Bathed in solar ultraviolet light , the gas molecules break down and are excited, producing a characteristic glow. This glow is responsible for visible light from the tail and astronomers using spectroscopes can identify the compounds involved. The close passage of Hyakutake presents an excellent chance to use this technique to explore the composition of its tail . In fact, the poisonous CO and CN compounds were seen in the spectrum of Halley's comet during its 1910 apparition. However, stretching for millions of miles, comet tails are extremely thin and tenuous and don't pose a danger to the Earth's atmosphere.
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