Off the Main Sequence - The Evolution of a Sun-like Star Stages 7 - 9

These stages have already been discussed: Stage 1: Interstellar Cloud Stage 2: A Collapsing Cloud Fragment Stage 3: Fragmentation Ceases Stage 4: A Protostar Collapsing cloud has ripped apart atoms, temperature at around 1,000,000 K. Stage 5: Protostellar Evolution Central temperature around 5,000,000 K Protons still cannot overcome Coulomb repulsion Stage 6: A Newborn Star Core temperatures reach 10,000,000 K – Fusion begins Off the Main Sequence - The Evolution of a Sun-like Star Stages 7 - 9

These stages have already been discussed: Stage 7: The Main Sequence Core temperatures around 15,000,000 K Hydrostatic equilibrium (radiation pressure balances gravity) stable size and burning of hydrogen Off the Main Sequence - The Evolution of a Sun-like Star 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). A star like the sun will remain on the Main Sequence for about 10 billion years Off the Main Sequence - The Evolution of a Sun-like Star Stages 7 - 9

Initial depletion of hydrogen occurs in the interior of the hydrogen burning core Off the Main Sequence - The Evolution of a Sun-like Star Stages 7 - 9

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 - The Evolution of a Sun-like Star Stages Helium cannot fuse because a temperature of around 10 8 K is required to overcome the coulomb repulsion to trigger fusion (helium has a charge of +2e).

Turn Off – the location on the H-R diagram where the star first leaves the main sequence as a result of hydrogen depletion in the core. Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch

Gravitational collapse of the core increases the temperature of the core. Heating of the gaseous envelope causes it to expand. Burning hydrogen in the expanding shell increases the luminosity Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch Hydrogen continues to burn outside of the central core.

Hydrogen Shell Burning – the fusion of hydrogen in shells of a star outside of the hydrogen depleted core. Because of the increase in pressure of the core due to gravitational collapse, the core temperatures will rise. The increase in thermal energy increases the temperature of the hydrogen burning (expanding) shell, which increases the amount of hydrogen fusion occurring in the shell. Therefore, the temperature goes up, along with a slight increase in luminosity. The star follows an almost horizontal path on the H- R Diagram. Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch

Heating of the gaseous envelope causes it to expand. Burning hydrogen in the expanding shell increases the luminosity somewhat. Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch Gravitational collapse of the core increases the temperature of the core. Increase in core temperature triggers more hydrogen fusion in the shell. Hydrogen continues to burn outside of the central core.

Gravitational collapse of the core increases the temperature of the core. Hydrogen continues to burn outside of the central core. Red Subgiant Branch Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch

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 - The Evolution of a Sun-like Star The Red Subgiant Branch

Gravitational collapse of the core increases the temperature of the core. Hydrogen continues to burn outside of the central core. Stage 8 – the end of the Red Subgiant Branch. The temperature of the core is very high, triggering intense hydrogen fusion in the burning shell. The huge pressures in the burning shell beings to push the cooler outer shell outward, starting a great change in radius. Off the Main Sequence - The Evolution of a Sun-like Star Stage 8 R ≈ 3 R sun

Outer envelop cools. Hydrogen burning still occurs in the shells at a rapid rate producing very high luminosity. Internal pressures cause a big increase in radius. Off the Main Sequence - The Evolution of a Sun-like Star The Red Giant Branch

Core density ≈ 10 8 kg/m 3 Core temp ≈ 10 8 K R ≈ 100 R sun Off the Main Sequence - The Evolution of a Sun-like Star Stage 9