Not only do we find hydrogen between the stars, but also a variety of interesting complex molecules which also emit radio energy at specific wavelengths.  These molecules are found in regions that are appropriately known as molecular clouds.  . The National Radio Astronomy Observatory keeps track of these molecules which include something as simple as carbon dioxide to molecules with more than a dozen atoms.  One of the nearest important molecular clouds can be found in the constellation of Orion, a busy place for star formation. But there is more than just gas between the stars.  We find an appreciable amount of dust.  This dust has a noticeable effect on starlight.
Molecular Clouds
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Carpe Caelum
Carpe Caelum Stellar Astronomy
Carpe Caelum Stellar Astronomy