Nuclear physics is the field of physics that studies the properties of atomic nuclei and their interactions. Nuclear reactions can involve the emission or absorption of particles or energy, including electromagnetic radiation like gamma rays. Radioactive decay occurs when an unstable nucleus undergoes a spontaneous transformation, emitting energetic particles and radiation in the process. Nuclear energy is harnessed through nuclear fission, where a nucleus is split apart, or nuclear fusion, where two nuclei combine to form a new, heavier nucleus. The challenge in using nuclear energy is ensuring that it is used safely and sustainably.