Comets are icy objects composed mostly of water ice, carbon dioxide ice, and other volatile compounds. When they approach the Sun, the heat from the Sun vaporizes these ices, creating a coma, or cloud of gas and dust, around the comet. The coma can be seen from Earth as a bright, fuzzy object with a tail that points away from the Sun.
The composition of a comet's coma can vary, but diatomic carbon is often a major component. Diatomic carbon molecules are made up of two carbon atoms bonded together, and they emit green light when they are excited by sunlight. This is why some comets appear green when viewed from Earth.
The amount of diatomic carbon in a comet's coma can vary over time. This is because the composition of comets can change as they travel through space, and the amount of sunlight that they are exposed to can also affect the composition of the coma. As a result, the green color of a comet can vary in intensity and may even disappear completely at times.
Comets that appear green are often called "green comets." Some famous green comets include:
* Comet Hale-Bopp, which was visible from Earth in 1997
* Comet Hyakutake, which was visible from Earth in 1996
* Comet McNaught, which was visible from Earth in 2007
