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The color purple

There’s wonderful stuff at one end of the rainbow.
Although our limited eyes cannot detect purple stars, the color purple is nevertheless part of a skywatcher’s vocabulary. It becomes especially noticeable when we deal with atmospheric aberrations, lensing effects, and the scattering of light.

Examples of the last include the violet flash at sunset, the purple light of twilight, the indigo bands of a rainbow, or the face of New Moon during a total solar eclipse. All these are fantastically purple, yet the opportunities to see them are generally uncommon.

Even simultaneous color-contrast illusions combined with a nervous atmosphere can trick us into believing we can see the color purple in some stars. The 19th-century English astronomer William Henry Smyth was a master of simultaneous color-contrast perception. He noted, for example, that double star Eta (η) Cassiopeiae has a bright white primary with a lilac secondary.

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