in spectroscopy, a pair of lines, characteristic of sodium, in the yellow region of the spectrum. Their separation is too small to be detected with a spectroscope of low resolving power. The line is the fourth prominent absorption line in the Sun's spectrum, starting from the red end, and accordingly is designated by the letter D. It has been resolved into two components, D₁ and D₂, corresponding to wavelengths 5895.924 and 5889.950 Å (angstrom = 10^-10 metre), respectively. An emission line appearing in the chromosphere, D₃, of wavelength 5875.62 Å, has been discovered. This line is caused by helium.