Caption: The formation of an absorption line spectrum.

Features:

1. The displayed spectra are in image representation rather than in intensity (which would be a plot of intensity versus wavelength).

2. The top band is a continuous spectrum such as would be emitted by a dense source.

3. The second band is line spectrum such as would be emitted by a dilute gas.

   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.

4. The lower band is an absorption line spectrum such as you would get from stars or collections of stars (e.g., galaxies).

   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.

5. Thus, they create dark lines that form an absorption line spectrum.

6. 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.

   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.

star file: nasa_spectra_002.html.