Molecules can only exist in the atmospheres of very cool stars or in the cooler regions of stars higher in the main sequence, such as in sunspots in our own sun. Most of the observed species are diatomics and are often transitory. Brown dwarfs, on the other hand, can be cool enough to support complex molecules such as methane. Two spectral classes, L (~1300 to ~2000 K) and T (below ~1300 K), have been created for them.

NOTE: Molecule identifications in the table below are intended to be representative, not exhaustive. As with any identification, certainties will vary; a molecule in italic font is a tentative detection. Sources indicate that about 25 species have been identified in the Sun.

See also: The Classification of Stellar Spectra (an informative discussion on star type) and the Wikipedia article on spectral classes.