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Why do comet dust tails often appear white or yellow?
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Comet
Comet McNaught with filamentary tail and the Moon over the Pacific Ocean, photographed from Paranal Observatory, Chile, January 2007.
Comet dust tails, also known as Type II tails, appear white or yellow because they consist of fine dust grains that reflect sunlight. Unlike ion tails, which glow blue due to the presence of ionized molecules, dust tails are formed when radiation pressure from the Sun pushes these tiny particles away from the comet's nucleus.1
Because these grains are solid matter, they scatter the full spectrum of incoming solar light, resulting in the characteristic white or yellowish appearance observed by astronomers. These tails typically form a broad, curved path that follows the comet along its orbit, distinct from the straight, narrow ion tails that point directly away from the Sun.1