reflection nebula
/ NEB-yuh-luh /
Not every glowing cloud in space makes its own light. Some shine the way the daytime sky shines — by scattering the light of a nearby source. A reflection nebula is a cloud of interstellar dust that does not emit its own light but glows because it is scattering the light of a star close by, the way a fine mist glows blue-white in a car's headlights.
Reflection nebulae appear when a star is bright enough to light up the surrounding dust but not hot enough to ionize the gas (which would instead make a red emission nebula). The tiny dust grains scatter blue light far more efficiently than red — for the very same reason the daytime sky is blue — so reflection nebulae glow a characteristic soft blue, even when the illuminating star is white or yellowish. The spectrum of a reflection nebula is telling: it is a faint copy of the star's own spectrum, complete with the star's absorption lines, because the dust is simply bouncing the starlight toward us rather than producing anything new.
Reflection nebulae are a vivid demonstration that 'dark' dust clouds are everywhere, even when no star happens to light them. They also help astronomers study the dust itself — the exact shade of blue and how the brightness fades with distance reveal the grains' size and composition. The classic example is the gauzy blue haze around the Pleiades star cluster, dust the cluster happens to be drifting through, caught and lit like fog in a streetlamp's beam.
The Pleiades star cluster is wrapped in a wispy blue glow — a reflection nebula of dust the cluster is passing through, lit not by its own light but by the brilliant blue starlight scattering off the grains.
The Pleiades' blue haze is starlight scattered by surrounding dust.
A reflection nebula shows the illuminating star's spectrum, complete with its absorption lines, because it only scatters light — whereas an emission nebula has its own bright emission lines. The blue color comes from the same scattering physics that makes the sky blue.