When a star first begins the long path toward becoming a red giant, a layer of hydrogen around the core begins to undergo fusion. If this layer was too cold to do fusion throughout the main sequence stage, why is it suddenly warm enough

The contraction in the star due to gravity

Explanation:

The energy of the stars come from the nuclear fusion happening in their core. This reaction generates outward radiation pressure which is balanced by the inward gravitational pull to keep the stars in shape. Very high pressure and temperature is needed for the fusion to start. When all the hydrogen is converted into helium the outward radiation pressure is overpowered by the inward gravitational force.

Because of this core starts contracting and the layer of hydrogen around the core comes in the region where pressure and temperature is optimum for the nuclear fusion to start. Thus nuclear fusion starts in the layer around the core. The main reason is the contraction due to gravity.

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

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