The brown dwarfs' formation.

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"General size comparison between a low mass star, a brown dwarf, and the planet Jupiter. Although brown dwarfs are up to 80 times more massive than Jupiter, their size would only be about 10-15% larger. Image via Wikimedia Commons". (https://earthsky.org/What are brown dwarfs?). In the image above: brown dwarfs are compared with planets and low-mass stars. Brown dwarfs are hardly larger than Jupiter. They are heavier than Jupiter. But not much. 

In the beginning, I must say that there are many types of stars under the term "brown dwarfs". There are at least "L", "T", and  The "late-M" dwarfs are the smallest versions of red dwarfs. The first founded brown dwarf, "Teide 1" is the "late-M" dwarf. So some researchers would classify Teide 1 as a red dwarf. 

Some researchers rather put "late-M" dwarfs to red dwarfs than brown dwarfs. Sometimes is suggested that the border between brown and red dwarfs is that the red dwarf's nuclear reactions are non-stop. And brown dwarf's nuclear reactions are full of breaks. In some theories in some lithium-fusion stars, the fission maintains fusion. And the main topic in the next text is lithium-fusion stars. 

That means the brown dwarf was born as a "late M" star. The mass of young brown dwarfs will be high enough to maintain non-stop fusion. Or the fissile material in lithium-star solid nuclei can maintain so high-energy fission that those stars keep fusion all the time. But in older versions, the brown dwarfs blow so much material into space, that they cannot create a fusion that continues without breaks. 

The brown dwarf is sometimes called a failed star. Or actually, it's a very cold, and lusterless star. The forming of a brown dwarf could have a connection with forming of rocky planets. There is a possibility that if there are lots of very heavy radioactive isotopes in the planetary cloud that could explain the formation of a brown dwarf. 

If there are lots of radioactive elements in the protoplanet that thing can form a "fission star", or otherways saying, the nuclear reaction can melt the planet's shell. But if that kind of planet starts to form in the area. There is enough gas. If there are lots of radioactive elements in the planet's mantle, that thing can make the situation. The planet's nucleus or the fissile area in the planet is very big. Or nuclear fission can happen in the planet's core. So, we can call that thing a "fissile star". 

In some theories whirling magnetic fields maintain the fusion in some brown dwarfs. And the temperature on the coolest brown dwarfs is 250 Kelvin. That means the surface temperature is 23 degrees below zero Celsius (or -23C). (https://science.howstuffworks.com/This Brown Dwarf Isn't a 'Failed Star' — It's a Magnetic Powerhouse). 

Sometimes is suggested that the border between brown and red dwarfs is that the red dwarf's nuclear reactions are non-stop. And brown dwarf's nuclear reactions are full of breaks. In some theories in some lithium-fusion stars, the fission maintains fusion. And the main topic in the next text is lithium-fusion stars. 

The brown dwarf is failed star. Or actually, it's a very cold, and lusterless star. The forming of a brown dwarf could have a connection with forming of rocky planets. There is a possibility that if there are lots of very heavy radioactive isotopes in the planetary cloud that could explain the formation of a brown dwarf. 

If there are lots of radioactive elements in the protoplanet that thing can form a "fission star", or otherways saying, the nuclear reaction can melt the planet's shell. But if that kind of planet starts to form in the area. There is enough gas. If there are lots of radioactive elements in the planet's mantle, that thing can make the situation. The planet's nucleus or the fissile area in the planet is very big. Or nuclear fission can happen in the planet's core. So, we can call that thing a "fissile star". 

The idea is that the brown dwarf gets energy from fission and fusion. So it's a little bit like some subcritical nuclear weapon. The key element in this idea is that the brown dwarf's nucleus is solid. Maybe it's similar to Earth. But there is a gas layer around that thing. 

The brown dwarf cannot maintain its fusion without fission. When the fusion reaction starts it expands the brown dwarf's size. When the brown dwarf expands, pressure around the nucleus decreases. 

Fusion ends. And then, the gas starts to fall to the fissioning nucleus. That increases pressure and launches the fusion. So brown dwarf requires fissile material for keeping its fusion going. 

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The main difference between brown dwarfs and other stars like red dwarfs is this. 

1) Gravitation in regular stars keeps nuclear fusion going. So the whirls and gravitation are forming a situation where temperature and pressure rise to the level that a fusion reaction can form. 

2) Brown dwarf is too light for starting and maintaining fusion. But the fissile material in those objects' solid nuclei can assist in fusion. In brown dwarfs, fusion doesn't continue all the time. There might be many things that are making this type of star possible. 

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That planet can get the gas layer. Of course, the gas or some other elements like lithium must be suitable for fusion. There is the possibility that also stars can get their energy from lithium deuteride. The brown dwarf is in that case long-lasting thermonuclear device. 

If the fission reactions start at the right moment. This reaction can ignite the fusion. When fusion ignites the size of brown dwarfs starts to expand. That decreases pressure and fusion will turn weaker. 

And then the brown dwarf starts to come down. That thing rises pressure and temperature in the brown dwarf's nucleus. That increases the splitting of the radioactive isotopes. And the fusion reaction's power increases.

Forming the brown dwarf requires that the gravitation of that planet is so strong that the gas layer will not fly to space. And that thing explains the strange objects called brown dwarfs. 

https://earthsky.org/space/definition-what-are-brown-dwarfs/

https://science.howstuffworks.com/brown-dwarf-isnt-failed-star-magnetic-powerhouse.htm

https://en.wikipedia.org/wiki/Brown_dwarf

https://en.wikipedia.org/wiki/Teide_1

https://shorttextsofoldscholars.blogspot.com/