I recently watched a science video about the classes of stars and how they generate energy and it listed brown dwarfs as fusing deuterium, saying that deuterium was easier to fuse than straight up hydrogen and that deuterium fusion was how they got their meger energy... It also mentioned that some of them fused lithium because it was easier to fuse than hydrogen and deuterium. Is this true? If so why is lithium not used as fuel in fusion reactors if it is easier to fuse?  

1 hour ago, Moontanman said:

I recently watched a science video about the classes of stars and how they generate energy and it listed brown dwarfs as fusing deuterium, saying that deuterium was easier to fuse than straight up hydrogen and that deuterium fusion was how they got their meger energy... It also mentioned that some of them fused lithium because it was easier to fuse than hydrogen and deuterium. Is this true? If so why is lithium not used as fuel in fusion reactors if it is easier to fuse?  

The lower mass of brown dwarves means there's not enough heat from contraction and not enough density to ever reach the conditions needed to sustain hydrogen fusion.  IIRC, terrestrial reactors fuse deuterium and tritium, on the basis of their availability and what conditions can be engineered.  Lithium could be used, but it's easier to fission lithium and use the tritium product.  (and sell the helium, or make a lot of party balloons?)

3 hours ago, Moontanman said:

It also mentioned that some of them fused lithium because it was easier to fuse than hydrogen and deuterium. Is this true?

Depends on what they mean by “easier”

The coulomb barrier is higher, owing to the larger charge, but it undoubtedly has a higher cross-section than p-p fusion, because that is too small to directly measure (the average proton takes more than a billion years to fuse in the sun)