What critical condition differentiates a true star from a brown dwarf?

The core reaching approximately 15 million Kelvin for sustained hydrogen fusion.

The defining characteristic separating a true star from a substellar object like a brown dwarf lies in the achievement of self-sustaining core fusion. For an object to become a recognized star, its core must achieve extremely high temperatures, specifically around 15 million Kelvin ($15 imes 10^6$ K). At this temperature, hydrogen fusion proceeds continuously, generating the sustained outward pressure necessary to permanently halt gravitational collapse. Brown dwarfs are often termed 'failed stars' because their cores never attain this required thermal threshold. Consequently, they cannot generate the fusion pressure needed to maintain equilibrium and remain supported predominantly by electron degeneracy pressure, a quantum mechanical effect, rather than sustained fusion output.