Dark nebula

As dark clouds or dark nebulae large clouds of interstellar matter are referred to in astronomy, which absorb the light located behind objects. They can be observed when they darken the background stars or completely hide (for example, Barnard 68), or if they cover parts of emission or reflection nebulae (eg Horsehead Nebula ).

The form of such dark clouds is very irregular, without clearly defined borders and with sometimes tortuous shape. The largest dark clouds are visible to the naked eye as dark spots against the lighter background of the Milky Way.

The hydrogen in this opaque cloud is in the form of molecules. The biggest clouds of this type, known as giant molecular clouds (giant molecular clouds, GMC), have a million times the mass of the sun. They make up a significant proportion of the mass in the interstellar medium, are up to 150 light -years wide and have an average density of 100 to 300 molecules per cm ³ and an internal temperature of only 7 to 15 K.

Molecular clouds consisting mostly of gas and some filamentary dust, but can also include a larger number of stars. Clouds centers are not in the visible light range visible, but can be carried out by microwave radiation of the molecules present. This type of radiation is not absorbed by dust and can escape from the cloud. The cloud material has agglomerations in different sizes of star size to light years large structures. The clouds have an intrinsic magnetic field that counteracts its self-gravity.

Giant molecular clouds ( GMCs ) play an important role in the dynamics of the galaxy: When a star near such a cloud passes, their attraction can cause a significant perturbation of its movement in space. After repeated encounters of this kind has a middle-aged star significant velocity components in different directions, instead of the nearly circular orbit of a newly formed star around the Milky Way center ( young stars have the same orbit as the GMC, in which they are incurred ). This gives astronomers another tool to determine the age of stars and helps to explain the observed thickness of the galactic disk.

The nature of dark clouds was discovered by the astronomer Edward Emerson Barnard. In the eponymous Barnard 's catalog of dark clouds over 300 of them are listed. So, for example, carries the famous Horsehead Nebula catalog number B 33