The hydrogen in your body ultimately traces back to the early universe, while many heavier elements were formed later inside stars or during violent stellar events. Stars fuse lighter nuclei into heavier ones during their lives. Massive stars can build elements through several stages, and stellar explosions and other cosmic events spread material into space. That material can become part of new stars, planets and eventually living things. The carbon in cells, oxygen you breathe and calcium in bones all have an astronomical history. Atomic physics therefore connects your body to processes that happened long before Earth existed.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

Atoms contain a tiny nucleus of protons and neutrons surrounded by electrons. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons; some isotopes are unstable and radioactive. The behaviour of a material therefore depends both on its electron structure, which controls most chemical behaviour, and on its nucleus, which matters in radioactive and nuclear processes.