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A comet is a small astronomical body containing volatile material and solid grains, capable of becoming active when heated. Its comet nucleus is the compact reservoir. As volatile ice undergoes sublimation, escaping gas carries dust outward and can make a diffuse cometary coma and a comet tail.^[ref]

A cometary dust tail responds strongly to radiation pressure, whereas ionized gas couples to the solar wind and magnetic fields. The visible tail is therefore not simply an exhaust trail opposite the direction of travel. An inactive distant comet can lack a prominent tail. Its Kepler orbit, later planetary scattering and material loss together determine the observable history.

A comet tail is an extended structure formed from material released by a comet. Its dust tail is shaped largely by radiation pressure, while an ion tail is shaped by the solar wind and magnetic field. The cometary dust tail is the solid-grain component.

A cometary dust tail is a superposition of synchrones and syndynes, rather than a material line emitted in one fixed direction. For grains larger than the stellar wavelength, $\beta \propto D^{-1}$: small grains have large $\beta$, are displaced rapidly in the anti-stellar direction, and make a relatively straight broad tail. Large grains have small $\beta$, remain closer to the comet's Kepler orbit, and make a more strongly curved dust trail. The observed position angle consequently depends on grain size, release time, orbital phase, and projection onto the plane of the sky.