The Genesis of Absorption Lines

Capella in close-up. Instrument: The Gimp.
Looking at the sun, with the usual precautions(!), we see a sharp disk. But the radius of the sun isn't well defined. The density of the gas continuously decreases towards the outside. The reason why we see a sharp disk is that the greatest amount of light comes from a special layer where the gas becomes transparent.

No light from the inside reaches us directly. The outer layers are transparent and don't add much light to the field of radiation.

This is true only in the spectral lines typical for the gas. Say H-alpha at 656.3 nm. A little step below or above this wavelength hydrogenium is transparent and we see deeper layers of the star.

The temperature is higher there and this layer is therefore more luminous at the considered wavelength in the neighbourhood of 656 nm.

So we see a dark line from hydrogenium in front of a bright continuum. From this absorption lines result.

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