
A spectral line produced by excited atoms emitting light at specific wavelengths, used to diagnose gas properties.An emission line is a bright feature in a spectrum produced when an atom or molecule transitions from a higher energy state to a lower one, emitting a photon of a specific wavelength. These lines are like fingerprints, unique to each element, and they provide a powerful tool for diagnosing the physical conditions of gas in space, such as its temperature, density, and chemical composition. Emission lines are observed in a wide range of astrophysical objects, from the glowing nebulae around dying stars to the violent outflows from quasars.
The connection between emission lines and chemical elements was established in the 1850s by Gustav Kirchhoff and Robert Bunsen, who showed that each element produces a unique set of spectral lines. This laid the foundation for spectroscopy as a tool in astrophysics.
When a gas is heated or energized, its atoms can absorb energy and jump to excited states. These states are unstable, and the atom quickly falls back to a lower energy level, releasing the energy difference as a photon. Because the energy levels of an atom are quantized, the emitted photons have discrete wavelengths, creating sharp lines in the spectrum. The intensity and width of these lines reveal information about the number of emitting atoms and their motion (via Doppler broadening), while the presence of certain lines indicates the temperature and ionization state of the gas.
Emission lines are used in astronomy to map the distribution of gas in galaxies, measure the expansion of the universe, and identify the composition of exoplanet atmospheres. In technology, they are the basis for fluorescent lamps and lasers.
The 'forbidden' emission lines seen in nebulae, like the green line of doubly ionized oxygen, are so named because they are extremely unlikely to occur in laboratory conditions on Earth, but in the near-vacuum of space, atoms can stay in these metastable states long enough to emit them.




