A Brown Dwarf is an object larger than a planet, but smaller than a star. Many celestial bodies exist that straddle the line, and are hard to categorize. The usual definition is above 13 Jupiter masses (at which point deuterium fusion begins) but below 80 Jupiter masses (at which point the normal hydrogen fusion of stars begins). Most brown dwarves have Lithium in them - but most actual stars burn lithium off (breaking it into helium). Detecting Lithium in the spectra is one way to know from far away whether a small fiery object is a star or a brown dwarf - this is known as the Lithium Test. Brown Dwarves lack nuclear fusion, and are instead heated by gravity and more mundane combustion].
Brown dwarfs change colors somewhat as they contract over the course of their lifetimes. They may actually appear red or magenta, instead of brown. The highest mass brown dwarfs start their lives as Class M Stars and will cool through the L, T, and Y spectral classes. Despite this contracting, they all have a radius roughly similar to that of Jupiter, so given their mass (13 to 80 times that of Jupiter), they must be very dense.
Brown dwarfs are frequently part of binary or larger star systems. However, they cannot be closer than 5 astronomical units from their companions star, or else they would be pulled in and consumed. This area where they can't exist is known as the Brown-Dwarf Desert.
- Helioforming - converting brown dwarfs into usable stars or protostars
- MACHOs - the notion that brown dwarfs may be a major form of Dark Matter
- Nemesis (star) - a specific hypothetical brown dwarf that orbits our sun every 26 million years
- RAMBOs - clusters of brown dwarfs
Surface Conditions (and possible life)
Brown Dwarfs never quite became stars, but many are likely to be quite warm. Many radiate thermal energy and light in the infrared range. Others emit x-ray pulses. Many will have lava on the surface or magma near it. Other brown dwarves, however, may be cool enough to have stable water on the surface. Scientists have observed via telescope Brown Dwarf Stars whose surface temperature lies somewhere between a warm summer day (here on earth) and a hot cup of coffee - thousands of degrees cooler than our own Sun.
Many Brown Dwarf stars are likely to experience volcanism and even lightning storms up topside. Since the lava and lightning bit sounds a little like the early conditions on the earth, it's possible that life may evolve on a brown dwarf. Even seemingly inhospitable environments here on Earth have extremophile life, so Brown Dwarf stars might have inhabitants on their surface.
Brown Dwarfs are extremely dense, and have heavy gravity. Native life would need to be sure-footed and strongly constructed, in addition to being adapted to extreme temperatures. If there is significant water on the surface of a brown dwarf then life there is likely to be aquatic, as being submersed in water would help alleviate the pressures and dangers of the high gravity. Don't beam down to the surface if you aren't prepared to be squished by your own weight.
Local life is unlikely to have eyes, but may have other sensory organs. They may be sensitive to vibrations, thermal variations, and magnetic or electrical fields. They may be pale in color (like animals found in deep caves on earth where the light doesn't shine) and would lack any biological need for camouflage or colorful displays. Perhaps picture a cold-blooded albino creature with the exoskeleton and strength of an ant (able to carry more than a hundred times its own body weight) combined with the heat-sensing organs of a pit viper and possibly capable of infra-red photosynthesis equivalent.
Game and Story Use
- A brown dwarf orbiting a larger star might have Earth-sized, habitable moons.
- An interstellar brown dwarf might have planets orbiting it. Though these would be too cold for life, an interstellar SF campaign could use the mining of some rare substance (perhaps the setting's Phlebotinum) from one of these planets.
- The brown dwarf itself can be the planet and the star, rolled in to one to simplify the GMs job. A rogue planet version of a brown dwarf could serve all your plotline needs.
- Ships start going missing along a well-traveled space lane. The PCs are sent to find out what went wrong. They discover that a nigh-indetectable rogue brown dwarf has drifted onto the route. The missing ships failed to notice it soon enough to prevent crashing into the large dark mass. The PCs search the surface for survivors, but instead find there's actually a new intelligent species on the planet. This species has no eyes, and thus no idea that there are other stars out there that life could come from. These aliens have found the wreckage and escape pods, but aren't sure what to make of it. The gravity on the surface is so high, that the PCs can't interact with the aliens without use of potent anti-grav tech or special EVA suits.
- Realistically, the most likely life on a brown dwarf would be unicellular, as that would really reduce the dangers and needs associated with larger life in such a place. I think it's safe to assume, however, that most gaming groups want their aliens on a little larger scale. But if your game is the sort of campaign where Bacteria can form sentient colonies, then a brown dwarf might be just the right place to spawn them.
- Extremely dense matter from a brown dwarf might be a valuable (or scientifically interesting) resource. Under the influence of all that gravity, heat and pressure some very interesting materials might form - think super-diamond (see also diamond planet). Mining it would be perilous, of course.