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Off the Main Sequence Stages 7 - 9 Off the Main Sequence Stages 7 - 9 While on the main sequence, the star is burning hydrogen. It’s luminosity is determined by the stars mass. The most intense fusion is occurring at the center regions of the core (highest pressure and temperature). Off the Main Sequence Stage 8 Initial depletion of hydrogen occurs in the interior of the hydrogen burning core Off the Main Sequence Stage 8 Once the hydrogen in the central core is depleted, hydrostatic equilibrium is lost and the central core begins to collapse as a result of strong gravity Off the Main Sequence Stage 8 Turn Off – the location on the H-R diagram where the star first leaves the main sequence as a result of hydrogen deletion in the core. Off the Main Sequence Stage 8 Heating of the gaseous envelope causes it to expand. Burning hydrogen in the expanding shell increases the luminosity Gravitational collapse of the core increases the temperature of the core. Hydrogen continues to burn outside of the central core. Off the Main Sequence Stage 8 Hydrogen Shell Burning – the fusion of hydrogen in shells of a star outside of the hydrogen depleted core. Off the Main Sequence Stage 8 Heating of the gaseous envelope causes it to expand. Burning hydrogen in the expanding shell increases the luminosity Gravitational collapse of the core increases the temperature of the core. Hydrogen continues to burn outside of the central core. Off the Main Sequence Stage 8 Stage 8 – Red Subgiant Stage Gravitational collapse of the core increases the temperature of the core. Hydrogen continues to burn outside of the central core. Off the Main Sequence Stage 8 Red Subgiant Branch – the portion of the H-R Diagram that corresponds to the hydrogen shell burning portion of the life of a star related to the expansion of the radius and increase in luminosity of the star before helium burning is triggered in the core of the expanding star. There is a decrease in the surface temperature of the star as it expands. Off the Main Sequence Stage 9 Outer envelop cools. Hydrogen burning still occurs in the shells at a rapid rate producing very high luminosity Off the Main Sequence Stage 9 Radius and luminosity will continue to grow as the hydrogen continues to burn in the shells. What happens next is determined by the mass of the star