Separating radiation by wavelength
A spectrometer separates radiation by wavelength and measures intensity, producing comparable data rather than merely a coloured picture. Prisms or gratings disperse the radiation, detectors record signals, and calibration is needed to connect positions and measured intensities with physical quantities.
References: [1]
Why spectral lines appear
Atoms and molecules exchange energy through particular transitions, producing absorption or emission at characteristic wavelengths. Dark lines against a continuous background and bright lines from emitting gas reflect different conditions. Temperature and ionization also affect the patterns observed.
Identifying composition remotely
Laboratory spectra provide reference patterns for comparison with stars and other remote objects. Composition can thus be investigated without collecting a physical sample. Reliable identification generally uses multiple features and a physical model rather than a single apparently matching line.
Line positions and motion
Shifts relative to laboratory wavelengths can reveal motion along the line of sight, supporting studies of stellar velocities and galaxies. The observed displacement must still be interpreted with the relevant physical context and model rather than treating every redshift as the same process.
References: [3]