few dozen to hundreds of kilometers, in view of the typical radius of several hundreds of thousands of kilometers of other measurement inaccuracies, is negligible.

In the case of, for example, of supergiants or in dense stellar winds of the density drop is considerably gentler pronounced and there can be the difference of the photospheric radius in the visual to the infrared region can be clearly measurable.

In some extreme types of stars, such as the Wolf -Rayet stars or LBVs, is the point at which the optical depth is less than the value of 2 /3, also already for visual light far in the supersonic part of the stellar wind. In such stars can not be spoken of a photosphere, and there are alternative definitions of the stellar radius, and thus the stellar temperature used.