What kind of reactions provide a star with energy?

The nuclear fusion reaction is the source of energy in a star. In nuclear fusion reactions, the nuclei of two atoms combine to create a new atom with the release of a lot of energy. The nuclear fusion reaction takes place in the core of the star where two hydrogen atoms fuse to become one helium atom.

²₁H + ³₁H --> ⁴₂He + ¹₀n + 17.59 MeV

In the fusion reaction above deuterium fuses with tritium to form helium and releases ~18 mega electron volt of energy.

periodic trends is an atom that gets larger as the number of electronic shells increase, the radius of atoms increases as you go down a certain group in the periodic table of elements. in general, the size of an atom will decrease as you move from left to right of a certain period.