Brown Dwarf

Stars that cannot reach the 15 million Kelvin required to begin thermonuclear fusion are known as brown dwarfs. These failed stars typically weigh between 13 MJ(Jupiter masses) and 84 MJ, or equivalently, roughly 0.012 M☉ and 0.08 M☉. An interesting property of brown dwarfs is that they always have a radius of around 1 Jupiter radius, with only around 15% variation between the least massive brown dwarfs and the most massive brown dwarfs. This is due to electron-degeneracy  in high mass brown dwarfs, and normal Coulomb pressure  in low mass brown dwarfs.

Brown dwarfs, if they are part of a multiple-star system, are very rarely found to orbit within 5 AU of their companion star. This phenomenon is known as the brown dwarf desert.

Physical Properties
Given the mass (in MJ), radius (in RJ) and surface temperature (in Kelvin), the following properties can be determined for brown dwarfs :

Luminosity

Density'Circumference'Surface AreaVolume



Surface Gravity



Escape Velocity

These give answers relative to Jupiter (except for luminosity, which is relative to the sun).

Brown dwarfs have a typical core density of around 10 g/cm3 to 1000 g/cm3, and this wide range of densities arises from the need for a brown dwarf to have roughly the radius of jupiter. Brown dwarfs have core temperatures less than or equal to about 3 Million Kelvin, and core pressures of around 100 000 Mbar.

Some brown dwarfs may exhibit the highly unusual, but highly awesome property of molten iron rain due to atmospheric convection.

Statistically, there is about 1 brown dwarf for every six stars.

Classification
Brown dwarfs can be subdivided based on their spectral type. Brown dwarfs start out as M dwarfs, and then move down the spectral types from L to T and finally to Y, each with a lower temperature and luminosity than the last. More massive brown dwarfs cool slower than less massive brown dwarfs.

M Dwarfs are the youngest and hottest of the brown dwarfs and occupy the late stage of the M spectrum (M6.5 or later). They are composed largely of the same materials as sunspots: carbon monoxide, molecular hydrogen and water vapour. The presence of lithium in their composition is often used as a test to differentiate them from M stars. They have effective temperatures (surface temperatures) greater than about 2500 K. Visually, an M dwarf would have a bright red appearance, similar to M stars (red dwarfs).

L Dwarfs are composed largely of the same materials as M dwarfs, but with significantly higher amounts of metal hydrides (FeH, CrH, MgH, CaH) and alkali metals (Na I, K I, Cs I and Rb I). They may also have quantities of lithium in their composition. They have effective temperatures between 1400 K and 2500 K. They would be a dull red in appearance.

T Dwarfs are composed, unlike their hotter cousins, largely of methane (CH4) and, like L dwarfs, have large amounts of alkali metals, although, unlike L dwarfs, they lack substantial amounts of metal hydrides. They typically range in effective temperature from 600 K to 1400 K, and visually would appear magenta.

Y dwarfs, the coldest and dullest of the brown dwarfs, have atmospheres composed largely of water and ammonia, possibly existing in the form of ice clouds. Due to the time it takes for brown dwarfs to cool to the Y stage, Y dwarfs are very rare, but more and more will form as the universe ages. These Y dwarfs have effective temperatures lower than 600 K, some of them are even at room temperature! They would be violet in appearance.

Habitability
It is entirely possible for a brown dwarf to have a planetary system, as brown dwarfs have been found to have planetary accretion disks surrounding them. The system would likely consist more of terrestrial planets than gas giants, due to the low mass of the accretion disks.

Whether or not these planets are habitable is a different matter entirely. The habitable zone of a brown dwarf would be very narrow, and would decrease with time due to the cooling of the brown dwarf. Planets would also have to have a very low eccentricity (on the order of 10-6) in order to avoid runaway greenhouse effects caused by strong tidal forces if they approach too close to the brown dwarf.

Planets around brown dwarfs are likely to be carbon planets.

Worldbuilding in Practice
"To the joy of astronomers all over the galaxy, the Okeanos Gateship is scheduled to construct a gate to Nisos within the next 15 years. Nisos, a class Y brown dwarf, is relatively close to the Agamemnon System, which is already colonised by humanity, and will act as a stepping stone to Clytemnestra. Nisos itself, however, is of more interest than simply being a 'stepping stone system'." "Nisos is a mid-mass brown dwarf of 34 Jupiter Masses and is about 5% larger than Jupiter, with a density of around 43.1 g/cm3. Nisos is very old and has cooled considerably during its lifetime. Its surface temperature is a frigid (by stellar standards) 483 K (roughly 210 °C)! Because of this, it is not very luminous (outputting only 0.00006% of the Sun's energy) and is a dark purple color. From its only planet, Abrota (which orbits just over 1 AU from Nisos), it is only 15% as bright as Jupiter is from Earth!""Nisos's atmosphere is composed largely of water and ammonia, with large methane bands which stretch across its surface.""When the Agamemnon-Nisos gate is built, Nisos will become the first Y class brown dwarf to be colonised by humanity: a milestone in the progress of science."