The beginning of the map shows how protons and neutrons agglomerate in order to form light nuclei, hydrogen, helium, lithium and berylium isotopes. All the nucleons have been drawn visible. The number of protons has been limited to 4, that of neutrons to 3. Radioactive isotopes are unstable and are thus absent from our environment. The proton (ordinary hydrogen) and helium-4 (alpha particle), which are stable, together form more than 99% of the mass of the universe.