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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 cometary coma is the extended cloud of gas and dust surrounding an active comet nucleus. Released material expands away from the nucleus and is altered by sunlight, chemistry and interactions with the solar environment.

Different spectral lines can trace different molecules or their dissociation products, while dust scatters sunlight and emits thermally. The resulting image combines source activity, transport and excitation. It is therefore a physical environment around the nucleus rather than a solid extension of its surface.

A comet nucleus is the solid central body of a comet, containing volatile ices mixed with refractory material. Solar heating can release gas and entrain dust, creating the surrounding cometary coma.

Its shape, rotation, surface properties and heat transport influence where activity occurs. Bright coma emission therefore does not directly measure nucleus size: the same nucleus can have very different apparent brightness at different distances from the Sun. A model of gas and dust production is needed to connect the surrounding signal with the body.