Dwarf brown
The brown dwarf form an original class of stars, intermediate mass between planets and stars.
Because of its too low mass, the temperature and the pressure in its heart are not sufficient to start or maintain the reactions of nuclear Fusion. It is to some extent a fallen through star. But contrary to planets, it radiates a little its own heat. Dwarf brown can, at a certain time, to have succeeded in starting of the reactions of fusion, but never not having reached a stable condition and to have ended up “dying out”.
The majority of dwarf brown only float in space, which confirms that they are formed like stars and not like planets. Today, it is considered that dwarf brown must have a mass higher than 13 times that of Jupiter and lower than 0,07 solar Masse because above this mass, the reactions of fusion can engage durably. The threshold of 13 masses joviennes was selected because it is about the lower mass to which a star can burn Deutérium. It is also about the higher limit in mass for the objects detected by radial speed in orbit around certain stars. Another definition of the limit separating planets from dwarf brown was also proposed, dwarf brown is distinguished from a giant planet, by the mechanisms which led to its formation. Dwarf brown is born from the fragmentation of a molecular cloud, just like the stars, and the planets are born in local collapse from a disc which is present around a star.
The discovery of dwarf brown surrounded by a disc protoplanétaire (see Cha 110913-773444) leaves suppose that the formation of planets, natural by-products of the stellar formation, is possible also around the dwarf brown ones.
One qualifies dwarf brown cold with 1000 °C, and of hot starting from 2000°C. Heat emitted by dwarf brown is nothing any more but the residue of its formation (this type of star not producing a energy). A brown dwarf young person will be thus rather hot, then will cool slowly during his existence. It is interesting to note that the brown dwarf young people have temperatures of surface similar to those of not very massive and older stars and are almost indifférentiables. It is only after a few tens to a few hundreds of million of years (depend on the mass of dwarf brown) that those reach the temperatures of the coldest stars (approximately 1800 K). When the dwarf brown ones reach ages of several billion years, they have temperatures of surface going from 400 K to 1000 K.
The dwarf brown ones are not easily observable, since they emit only one weak radiation in the Infrarouge.
It is only since the medium of the Années 1990 that one could establish their existence.
Various types of dwarf brown
The dwarf brown ones are subdivided in several spectral types:
Dwarf M: Spectral type of dwarf brown hottest and stars of very low mass.
Dwarf L: One knows a few hundreds of dwarf L, those present spectroscopic signatures of grains of dust in their atmosphere and seem to have a “cycle of the rain” with their grains of dust.
Dwarf T: Coldest of dwarf brown known, with temperatures of surface of less 1200K. Approximately 60 dwarf T (November 2005) are known whose majority are in the immediate vicinity of the Sun. Dwarf T have strong spectroscopic signatures of methane and carbon monoxide. Coldest dwarf T known has a temperature of 750K.
Dwarf Y?: A class was already planned for a type of dwarf brown which was not observed yet but whose existence is envisaged by the ideal models: dwarf the Y. Those would have a temperature about 500K and the spectroscopic signatures of the Ammoniac.
Simple: Brown dwarf
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