The preferential scattering and absorption of blue light over red light by interstellar dust, causing distant objects to appear redder than their true color. This occurs because shorter wavelengths (blue) are scattered more efficiently than longer wavelengths (red).
From Old English 'read' meaning red, combined with the suffix '-ening' indicating a process. The astronomical usage developed in the early 20th century when astronomers realized that interstellar dust was systematically altering stellar colors.
Interstellar reddening works exactly like Earth's sunsets - both are caused by small particles scattering blue light away while letting red light pass through! The difference is that in space, this 'sunset effect' can make a blue giant star look like a red star from billions of miles away.
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