Brown dwarfs are objects that are too massive to be called planets, but not massive enough to sustain nuclear fusion in their cores, as stars do. They are therefore sometimes referred to as "failed stars" or "super-planets."

The boundary between brown dwarfs and stars is not clearly defined, and astronomers use various criteria to distinguish between them. One common criterion is the mass limit for hydrogen fusion, which is about 0.08 solar masses. Objects below this mass are generally considered to be brown dwarfs, while objects above it are considered to be stars.

Brown dwarfs can form in the same way as stars, through the collapse of a cloud of gas and dust. However, they do not accumulate enough mass to reach the critical temperature and pressure required for hydrogen fusion to ignite in their cores. They instead cool and fade over time, and may eventually become black dwarfs.

Brown dwarfs are of interest to astronomers because they provide insights into the formation and evolution of stars and planets. They also help us to understand the diversity of objects that exist in the universe.