dark nebula

(redirected from Giant Molecular Cloud)
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Related to Giant Molecular Cloud: Dark nebula, interstellar medium

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]
(astronomy)
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.
References in periodicals archive ?
The largest of these structures, fittingly known as giant molecular clouds (GMCs), range from 15 to more than 600 light-years across.
The giant molecular clouds in the Milky Way are concentrated into just such a ring 13,000 light-years in radius - about half of the Sun's distance to the galactic center.
The researchers have revealed that the two giant molecular clouds collide with one another at exactly the bottom of the "pigtail" molecular cloud.
These two quantities are expected to be correlated since this is gas in giant molecular clouds, which are the birth places of stars.
Thomas Dame and Patrick Thaddeus (Harvard-Smithsonian Center for Astrophysics) traced millimeter-wave emission from the carbon monoxide (CO) in giant molecular clouds to identify an arm segment on the far side's outer margin, roughly 50,000 light-years from the Milky Way's center.
Specifically his question is whether, as the solar system moves through the giant molecular clouds (GMCs) that populate interstellar space, the Oort cloud can be alternately stripped of its contents by the gravity of such clouds and then replenished from the matter in the GMCs.
The common denominator is the presence of large reservoirs of cold, dark gas and dust called giant molecular clouds (GMCs).
Open clusters are formed when giant molecular clouds collapse.
The Orion Nebula, starburst knots in the spiral arms of galaxies, and the giant molecular clouds in Ophiuchus are among the beautiful nurseries that we explore in an effort to learn about the often hidden acts of stellar creation.
Using optical, X-ray, and radio data, they conclude that "dark bursts," those that leave no visible-light afterglow, are dark only because they are hidden behind a galaxy's dust clouds--in particular, very dense giant molecular clouds.
Ninety percent of its dust grains retain a temperature of only 20[degrees] Kelvin or less, shielded in giant molecular clouds from radiation emitted by nearby stars or lurking as part of the diffuse medium between stars.
But to Schweizer the Hubble observations suggest that globulars can be spawned by giant molecular clouds (GMCs) when galaxies collide.