Spectroscopy is the study of light spectra. When light passes through a prism or a diffraction grating, it spreads into its component wavelengths. The resulting spectrum contains dark lines called absorption lines or bright lines called emission lines. Each element produces a unique pattern of lines. By reading those patterns, astronomers can determine what a star, galaxy, or planet is made of.

The technique was developed in the 19th century by Gustav Kirchhoff and Robert Bunsen, who showed that each element has a characteristic spectral fingerprint. Astronomers quickly applied it to the Sun, discovering helium before it was found on Earth. Since then, spectroscopy has become the most important tool in astronomy.

Spectroscopy reveals more than composition. The shift of spectral lines reveals motion. The broadening of lines reveals temperature and pressure. The splitting of lines reveals magnetic fields. The shape of the continuum reveals the presence of dust. Almost everything astronomers know about the physical properties of celestial objects comes from spectroscopy.

Modern spectrographs are incredibly sensitive. They can detect planets by measuring a star's wobble. They can measure the expansion of the universe. They can search for biosignatures in exoplanet atmospheres. The spectrum is a cosmic barcode, and astronomers have learned to read it.