Wikipedia discusses this matter. Essentially, the mass limit of brown dwarfs is defined in a way that excludes proton-proton fusion, which requires higher temperature and pressure (thus stronger gravitational compression and more mass) than some other fusion reactions that do occur in brown dwarfs. In addition to deuterium-deuterium, sufficiently massive brown dwarfs can fuse protium (hydrogen-1) with lithium-7, which produces two nuclei of helium-4 ($^7\text{Li} +\text{ }^1\text{H}\to 2\text{ }^4\text{He}$) with release of energy.

None of the fusible constituents is very abundant in typical gas-cloud compositions, so many brown dwarfs seen today have already exhausted their fusion supply and are simply glowing with their residual temperature as they cool off, like white dwarfs.

It might be added that the most massive stars also do not primarily fuse protons directly. With sufficient temperature and pressure various CNO cycles take over, these requiring more activation energy than proton-proton fusion but going faster once this energy is supplied. Unlike the deuterium- and lithium-based fusion reactions occurring in brown dwarfs, the CNO cycles do not deplete any reactants other than protium; the carbon, nitrogen and oxygen isotopes are interconverted cyclically. Thus stars massive enough to undergo sugnificant CNO cycling can sustain it.

Wikipedia discusses this matter. Essentially, the mass limit of brown dwarfs is defined in a way that excludes proton-proton fusion, which requires higher temperature and pressure (thus stronger gravitational compression and more mass) than some other fusion reactions that do occur in brown dwarfs. In addition to deuterium-deuterium, sufficiently massive brown dwarfs can fuse protium (hydrogen-1) with lithium-7, which produces two nuclei of helium-4 ($^7\text{Li} +\text{ }^1\text{H}\to 2\text{ }^4\text{He}$) with release of energy.

None of the fusible constituents is very abundant in typical gas-cloud compositions, so many brown dwarfs seen today have already exhausted their fusion supply and are simply glowing with their residual temperature as they cool off, like white dwarfs.

It might be added that the most massive stars also do not primarily fuse protons directly. With sufficient temperature and pressure various CNO cycles take over, these requiring more activation energy than proton-proton fusion but going faster once this energy is supplied. Unlike the deuterium- and lithium-based fusion reactions occurring in brown dwarfs, the CNO cycles do not deplete any reactants other than protium; the carbon, nitrogen and oxygen isotopes are interconverted cyclically. Thus stars massive enough to undergo significant CNO cycling can sustain it.

Wikipedia discusses this matter. Essentially, the mass limit of brown dwarfs is defined in a way that excludes proton-proton fusion, which requires higher temperature and pressure (thus stronger gravitational compression and more mass) than some other fusion reactions that do occur in brown dwarfs. In addition to deuteriu-deuterium, sufficiently massive brown dwarfs can fuse protium (hydrogen-1) with lithium-7, which produces two nuclei of helium-4 ($^7\text{Li} +^1\text{H}\to 2\text{ }^4\text{He}$) with release of energy.

None of the fusible constituents is very abundant in typical gas-cloud compositions, so many brown dwarfs seen today have already exhausted their fusion supply and are simply glowing with their residual temperature as they cool off, like white dwarfs.

It might be added that the most massive stars also do not primarily fuse protons directly. With sufficient temperature and pressure various CNO cycles take over, these requiring more activation energy than proton-proton fusion but going faster once this energy is supplied. Unlike the deuterium- and lithium-based fusion reactions occurring in brown dwarfs, the CNO cycles do not deplete any reactants other than protium; the carbon, nitrogen and oxygen isotopes are interconverted cyclically. Thus stars massive enough to undergo sugnificant CNO cycling can sustain it.

Wikipedia discusses this matter. Essentially, the mass limit of brown dwarfs is defined in a way that excludes proton-proton fusion, which requires higher temperature and pressure (thus stronger gravitational compression and more mass) than some other fusion reactions that do occur in brown dwarfs. In addition to deuterium-deuterium, sufficiently massive brown dwarfs can fuse protium (hydrogen-1) with lithium-7, which produces two nuclei of helium-4 ($^7\text{Li} +\text{ }^1\text{H}\to 2\text{ }^4\text{He}$) with release of energy.

None of the fusible constituents is very abundant in typical gas-cloud compositions, so many brown dwarfs seen today have already exhausted their fusion supply and are simply glowing with their residual temperature as they cool off, like white dwarfs.

It might be added that the most massive stars also do not primarily fuse protons directly. With sufficient temperature and pressure various CNO cycles take over, these requiring more activation energy than proton-proton fusion but going faster once this energy is supplied. Unlike the deuterium- and lithium-based fusion reactions occurring in brown dwarfs, the CNO cycles do not deplete any reactants other than protium; the carbon, nitrogen and oxygen isotopes are interconverted cyclically. Thus stars massive enough to undergo sugnificant CNO cycling can sustain it.