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Concept
Chromosphère
Résumé
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Sun's atmosphere, the chromosphere is roughly in height, or slightly more than 1% of the Sun's radius at maximum thickness. It possesses a homogeneous layer at the boundary with the photosphere. Hair-like jets of plasma, called spicules, rise from this homogeneous region and through the chromosphere, extending up to into the corona above.
The chromosphere has a characteristic red color due to electromagnetic emissions in the Hα spectral line. Information about the chromosphere is primarily obtained by analysis of its emitted electromagnetic radiation.
Chromospheres have also been observed on stars other than the Sun. On large stars, chromospheres sometimes make up a significant proportion of the entire star. For example, the chromosphere of supergiant star Antares has been found to be about 2.5 times larger in thickness than the star's radius.
The density of the Sun's chromosphere decreases exponentially with distance from the center of the Sun by a factor of roughly 10 million, from about 2e-4kg/m3 at the chromosphere's inner boundary to under 1.6e-11kg/m3 at the outer boundary. The temperature initially decreases from the inner boundary at about 6,000K to a minimum of approximately 3,800K, but then increasing to upwards of 35,000K at the outer boundary with the transition layer of the corona (see ).
The density of the chromosphere is 10−4 times that of the underlying photosphere and 10−8 times that of the Earth's atmosphere at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish colour is revealed. The colour hues are anywhere between pink and red. Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere.
The chromosphere's spectrum is dominated by emission lines.
Source officielle
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