Brown dwarfs

Monday, July 20, 2009

Brown dwarfs form like stars so you can't call them planets, but they have a mass that is to small to sustain the nuclear fusion process that takes place inside a star, so they aren't really stars either. As such an object contracts under the influence of it's own gravity, it doesn't reach the temperature that is needed to start the nuclear fusion from H-nuclei into He-nuclei. But it does reach a high enough temperature for the fusion of protons into deuterium. Because of this fusion it emits light during the first period of it's lifetime. It also emits light because of the fall energy that is produced as a result of the contraction of the gas, but this forms only a minor contribution to the total emission of light. Because very little or no fusion takes place, the core of the star can't build up enough pressure to prevent the star from further contraction under the influence of the gravitational forces that are working on the gas. The gas in the center of the star gets so dense that it degenerates. Now the star won't contract any further because of the pressure that is build up by the degenerated matter. This pressure is called electron degeneracy pressure (EDP) (Kulkarni, S.R.). The origin of this pressure is explained by quantum mechanics as arising from oscillations of confined electrons. Because the fusion and contraction have stopped, the only light emission that is left is due to the cooling of the atoms of the star. During the rest of it's lifetime the star will get dimmer and dimmer and eventually and up as a cool object emitting light in the infrared. The chemical composition of the atmosphere of a brown dwarf strongly depends on it's temperature. But for an atmosphere similar to that of the first detected brown dwarf (gl229B), chemical equilibrium calculations indicate that the upper layers of it's atmosphere mainly exist out of methane (CH4), ammonia (NH3), water (H2O) hydrogen sulfide (H2S) and phosphine (PH3). However deep in the atmosphere methane is "replaced" by carbon monoxide (CO) and ammonia is "replaced" by nitrogen (N2) (Marley, M.S.).

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