What characteristic defines brown dwarfs?

Brown dwarfs are defined by their low mass, which ranges between that of the largest planets and the smallest stars. Specifically, they have masses between about 13 and 80 times that of Jupiter. This mass is insufficient for sustaining hydrogen fusion in their cores, which is the process that powers true stars. Instead, brown dwarfs are often referred to as "failed stars" because they do not reach the temperatures and pressures necessary for hydrogen nuclei to fuse into helium, a process that defines typical stars.

This characteristic of being unable to achieve hydrogen fusion is what primarily distinguishes brown dwarfs from stars. Unlike higher mass stars, brown dwarfs cool over time and do not initiate the fusion process, which is critical in defining the lifecycle of normal stars. Their unique position between stars and planets in terms of mass and fusion capabilities makes them an interesting area of study in astrophysics.