A cloud of interstellar dust that appears blue because it scatters light from nearby stars, with shorter blue wavelengths scattered more efficiently than longer red wavelengths. These nebulae do not emit their own light but simply reflect and scatter starlight from embedded or nearby stars.
From Latin 'reflectere' (to bend back) and 'nebula' (cloud), coined in the early 20th century when astronomers realized some nebulae showed stellar spectra rather than emission lines. The term emphasizes the physical process of light scattering rather than atomic emission.
Reflection nebulae are blue for the same reason Earth's sky is blue - Rayleigh scattering! The tiny dust grains preferentially scatter blue light, creating some of the most beautiful blue clouds in space, like the ghostly wisps around the Pleiades star cluster.
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