What ability is fundamentally missing in brown dwarfs, preventing them from being classified as true stars?
Answer
They cannot sustain core hydrogen fusion
The critical differentiator between a true star and objects like brown dwarfs is the capacity for sustained nuclear fusion in the core. Brown dwarfs are sometimes referred to as 'failed stars' because, despite possessing significant mass, they lack the requisite minimum mass—greater than about 80 times the mass of Jupiter—needed to generate the core pressure and temperature high enough to initiate and maintain the thermonuclear reaction where hydrogen fuses into helium. While they may briefly fuse deuterium, they cannot perform the core hydrogen fusion that defines a self-luminous, energy-generating star.
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