Asked by Sam Harvey
The heart, or nucleus, of a comet is a collection of frozen water and gases as well as other carbon-based materials. As a result, comets far away from our Sun are effectively black since they have one of the lowest albedos – a measure of how much light they reflect – of any object we have observed.
As a comet gets closer to the Sun, some of these frozen gases sublimate creating the coma – the envelope of atmosphere that surrounds a comet. These gases can reflect sunlight and turn our dark object into a bright, yellow-white body. One of the two tails a comet produces, the ion tail – a collection of charged particles pushed away by the solar wind – will begin to glow with a blue tint.
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