dark nebula

Most nebulae are noticed because they glow. A dark nebula is noticed by its absence of light — it is a cloud so thick with dust that it blots out the stars behind it, appearing as a starless, inky hole or a black silhouette against a brighter background. Early astronomers thought these were genuine holes in the heavens; in fact they are some of the densest, coldest clouds in the galaxy.

A dark nebula is the same interstellar dust and gas as everywhere else, just packed densely enough that the dust totally absorbs and scatters away the visible light trying to pass through. These clouds are cold — often just 10 to 20 kelvin, close to the lowest temperatures in nature — and dense enough that hydrogen pairs up into molecules, so dark nebulae and the cold molecular clouds largely overlap. Their opacity is exactly what makes them important: shielded from harsh starlight, their cold, quiet interiors are where gravity can win and stars are born. The smallest, roundest ones, called Bok globules, are compact knots that may be on the verge of collapsing into stars.

Because dust blocks visible light but lets infrared and radio through, what looks pitch-black to the eye is alive with activity when viewed in those longer wavelengths — infrared cameras peer inside and find protostars glowing in the dark. The famous Horsehead Nebula is a dark nebula, its horse-head shape a dense pillar of dust seen against the red glow of an emission nebula behind it. The lesson is that 'dark' in astronomy often means 'full,' not 'empty.'