What class of celestial bodies represents the quintessential "failed stars," sitting between the largest planets and the smallest true stars?

Answer

Brown dwarfs

Brown dwarfs are defined as objects that fall short of stellar ignition criteria, occupying the mass range above gas giants but below the minimum mass required for sustained hydrogen fusion.

What class of celestial bodies represents the quintessential "failed stars," sitting between the largest planets and the smallest true stars?
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