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Dark nebulae can be seen if they obscure part of a emission or reflection nebula (eg. the Horsehead Nebula) or if they block out background stars (eg. the Coalsack Nebula).
The form of such dark clouds is very irregular: they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes. The largest dark nebulae are visible to the naked eye, appearing as dark patches against the brighter background of the Milky Way.
The hydrogen of these opaque dark clouds exists in the form of molecular hydrogen. The largest nebulae of this type, the so-called giant molecular clouds (GMC), are more than a million times as massive as the Sun. They contain much of the mass of the interstellar medium, are some 150 light-years across, and have an average density of 100 to 300 molecules per cubic centimetre and an internal temperature of only 7 to 15 KThe kelvin (symbol: K is the SI unit of temperature, and is one of the seven SI base units. It is defined by two facts: zero kelvin is absolute zero (when molecular motion stops), and one kelvin is the fraction 1/273. 16 of the thermodynamic temperature o. Molecular cloudA Molecular cloud is a type of interstellar clouds whose density and size permits the formation of molecular hydrogen, H. However, this molecule is difficult to detect, and the molecule most used to trace the H is CO ( carbon monoxide). The ratio betweens consist mainly of gas and dust but contain many stars as well. The cloud cores are completely hidden from view and would be undetectable except for the microwaveThis page is about the radiation; for the appliance, see microwave oven. Microwaves are electromagnetic waves with a wavelength longer than infrared light, but shorter than radio waves. Microwaves, also known as Super High Frequency (SHF signals, have wav emissions from their constituent molecules. This radiation is not absorbed by dust and readily escapes the cloud. The material within the clouds is clumped together in all sizes, with some clouds ranging down to the masses of individual stars, small clumps may extend about one light-year across. The clouds have an internal magnetic fieldIn physics, a magnetic field is an entity produced by moving electric charges ( electric currents) that exerts a force on other moving charges. The quantum-mechanical spin of a particle produces magnetic fields and is acted on by them as though it were a that provides support against their own gravityThis article covers the physics of gravitation. See also gravity (disambiguation). Gravitation is the tendency of masses to move toward each other. The first mathematical formulation of the theory of gravitation was made by Sir Isaac Newton and proved ast.
GMCs play an important role in the Galaxy dynamics: when a star passes near a GMC, the considerable gravity pull will perturb the star's orbit by a not-insignificant amount. After repeated near encounters, a middle-aged star will have significant velocity components in all directions, instead of an almost circular orbit like a newborn star (this is because the newborn star inherits the circular orbit of the GMC where it was born). This gives the astronomer another tool to estimate star ages, and helps to explain the thickness of the galactic disk .
In the inner regions of dark nebulae important events take place: the formation of starsStar formation is the process by which gas in molecular clouds gets transformed into stars. In the current paradigm of star formation, cores of molecular clouds (regions of especially high density) become gravitationally unstable, fragment and begin to co.