Brown Dwarf

This article covers the Substellar Object types Brown Dwarf and Sub-brown Dwarf, objects that are too small to fuse light Hydrogen ("protium" - ₁H¹) and too large to be a Gas Giant that formed around a rocky core thru planetesimal accretion.

• Please see Star: Origin & Formation section at article Star.

Description (Specifications)

Large Gas worlds can form thru one of two primary mechanisms which define the categorization of the object.

1. A "Gas Giant" results from an initial rocky core formed thru the planetary formation process via the accretion of planetesimals. When such a core becomes massive enough and forms far enough out in the colder regions of the star system to trap and retain a substantial Hydrogen atmosphere, the result is the formation of a Gas Giant planet.
2. A "Brown Dwarf" properly results from the gravitational collapse of a nebular gas cloud of Molecular Hydrogen in the same manner as is common in typical star formation. If the body eventually formed is of mass below about 85 M_J it will have insufficient final mass to initiate the normal proton-proton core fusion of light Hydrogen characteristic of typical Main-sequence dwarf stars. If the mass is nevertheless greater than about 13 M_J however, it will have sufficient final mass to initiate the process of Deuterium fusion, which will occur for a brief period early in the Brown Dwarf's lifespan before using up this fuel resource. If the body lies between 65 M_J and 85 M_J it will also briefly engage in Lithium burning before using up that fuel resource as well. Resultant objects of less than 13 M_J have insufficient mass to initiate any kind of thermonuclear reaction at all and are referred to as "Sub-brown Dwarfs".

Once a Sub-brown Dwarf or Gas Giant reaches about 170,000km diameter, it may become more massive, but it doesn't become much larger - the core density just increases instead. These become very hot due to gravitational heating and reach temperatures at which Deuterium (₁H² or "D") can begin to fuse, forming a proper Brown Dwarf. All Brown Dwarfs are too light to fuse light Hydrogen (₁H¹) as a fuel by any reaction pathway. They may fuse heavy Hydrogen ("Deuterium" - ₁H² or "D") in their early history, but due to the relatively small fraction of heavy Hydrogen by volume found in a typical sample of Hydrogen, the time spent fusing Deuterium is generally very short before Deuterium burning shuts down, after which the Brown Dwarf will slowly radiate its heat away via gravitational contraction, slowing cooling from its initial spectral class down thru the cooler spectral classes toward thermal equilibrium with its local environment, much as Gas Giants and Sub-Brown Dwarfs do.

The most massive Brown Dwarfs overlap with the lowest mass proper stars, occupying the spectral category of ~ M6.5 down thru M9.5. Those Brown Dwarfs that are massive enough (and hot enough) to initiate Lithium burning generally fall within this range (though not all are). Less massive (and cooler) Brown Dwarfs initially range thru spectral Types L, T and Y, respectively, and as they subsequently cool they slowly devolve down thru this sequence. Sub-brown Dwarfs are sometimes classified below Type-Y as Type-H.

History & Background (Dossier)

Brown Dwarfs are relatively common. In general, Brown Dwarfs are theoretically proposed and detected well before the era of spaceflight by most sophont species. The ancient Terrans, for example, learned of them about a century before the dawn of their own space age.

A Brown Dwarf may resemble a Gas Giant in many ways. Anyone attempting to harvest fuel from the atmosphere of a Brown Dwarf should keep careful watch on their manoeuvre drive as their gravity is strong and requires at least 3-G of acceleration or greater to regain orbit. This however does not take into account the extreme temperatures and immense winds that are typically encountered in Brown Dwarf atmospheres.

References & Contributors (Sources)

This page uses content from Wikipedia. The original article was at Brown_dwarf. The list of authors can be seen in the page history. The text of Wikipedia is available under the Commons Attribution-ShareAlike 3.0 Unported License.

This list of sources was used by the Traveller Wiki Editorial Team and individual contributors to compose this article. Copyrighted material is used under license from Mongoose Publishing or by permission of the author. The page history lists all of the contributions.

• Author: BackworldTraveller