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Physicochemistry

Ionization is in general produced by the Photon S with great energy emitted by a star young person and hot being in the vicinity; for example the radiation UV of a star of standard OB. Usually, a young star will ionize part of the cloud which generated it, but only the massive and hot stars can provide the quantity of energy required to ionize a significant part of an interstellar cloud. Often, a whole cluster of young stars carry out work. This ionization overheats the interstellar Milieu surrounding; the typical temperature is in the neighborhoods of 10 000 K but depends on the composition and the density of the medium.

The color of nebulas depends on their Chemical composition and the intensity of their ionization. Many nebulas in emission are with dominant red, the color of the line of the Hydrogène alpha with 656,3 Nanomètre S of Wavelength, because of the strong presence of Hydrogène in interstellar gases and of sound Potentiel of ionization relatively low. If ionization is more intense, others elements can be ionized and nebulas can emit not only in other nuances of red (Soufre II to 671,9 and 673,0 Nm), but also in the green (Oxygène III to 495,9 and 500,7 Nm) and in blue (Hydrogène beta to 486,1 Nm). Thus, by examining the spectrum nebulas, the astronomers can deduce their chemical composition. The majority of nebulas in emission are made of approximately hydrogen 90%, the remainder being Hélium, oxygen, Azote and other elements.

Classification of nebulas in emission

One can classify nebulas in emission in four categories.

The diffuse nebulas in emission or HII/Hα areas

The Nebulous of Orion (M42), the Nebulous Omega (M17) and the Nébuleuse of the Lagoon (M8) appear among most beautiful diffuse nebulas in emission visible since the northern Hemisphere.

One often opposes them to the nebulas by reflection and the obscure nebulas although they are often associated. Such combinations provide some objects interesting to observe. The majority of most brilliant of these nebulas bear the name of it to what they can visually resemble, like the Nébuleuse of North America (NGC 7000) or the Nébuleuse of the Cone (NGC 2264), delicately modelled by obscure nebulas which mask nebula with emission partially. Other diffuse nebulas are made at the same time of components in emission and components by reflection, like the Nébuleuse Trifide (M20).

By defect, when one speaks about “nebula in emission”, one implies “nebulous most of the time diffuses in emission”. Indeed, the three following types have a clean denomination which supposes the emissive character of nebula. One will prefer all the same the term of area HII (to pronounce “axe two”) or of area Hα (“axe alpha”) in order to raise ambiguity.

Planetary nebulas

Among most famous, let us quote the Nébuleuse of Haltère (M27), the Anneau of the Quadrant (M57) or the Cats eye (NGC 6543).

The remanent ones of supernova

Let us quote the Dentelles of the Swan and the Nébuleuse of the Crab (M1).

Bubbles of Wolf-Rayet

Not very frequent, some of them remains relatively accessible, like NGC 2359 and NGC 6888.

See too

Nebulous
Nebulous Area HII
Remanent planet gear
of obscure supernova
Nebulous
Bubble of Wolf-Rayet

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