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On the evening of 6 Feb 2018 (and for a few nights before and after), blue comet C/2016 R2 appeared in the sky near the majestic blue star cluster and nebulosity known as Messier 45, The Pleiades. The most unusual thing about this nice juxtaposition is that the comet is blue. As you can see from the other pictures in this gallery, comets are usually green as a result of the abundance of carbon, but this comet had an unusually high amount of CO (carbon monoxide), which glows blue.