As comets move close to the Sun, they develop tails of dust and ionized gas. Comets have two main tails, a dust tail and a plasma tail. The dust tail appears whitish-yellow because it is made up of tiny particles — about the size of particles of smoke — that reflect sunlight. The plasma tail is often blue because it contains carbon monoxide ions. Solar ultraviolet light breaks down the gas molecules, causing them to glow. Plasma tails can stretch tens of millions of kilometers into space.
The more you know…
For more on plasma, watch the TED-Ed Lesson Solid, liquid, gas and … plasma? – Michael Murillo
Animation by Tomás Pichardo Espaillat