A comet’s tail begins with a change of state
Far from the Sun, a comet is a small, cold body made of ice, dust, and rock. As it approaches, sunlight warms its surface. Some ice turns directly into gas—a process called sublimation—carrying loose dust into space. The escaping material forms a hazy cloud around the solid nucleus, known as the coma.
The nucleus is tiny compared with the coma, but it continually supplies fresh gas and dust while sunlight keeps driving the release. That activity makes a comet look dramatically larger and brighter as it nears the Sun.
Two forces make two kinds of tail
Sunlight does more than heat the comet. Its pressure nudges dust grains away from the Sun. Because dust grains have weight and retain some of the comet’s orbital motion, their tail often bends into a broad curve.
Gas behaves differently. Sunlight can strip electrons from some gas atoms, turning them into electrically charged particles. The solar wind—the stream of charged particles flowing outward from the Sun—sweeps this ionized gas away, forming a narrower, straighter ion tail. The two tails can point in slightly different directions because light and the solar wind act on different material in different ways.
The tail tracks the Sun, not the journey
Both tails point generally away from the Sun, even as the comet races along its orbit. That means a tail is not a trail of exhaust left along the comet’s path: it is material being continually pushed outward from the Sun-facing environment. As the comet swings around the Sun and heads back into colder space, the supply of escaping gas and dust dwindles, and its bright coma and tails fade.
