  The star formation history of galaxies is a complex process usually
considered to be stochastic in nature, for which we can only give average
descriptions such as the color-density relation. In this work we follow
star-forming gas particles in a hydrodynamical N-body simulation back in time
in order to study their initial spatial configuration. By keeping record of the
time when a gas particle started forming stars we can produce gas-star
isochrone surfaces delineating the surfaces of accreting gas that begin
producing stars at different times. These accretion surfaces are closely packed
inside dense regions, intersecting each other, and as a result galaxies inside
proto-clusters stop accreting gas early, naturally explaining the color
dependence on density. The process described here has a purely gravitational /
geometrical origin, arguably operating at a more fundamental level than complex
processes such as AGN and supernovae, and providing a conceptual origin for the
color-density relation.
