dark nebula(redirected from Giant Molecular Cloud)
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dark nebula(dark cloud; absorption nebula) A cloud of interstellar gas and dust that is sufficiently dense to obscure partially or completely the light from stars and other objects lying behind it and sufficiently large and suitably located to produce a noticeable effect. These nebulae can be observed as dark extrusions in front of bright (emission or reflection) nebulae or as blank regions or regions with a greatly diminished number of stars in an otherwise bright area of sky. In external galaxies they are often observed against the bright spiral arms, where they appear as dark dust lanes. In our own Galaxy the dark clouds in Taurus are the nearest sites of star formation to the Sun. Although the absorption is caused by cosmic dust, the dark nebulae are composed predominantly of molecular hydrogen. Small dark nebulae, called Bok globules, can sometimes be seen in large numbers superimposed on bright nebulae. Although having no optical features dark nebulae can be studied through their radio and infrared emissions. The Coalsack, Horsehead nebula, and the Great Rift are dark nebulae.
dark nebula[¦därk ‚neb·yə·lə]
A cloud of solid particles which absorbs or scatters away radiation directed toward an observer and becomes apparent when silhouetted against a bright nebula or rich star field. Also known as absorption nebula.