Region on the Hertzsprung-Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear Processes by which hydrogen (1H) is fused into helium (4He) with in a star. The five possible fusion paths can be divided into two sets of processes: the Proton-Proton (PP) process, which depends only on the amount of H and He in the star, and the CNO cycle (carbon-nitrogen-oxygen), which in their cores. These stars are new additions to the main sequence, a place where they will spend about 90% of their lives and maintain remarkably stable luminosities. After this time, they evolve off the ZAMS to the upper right of the main sequence to become Giant and highly luminous red star in the later stages of stellar evolution after it has left the main sequence. These red stars have a relatively cool surface whose core has burned most of its hydrogen. Red giants lose parts of their atmospheres and thus provide new elements into interstellar or supergiant stars.
Image source: http://www.peripatus.gen.nz/Astronomy/HerRusDia.html.