The process of splitting light into its energy spectrum of electromagnetic radiation (like the colors of a rainbow) to produce wavelength specific emission lines that are analyzed is the science of spectroscopy. A black object produces a continuous spectrum void of any spectral lines. A hot, transparent gas produces emission lines against are dark background. A cool transparent gas in front of a black body produces absorption lines. The spectral study of stars and galaxies are based on those three identities by focusing light unto the diffraction grating (a very thin slit) that is captured on camera for digital analysis.