Sun and the Fraunhofer lines

In physics and optics, the Fraunhofer lines are a set of spectral absorption lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.

In 1802, the English chemist William Hyde Wollaston was the first person to note the appearance of a number of dark features in the solar spectrum.
 In 1814, Fraunhofer independently rediscovered the lines and began to systematically study and measure the wavelengths where these features are observed. He mapped over 570 lines, designating the principal features (lines) with the letters A through K and weaker lines with other letters.

Wavelengths of the visual spectrum, 380 to about 740 nanometers (nm).Dips in intensity are observed as dark lines (absorption) at the wavelengths of the Fraunhofer lines, The "spectrum of blue sky" presents across 450 – 485 nm, the wavelengths of the color blue.

 Modern observations of sunlight can detect many thousands of lines.

Solar spectrum with Fraunhofer lines as it appears visually.

About 45 years later Kirchhoff and Bunsen noticed that several Fraunhofer lines coincide with characteristic emission lines identified in the spectra of heated elements.

It was correctly deduced that dark lines in the solar spectrum are caused by absorption by chemical elements in the solar atmosphere.

 Some of the observed features were identified as telluric lines originating from absorption by oxygen molecules in the Earth's atmosphere.

The Fraunhofer lines are typical spectral absorption lines. Absorption lines are dark lines, narrow regions of decreased intensity, that are the result of photons being absorbed as light passes from the source to the detector. In the Sun, Fraunhofer lines are a result of gas in the photosphere, the outer region of the sun. The photosphere gas has lower temperatures than gas in the inner regions, and absorbs a little of the light emitted from those regions.

Fraunhofer lines stand as evidence for the plasma Sun

The Fraunhofer lines also stand as evidence for the plasma Sun in still another way. They tell us in a clearly measurable fashion that extremely heavy atomic elements exist in the solar corona, such as mercury that is almost 4 times heavier than iron. With the mercury having a dynamic existence in the corona, like all the other elements that cause absorption lines, the question needs to be asked, what happens to them, especially the heavy mercury? Do we have evidence of its falling out from the solar wind? The answer is yes.

The black lines in the Sun's spectrum :

The black lines in the Sun's spectrum are caused by gases on, or above, the Sun's surface that absorb some of the emitted light. ... Fraunhofer lines, on the Sun's absorption spectrum. The letters correspond to various elements (such as helium, sodium) that cause the lines.

Uses of  Fraunhofer lines :

Because of their well–defined wavelengths, Fraunhofer lines are often used to characterize the refractive index and dispersion properties of optical materials.

The Fraunhofer lines are, indeed, a lifeline of solar physicists. The depths of the absorption lines provide information about temperature, and the wavelength shifts of the lines tell us the motion of gas