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ultraviolet
Light that is so blue humans cannot see it. A band of the electromagnetic spectrum between the visible and the X-ray. Photons of ultraviolet light are more energetic than photons of visible light.

umbra
The dark central region of a sunspot.

undae
Dunes (literally 'waves').

Uranus
Uranus is the seventh planet from the sun. It is the third largest planet in our solar system. Uranus is named for the Roman god who is father of the Titans.
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