Hier is een uitsplitsing:

* kernfusie: Stars generate energy through nuclear fusion, where hydrogen atoms fuse together to form helium, releasing immense amounts of energy. This process requires immense pressure and heat, which are only achievable in the cores of massive objects like stars.

* Mass Threshold: There's a critical mass threshold for a star to form. This threshold is roughly 8% of the mass of our Sun. Objecten onder deze drempel, hoewel ze zich kunnen vormen uit dezelfde instortende wolk van gas en stof als sterren, missen de zwaartekrachtpull om de nodige druk en temperatuur te bereiken voor aanhoudende fusie.

* Brown Dwarf Fusion: While brown dwarfs don't achieve sustained fusion, they can undergo deuterium fusion for a short period. Deuterium is a heavier form of hydrogen, and it can fuse at lower temperatures and pressures than regular hydrogen. However, deuterium is relatively rare, and this fusion process eventually runs out, leaving the brown dwarf as a substellar object.

Samenvattend:

* Brown dwarfs are "failed stars" because they are below the mass threshold needed for sustained hydrogen fusion.

* They can undergo deuterium fusion briefly, but this is not a long-term energy source.

* Their lack of sustained fusion prevents them from shining brightly like true stars.