A nebulous (of the Latin the nebula , “cloud”) indicates, in Astronomie, a Celestial object of diffuse aspect composed of Gaz rarefied and/or interstellar dust. Nebulas are studied by astrophysicists specialized in the study of the interstellar Milieu.

History

Until the Years 1920, before Edwin Hubble shows only the aspect then nebulous of certain stars (the Galaxie S) was due only to the insufficient resolution of the instruments of then, the term of nebulous was used to indicate all Celestial object diffuse aspect.

Since then, one generally understands by nebulous any area of the interstellar Milieu particularly rich either in ionized gases (of the Hydrogène), or in interstellar dust, or of both.

Types of nebulas and classification

One can differentiate six types of nebulas. Roughly speaking more frequent with rarest:

; Obscure nebulas: Composed of dust and inert gas, they partly absorb the light which crosses them and thus veil what is behind them. In the field of the visible one, one can detect them only by contrast on a star field or a nebula.

See also: Nebulous obscure

; Planetary nebulas: In fact nebulas in emission are the mark of a star of low mass at the end of the lifetime and precede the destiny of the Sun. Often small, round and relatively brilliant, the astronomers a long time confused them with planets, from where them name.

See also: Nebulous planet gear

; Nebulas by reflection: Identical in kind to obscure nebulas, they reflect however partially the light of a star located in the vicinity.

See also: Nebulous by reflection

; Areas HII: Sometimes spectacular, these nebulas in emission is sometimes described like pouponnières of stars and is often associated with a open Amas of young stars or in formation.

See also: Area HII

; The remanent ones of supernova: These very wide nebulas in emission is the result of the violent explosion of a star of high mass. They often raise a characteristic filamentous structure which evokes lace.

See also: Remanent of supernova

; Bubbles of Wolf-Rayet: From an aspect close to that of the vestiges of supernova, these gas bubbles result from the progressive expulsion of the external layers of an extremely hot and massive star.

See also: Bubble of Wolf-Rayet

One can classify these nebulas according to two criteria: morphology and the physico - Chemistry. The last approach, more recent, is justified naturally much on the astrophysical level.

Morphological tree

• diffuse Nebulas

• Areas HII
• Nebulas by reflection
• obscure Nebulas
• planetary Nebulas
• filament Nebulas
• Vestiges of supernova
• Bubbles of Wolf-Rayet

Physicochemical tree

• Interstellar dust

• Nebulas by reflection
• obscure Nebulas
• Nebulas in emission
• planetary Areas HII
• Nebulas
• Bubbles of Wolf-Rayet
• Vestiges of supernova

Characteristics of diffuse nebulas

The matter which constitutes a nebula is very rarefied: there are in a nebula a few hundreds of Molécule S per cubic centimeter only - in the terrestrial atmosphere one finds, for same volume, of the billion billion molecules.

Nebulas are not homogeneous and have a complex structure.

See also: molecular Cloud

Role in the stellar formation

Nebulas can crumble Gravitation nellement and to form systems of star S. the Solar system would have been thus formed starting from nebulous a known as '' primitive '' or '' solar ''. This scenario was evoked for the first time during second half of the 18th century by Kant and Laplace.

See also: stellar Formation