What phenomenon causes a star's light reaching Earth to have its blue component diminished, making it appear redder than its actual color?

Answer

Interstellar reddening caused by cosmic dust

Interstellar dust preferentially scatters shorter wavelengths, like blue and violet light, more effectively than longer wavelengths like red and orange, leading to an apparent shift toward red upon arrival at the observer.

What phenomenon causes a star's light reaching Earth to have its blue component diminished, making it appear redder than its actual color?
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