Figure: A typical stellar spectrum

This figure shows a typical stellar spectrum from the SDSS spectral database.

If thermal radiation were the only source of light from a star, the star's spectrum would be a nice smooth curve. However, actual spectra observed from stars have a series of peaks and valleys as shown in the spectrum below, meaning some of their light comes from "non-thermal" radiation - light emitted or absorbed by a process other than random jostling of atoms.

Credit: SLOAN Digital Sky Survey

http://cas.sdss.org/dr4/en/proj/advanced/spectraltypes/