Brown dwarf

A brown dwarf is a failed star. It is a celestial body too small to fuse hydrogen, but still able to produce some energy by fusing deuterium into helium. Even if their light is too dim for plants, still they generate enough heat to warm a planet on a close orbit.

Formation and lifetime
The life of a brown dwarf can be separated into 4 different stages:

1 - formation (M class) Brown dwarfs might form in a similar way stars are created: from a cloud of gas and dust. Only that the cloud is too small. Some of them orbit around a host star, so they were created like planets. In either ways, when matter collided, it should have generated enough heat to start a nuclear reaction. If the initial temperature was high enough, it could be sufficient to start fusing hydrogen into helium. If that is the case, the brown dwarf will increase size, so that pressure and temperature will decrease below hydrogen fusing, then it will slowly contract. In this phase, the dwarf might look and behave like very dim M - type stars, they might even have flares.

2 - deuterium fuse (L class) In this phase, a brown dwarf creates a limited amount of heat by fusing deuterium. The process does not generate too much energy. Deuterium is an isotope of hydrogen, with one proton and one neutron. Since deuterium is found more rare, it will be exhausted fast. Depending on mass, some brown dwarfs might not be able to use all their deuterium. If they are too small, internal pressures will drop fast enough, so that up to 70% of deuterium will remain unused. Larger brown dwarfs will use all (or almost all) their fuel.

3 - methane (T class) When nuclear reactions are no longer taking place, the brown dwarf is cooling gradually. As the internal heat is diminishing, surface temperature is also decreasing. At some point, temperature drops below 1000 K and methane starts to get formed in the upper atmosphere. The brown dwarf still emits a dim red light, but its spectra contains some blue, from heated methane.

4 - black object (Y class) At some point, a brown dwarf is so cold that it no longer emits visible light. It becomes like a planet, like a giant Jupiter, with a similar diameter, but with much higher mass.

A brown dwarf can be directly created into M - class. However, if accretion process is slow enough and matter has time to cool itself, it might simply result a lower-class dwarf.

Sub - brown dwarf is an object that has not enough mass to fuse deuterium, but looks similar. In theory, when it was created, internal temperature could be enough to sustain deuterium fusion, but pressure was not enough to maintain the process.

Physical characteristics
To get an overall look at brown dwarfs, please go here.

A brown dwarf has a diameter similar to Jupiter (slightly larger in its upper stages). Its mass is between 15 and 65 times the mass of Jupiter. Temperature varies, as shown below:


 * M - class: ~2500 K
 * L - class: ~1300 K
 * T - class: ~700 K
 * Y - class: ~200 K
 * M - class dwarfs have a larger diameter (probably 1.5 times the diameter of Jupiter). L - class have their name from lithium. This is because lithium decays easy in the conditions found in their core. Spectral analysis found metals in their absorption lines, this is because at their temperatures many metals evaporate and can be found as volatiles in the upper atmosphere. On the other hand, T - class dwarfs are cold enough to allow creation of methane, found in high amounts in their upper atmosphere. The Y - class is different. When they are cold enough, water clouds can be formed.

Light
M - class brown dwarfs are flare stars, eruptive, with strong and dangerous flares, like the dim M - type stars. For example, DEN 1048-3956 was detected because a massive flare. When flares occur, light output might increase 100 times in less then a minute. A flare also comes with a massive solar wind that will destroy existing ecosystems. Since their average energy output, a habitable zone should be at maximum 5 million km away. At that distance, the effect of a flare would be catastrophic.

L and T - class brown dwarfs have a dim, red light. To have a better idea how their light looks like, their light output is similar to a burning cigarette. An L - class one will look like a cigarette when someone is inhaling, while a T - class dwarf is like a cigarette resting on an ashtray. Their light is so dim that it's hard to find your way around, on a nearby planet. Through a telescope, the Epsilon Indi brown dwarfs (T class) are hardly visible in red, clearly visible in infrared and almost invisible in blue.

Y - class brown dwarfs emit no visible light, only infrared.