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UV radiation ranges approximatively from 10nm (extreme UV) to 380nm (near UV).
Ultraviolet line spectrum measurements are used to discern the chemical composition, densities, and temperatures of interstellar medium, and the temperature and composition of hot young stars. UV observations can also provide essential information about the evolution of galaxies.
The ultraviolet Universe looks quite different from the familiar stars and galaxies seen in visible light. Most stars are actually relatively cool objects emitting much of their electromagnetic radiation in the visible part of the spectrum. Ultraviolet radiation is the signature of hotter objects, typically in the early and late stages of their evolution. If we could see the sky in ultraviolet light, most stars would fade in prominence. We would see some very young massive stars and some very old stars and galaxies, growing hotter and producing higher-energy radiation near their birth or death. Clouds of gas and dust would block our vision in many directions along the Milky Way.
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