Even something too small to emit visible light will probably give off radio waves and infra-red radiation (Jupiter does).

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These can be made to emit white light by coating the device with a phosphor compound that absorbs part of the blue light and re-emits it as yellow.

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But LEDs are made from a semiconductor material, and are able to emit much more light for the same amount of electricity.

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Active galaxies emit jets of light - up to trillions of times more energetic than the light we see - and a blazar is one with a jet pointing toward the Earth.

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In a report in the journal Nature, US researchers say they have worked out how black holes emit jet streams of particles at close to light speed.

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Mercury arc lamps (the kind used in football stadiums) are not much better, since they emit a lot of ultraviolet light, which contributes to astronomers' woes without illuminating the action in a way that the human eye can detect.

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It was also a big week for lighting news as we learned that compact fluorescent light bulbs emit potentially harmful UV radiation -- another good reason to make the switch to LEDs.

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Fluorescent bulbs emit a pallid, bleached light that is okay for hallways and porches, but which fails to flatter and provide a good quality of light for living rooms.

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