Caption: The formation of an emission line spectrum and an absorption line spectrum.
Features:
The lines are the atomic spectral lines and/or molecular spectral lines.
The spectral lines are charateristcs of the particular atoms and/or molecules that make up the gas.
The study of line spectra---which is called spectroscopy---is overwhelmingly the most important form of chemical analysis.
You do NOT need a sample of the material. You just need light from the material.
It could come from your lab bench or from across the observable universe.
These transitions are changes in the internal state of atoms and molecules that lead to the emission or absorption of light.
The transitions are conventionally called lines.
The atoms and/or molecules (including ionized atoms and molecules) in the low-density stellar atmosphere above the photosphere are colder (i.e., less excited) than the photosphere. They will absorb the photospheric emission in their lines.
For stars, such spectra are called stellar spectra.
The photosphere produces a continuous spectrum that closely approximates a blackbody spectrum.
The photosphere is often called the surface of a star, but that is just convenient usage. Stars do NOT have sharp surfaces. Above the photosphere is the stellar atmosphere.
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