The distribution of light separated in order of some varying characteristic such as wavelength or frequency. The "electromagnetic spectrum" refers to the full range of possible frequencies and wavelengths of light. If we "take a spectrum" of a star we analyze its light according to wavelength or frequency by, say, passing the light through a prism. A "spectral line" refers to emission or absorption at a particular wavelength of light.
FIGURE
Different wavelengths of the fluctuating electromagnetic waves
correspond to different types of light in the electromagnetic
spectrum. The visible portion of the spectrum is a small region from
about 400 to 700 nanometers (billionths of a meter). Different
astronomical objects emit most of their radiation in different parts
of the spectrum. Today's astronomer studies the sky in all
wavelengths.