The brown dwarf family of stars contains the lowest mass objects in the universe that are classified as stars. These stars range from thirteen times the mass of Jupiter (theoretically the lowest mass needed to fuse deuterium) to 80 times the mass of Jupiter. Because of their relatively low mass, some brown dwarf stars are unable to sustain fusion at their cores and may be more similar to massive gaseous planets than traditional stars.

Within the cores of brown dwarf stars the gravitational pressure of the star is often unable to overcome the electrostatic repulsion between atoms, resulting in a state of inert equilibrium. Brown dwarf stars with larger masses are able to overcome the repulsion between atoms at their cores and sustain small pockets of fusion where deuterium, an isotope of hydrogen with a lower fusion point, is fused into helium. The fusion lifespan of these stars is often short, as deuterium makes up just over 0.1% of all the hydrogen present within the brown dwarf. Once the deuterium within the star is exhausted, it will once again reach equilibrium with the electrostatic forces within its core and become an inert body of gas.

Because of their low mass and luminosity, these stars can be extremely difficult to find, the first being identified in 1995. Although they can be elusive to detection, brown dwarf stars can be differentiated from massive rogue planets by their surface temperature. A brown dwarf star which hosts an active fusion reaction can have surface temperatures exceeding 1000k, far hotter than what a rogue planet could sustain. Brown dwarf stars that cannot sustain fusion will slowly lose their energy through black body radiation and may cool to planetary temperatures over extremely long periods of time, but would still radiate energy left over from the formation process.

Bodies orbiting these stars are almost assuredly frozen and barren on their surfaces, as brown dwarf stars do not have the luminosity needed to warm planets through solar radiation like our own star. Planets that exist in orbits near to the star may instead be intensely geologically active thanks to tidal forces similar to the forces experienced by the Jovian moons.

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