Brown dwarfs don't become real stars because they lack the mass needed to ignite sustained nuclear fusion in their cores, which is the defining characteristic of a star.

Here's a breakdown:

* Nuclear Fusion: 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. Objects below this threshold, even though they might form from the same collapsing cloud of gas and dust as stars, lack the gravitational pull to reach the necessary pressure and temperature for sustained fusion.

* 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.

In summary:

* 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.