zero-age main sequence The region on the Hertzsprung—Russell diagram, as predicted by theoretical models, where stars are located at the onset of nuclear burning in their cores.

zodiac The twelve constellations through which the Sun moves as it follows its path on the ecliptic.

zonal flow Alternating regions of westward and eastward flow, roughly symmetrical about the equator, associated with the belts and zones in the atmosphere of a jovian planet.

zone Bright, high-pressure region, in the atmosphere of a jovian planet, where gas flows upward.