Star atmosphere
The visible outer areas of a star are called the star atmosphere . Physically more precise, the term includes the layers of stars of various kinds that are transparent to light .
Guided by observations of the sun , the best-studied star, a distinction is made between 3–4 hot gas layers from inside to outside:
Photosphere
The photosphere (Greek: envelope of light ) is the lowest, densest and coolest layer of the stellar atmosphere. The radiant heat coming from the inside of the sun penetrates through them as visible light to the outside.
It dominates the visible starlight, which passes through the higher layers largely unaffected. The photosphere of the sun has an effective temperature of about 5800 K. It shows phenomena such as sunspots , bright sun flares and granulation , a grainy structure of the sun's surface caused by convection . The photosphere spectrum is determined by properties such as temperature (essential for the spectral class ), acceleration due to gravity (determining the luminosity class ), and the content of heavy elements compared to hydrogen and helium ( metallicity ). Precise physical models of stellar atmospheres and their spectra are therefore an important tool in astrophysics .
Chromosphere
The chromosphere (Greek color envelope ) is the gas layer that adjoins the top. It takes its name from the red light that is briefly visible during a total solar eclipse .
It is usually completely outshone by the photosphere. Its temperature rises again after a minimum at the upper edge of the photosphere, its spectrum consists of narrow emission lines , in particular H-alpha at the wavelength of 656.3 nm, which corresponds to deep red light.
Above the chromosphere (near the sun) a transition layer to the corona is sometimes defined.
Corona (or solar corona)
The corona is the "halo" visible during solar eclipses. It consists of very thin gas that is over a million Kelvin hot. This is heated by various mechanisms and can reach several solar radii into the room. The structure of the "rays" depends on the current solar activity . At the sunspot minimum, the corona has a rather round outline, while at the spot maximum it appears elongated in the direction of the equator. This is related to the course of the magnetic field lines , which strongly influence the ionized gas.
In the lower part of the solar corona, the chromosphere and eruptive flares constantly throw up small spicules and high-rise protuberances , which often only sink back down to the photosphere after several days.
Atmospheres at other stars
While these atmospheric layers are well explored in the case of the sun, in the case of other stars, due to their great distance, one can usually only investigate the photosphere more closely. As a rule, only their spectrum is bright enough for this; that of the layers above is almost completely outshone.
The existence of star chromospheres and coronae mainly follows from theoretical models of the star structure - see there. However, similar phenomena have been observed in some star types, which supports the current theory of star structure. So -called star spots have been postulated on nearby giant stars from fluctuations in brightness , the nature of which is likely to correspond to our sunspots .
Some variables show eruptions of matter which are interpreted as corona phenomena or extremely violent solar winds , and young stars regularly emit clouds of gas that can be compared to the mechanism of spicules .
literature
- James B. Kaler: Stars and their spectra , Spectrum Academic Publishing 1994, ISBN 3-86025-089-2
- Albrecht Unsöld : Physics of the stellar atmospheres, with special consideration of the sun. Springer, Berlin 1938.