“[Brown dwarfs] appear to live a more exciting life than we presumed. They are too big to be planets and two small to be stars, but it does appear that if you watch one it has very active events… there is action going on.” -Lars Bildsten
For every star that’s out there in the Universe, for every object that ignited hydrogen fusion in its core, there are many other objects out there that never got massive enough to do so. The largest failures, those that gathered between 13 and 80 Jupiter masses’ worth of material, are known as brown dwarfs. They achieve deuterium fusion in their core, but never cross that critical threshold to become true stars.
Many brown dwarfs, just like many stars, though, come in binary systems. If you were to add up all the mass in some of these systems, it would, in fact, be enough to create a star out of. The closest brown dwarf binary to us, Luhman 16, has exactly the right conditions that it could create a star if a merger ever took place.