Otto von Bismarck
In Astronomy, the dark nebulas , obscure nebulas or nebulas of absorption are areas where dust of the interstellar Milieu seems to concentrate in large clouds which appear in areas low in star S.
Dark nebulas can be seen whether they darken part of a Nébuleuse in emission or of reflection, like the Nébuleuse of the Head of Horse or the Nébuleuse of the Cone in the Constellation of the Cocher, or if they block the light of stars in background, such as for example the Sac of coal in the constellation of the Croix of the South.
The shape of such clouds is very irregular: they do not have any clearly defined external border and take sometimes contorsionnées forms. Greatest dark nebulas are visible with the naked eye, seeming dark zones on the more luminous bottom of the Milky Way.
The Hydrogène of these dark and opaque clouds exists in molecular form (H2). Greatest nebulas of this type, the giant molecular clouds, are several million times more massive than the sun. They contain most of the mass of the interstellar environment, have a size of approximately 150 light-years, a Densité of matter Moyenne from 100 to 300 Molécule S by Cubic centimeter and an internal temperature of only from 7 to 15 K. The molecular clouds are made up mainly of gas and of dust but can also contain much stars. The heart of these clouds are completely hidden with the sight and would be undetectable if their constitutive molecules did not emit in the field of the Micro-onde S. This radiation is not absorbed by dust and can thus cross these clouds easily. The matter in these clouds is grouped in compact masses of all sizes; smallest having hardly the typical size of stars and others extending over one light-year. These clouds has a Magnetic field intern which prevent them from crumbling under the effect of their clean Gravitation.
The giant molecular clouds play a big role in the Dynamique of the galaxies: when a star passes close to a giant cloud, the considerable gravitational attraction of this one will significantly disturb the Orbite of star. After multiple similar meetings, a star of a certain age will have significant components speed in all the directions, contrary to the almost circular orbit that the young stars inherit the clouds which generated them. This gives to the astronomer another tool to estimate the age of stars and contributes to explain the thickness of the galactic disc.
The internal areas of dark nebulas are the seat of significant events: the Formation of the stars.
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