Spectral lines of helium. An element's spectral lines are specific wavelengths of light that it emits due to electrons transitioning between different energy levels in the atoms, emitting light of a characteristic wavelength as they move to a lower energy level. As each element has its set of levels at unique energies, the emission spectrum of an element is like a fingerprint. Helium was discovered by studying the spectrum of sunlight, where astronomers found a spectral line unknown amongst elements on Earth. This was the strong yellow line seen here, at a wavelength of 587.49 nanometres. Due to this, helium takes its name from the Greek word for the Sun.