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Related to emission nebula: planetary nebula, Reflection nebula
emission nebulaA region of hot interstellar gas and dust that shines by its own light. Most of the gas atoms in the cloud are ionized – stripped of one or more electrons. The electrons liberated by the ionization process are free to move about the nebula. The light, and other radiation, emitted by the nebula results from subsequent interactions between ions and free electrons, and from the decay (de-excitation) of excited atoms.
There are three classes of emission nebula, all of extremely low gas density. In H II (ionized hydrogen) regions, such as the Orion nebula, the gas is ionized by ultraviolet (UV) radiation from nearby hot young O and B stars; if there is sufficient gas close to one or more such stars a luminous nebula will form, surrounded by cooler neutral gas. The size is dependent on gas density and ultraviolet flux (see Strömgren sphere), ranging up to several parsecs. The temperature is maintained at about 10 000 K by forbidden line radiation, mostly of oxygen and nitrogen. In planetary nebulae, typified by the Ring nebula, the gas is ionized by UV radiation from the remnant of a dying star lying within the nebula; the nebula comprises gas expelled from this star. In supernova remnants the ionization mechanisms are very complex: the atoms can be ionized by ultraviolet synchrotron radiation, as in the Crab nebula, by interactions following the collision of an expanding remnant with surrounding interstellar gas atoms, as in the Cygnus Loop, or possibly by other processes.
Emission nebulae contain primarily free electrons and hydrogen ions, together with some helium ions and trace amounts of oxygen, carbon, nitrogen, and other elements. These components can interact in various ways. Firstly, an electron can be recaptured by an ion, usually a hydrogen ion. This recombination leads to the emission of radio, infrared, and optical continuum radiation, with line radiation then being emitted as the electron cascades down to lower energy levels. Secondly, an electron can collide with an ion, such as ionized oxygen, without being captured. Instead, energy is transferred from electron to ion, which becomes excited into an energy level just above the ground state. The excited ion returns to its normal energy state by emitting light at discrete wavelengths: at higher densities these transitions would be ‘forbidden’ (see forbidden lines). Thirdly, free electrons can be decelerated when they pass near an ion, energy being radiated away as weak bremsstrahlung – mainly radio waves.
The spectacular colors, predominantly red and green, of an emission nebula are due to recombination and excitation. The most intense visible radiation from hydrogen is the red hydrogen-alpha emission; red light is also emitted during a forbidden transition of singly ionized nitrogen. The strongest emission, however, results from a forbidden transition of doubly ionized oxygen (O++ or O III), the emission being green; the O III ions can only be produced by highly energetic UV photons. See also nebula.