Because Li and Be form natural stopping points on the table of isotopes for hydrogen fusion, it is believed that all of the higher elements are formed in very hot stars where higher orders of fusion predominate. A star like the Sun produces energy by the fusion of simple H into He through a series of reactions. It is believed that when the inner core exhausts its H fuel, the Sun will contract, slightly increasing its core temperature until He can fuse and become the main fuel supply. Pure He fusion leads to Be, which decays back to 2 He; therefore the He must fuse with isotopes either more or less massive than itself to result in an energy producing reaction. When He fuses with H or H, it forms stable isotopes Li and Li respectively. The higher order isotopes between Li and C are synthesized by similar reactions between hydrogen, helium, and lithium isotopes.