Image 1 Caption: The formation of stellar spectra illustrated.

Features:

1. The stellar photosphere produces a continuous spectrum that approximates a blackbody spectrum.

   Recall, the photosphere is the layer of a star where the star becomes sufficiently transparent that photons can escape about half the time to infinity.

2. The stellar atmosphere (which is above the photosphere) is low density and is too weak to absorb or emit much of a continuous spectrum. However, spectral lines (i.e., both atomic spectral lines and molecular lines) change the continuous spectrum from the photosphere by absorption in narrow wavelength bands.

3. Why absorption? The part of stellar atmosphere immediately above the photosphere is usually colder than the photosphere. Recall heat energy spontaneously flows form hot to cold in obedience to the 2nd law of thermodynamics. Here, it flows from the relatively hot continuous spectrum into the relatively cold stellar atmosphere gas, but only through the channels that are sufficiently strong: i.e., the relatively strong spectral lines.

4. The energy absorbed in the stellar atmosphere gas is emitted in some other wavelength bands, usually in the infrared where it is NOT usually observable from ground-based astronomy.

   

5. Image 2 Caption: The spectral lines dig troughs in the continuous spectrum to create what are called absorption lines or just spectral lines in a second meaning of the term. The overall spectrum is called an absorption line spectrum.

Images:

1. Credit/Permission: © David Jeffery, 2015 / Own work.
   Image link: Itself.
   
2. Credit/Permission: © David Jeffery, 2003 / Own work.
   Image link: Itself.