As a star burns its nuclear fuel, it creates heavier elements that sink to the core. This increases the core's density and temperature, causing the star to expand and cool its outer layers, leading to its transformation into a red giant. This expansion results from the decrease in gravitational pull from the core due to fusion-produced ash accumulation (mainly iron or other elements heavier than Helium at the end of their lifecycle), along with an increase in radiation pressure from the higher core temperature as fusion accelerates in response to the loss of gravitational pressure that normally opposes this increase in core-fusion rates when heavier elements accumulate there rather than burning.