Spectral lines are distinct lines that appear in a spectrum when light is emitted or absorbed by atoms or molecules. Each element has a unique set of spectral lines, which correspond to specific wavelengths of light. These lines can be observed in both emission spectra, where light is emitted, and absorption spectra, where light is absorbed by a substance. Scientists use spectral lines to identify the composition of stars and other celestial objects. By analyzing the patterns of these lines, researchers can determine the presence of elements like hydrogen, helium, and carbon, as well as their abundance and physical conditions in space.