The part of a comet containing dust and gas released from the comet’s head. Many comets fail to develop a tail but, when present, tails are always directed away from the Sun, so that comets move tail-first after perihelion. Tails do not normally develop until a comet is within about 2 au of the Sun, and are usually most impressive shortly after perihelion. Comet tails have two main components: the Type I or gas tail (also known as the ion tail or plasma tail), and the Type II or dust tail. Some comets may have a third form of tail, known as Type III, of neutral sodium, which lies between the gas and dust tails. This was first seen in 1997 in Comet Hale–Bopp. Gas tails consist of ionized gas carried away from the coma by the solar wind, and is more or less straight. They can reach 10⁸ km or more in length. Gas tails appear bluish or greenish and are dominated by emission from singly ionized carbon monoxide (CO⁺) at 420 nm wavelength, resulting from excitation by solar ultraviolet radiation. They are subject to disconnection events. The dust tail, by contrast, appears yellowish because it shines by reflected sunlight, and often appears markedly curved. Dust tails are usually shorter than gas tails, but can still reach 10⁷ km. They consist of micrometre-sized solid particles which are pushed away from the head by radiation pressure on parabolic trajectories. Larger particles of dust (millimetre- to centimetre-sized) shed by comets give rise to meteor streams. See also antitail.