A spectral classification applied to cool, faint stars at the bottom end of the main sequence. Objects that fall into this category are the lowest-mass red dwarfs and the sub-stars known as brown dwarfs. L dwarfs have surface temperatures in the range about 2200–1400 K. Their spectra show absorption caused by hydrides, i.e. hydrogen attached to atoms of metals such as iron and chromium, as well as features due to neutral alkali metals such as potassium, caesium, and rubidium. The presence of strong lithium absorption lines in the spectrum is proof that the object is a brown dwarf. Masses are estimated in the range 0.06–0.09 Suns (i.e. 60–90 Jupiters).