1.A luminous, hot solid, or dense, hot gas, emits light of all wavelengths and so produces a continuous spectrum of radiation.2.A low density hot gas emits light, the spectrum of which consists of discreet, bright lines. This emission spectrum is indicative of the chemical composition of the gas.3.A cool, low pressure gas absorbs light at specific wavelengths or colors. This occurs in all normal stars. This absorption spectrum reveals the presence of chemical elements.
The spectrum of hydrogen is most important to us, since that is the primary component of stars. Electrons change energy is specific patterns in hydrogen, creating different series of lines. The one we might pay attention to is the Balmer series.Detailed study of the spectra of stars led us to classify them with a simple seven letter sequence.