Nuclear physics is the branch of science which looks inside atoms. It looks at the nucleus, and what effect different interactions have on it.
These interactions come in 2 main forms: nuclear fission and nuclear fusion. Nuclear fission is where the nucleus breaks apart into smaller pieces. Nuclear fusion is where the nucleus gets bigger by fusing, or 'gluing', with another nucleus. Both of these release energy.
A nucleus sits at the centre of an atom. It was only found in 1911 by New Zealand physicist Ernest Rutherford. The nucleus contains 2 types of particles known as neutrons and protons.
The proton has a positive electric charge. The neutron has no charge at all. Between them, they make up most of the mass of an atom as the surrounding electrons are tiny in comparison.
The nucleus has a huge impact on the Universe we see. All the stars we see are powered by nuclear fusion. It is the fusion of nuclei which gives the star energy, and light.