Why are brown dwarfs characterized as objects that failed to form a true star?
Answer
They fall below the minimum mass required to initiate sustained hydrogen fusion
The term 'failed star' is applied to brown dwarfs because they start the formation process similar to a true star—as a collapsing core—but they ultimately fall short of the mass requirement necessary to establish the defining feature of a star: the sustained, self-regulating fusion of ordinary hydrogen ($ ext{}^1 ext{H}$). While they may briefly fuse deuterium, this activity is temporary and insufficient for long-term stability. They are gravitationally bound, massive spheres that hover just under the stellar threshold of $0.08$ solar masses, leading to contraction and cooling rather than main-sequence luminosity.
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