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

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

Red Subgiant Branch – hydrogen shell burning Off the Main Sequence - The Evolution of a Sun-like Star The Red Subgiant Branch

Stage 8 – the end of the Red Subgiant Branch. Off the Main Sequence - The Evolution of a Sun-like Star Stage 8 R ≈ 3 R sun

Red Giant Branch – * Outer envelop cools. * Rapid Hydrogen burning. * 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

The Helium Flash and Stages 10 and 11 M < 8 Mסּ

Electron degeneracy pressure: the pressure exerted by electrons caused by the confinement in the small core of a star that has left the main sequence. The Helium Flash and Stage 10 M < 8 Mסּ

Important point: Degeneracy pressure is not directly related to gravitational effects, that is, it is not a thermal pressure. Under thermal pressure, as the core heats up, the resulting increase in pressure would expand the core. The expansion would cause cooling, and moderate the rate at which nuclear fusion occurs. The Helium Flash and Stage 10 M < 8 Mסּ

The Helium Flash The Red Giant: The radius can grow to 100 R sun (larger than the radius of Mercury’s orbit).

The Red Giant Helium Flash: The rapid release of energy which occurs at the end of stage 9 that is generated in conjunction with degeneracy pressure in the core of the red giant.

The Reactions The Triple Alpha Process T > 100 million K 3 ( 4 He )  12 C

The Helium Flash The Tip: The point on the H-R diagram where the helium flash occurs.

M < 8 M  After the rapid release of energy in the helium flash, the luminosity of the star will drop, while the surface temperature will grow in reaction to the enormous temperature of the core.

M < 8 M  The rapid generation of energy and increase in temperature will expand the core, so that degeneracy pressure no longer dominates and thermal pressure dominates. The star will once again reach hydrostatic equilibrium, and fuse helium. Helium fusion will start to occur when the core temperature reaches 10 8 K.

Stage 10 - Helium Fusion M < 8 Mסּ Stage 10 – the start of continuous helium fusion

Stage 10 - Helium Fusion M < 8 Mסּ Horizontal branch: Continuous helium fusion. The horizontal branch is a “parallel” to the main sequence. Stage 10 at the start of the horizontal branch.

Stage 10 - Helium Fusion M < 8 Mסּ Horizontal branch: The star will remain on the horizontal branch, burning helium, for tens of millions of years.

Stage 11 – Asymptotic Giant Branch Asymptotic giant branch As the helium depletes, the star will basically reproduce it’s the path that took it to stage 9. Helium and hydrogen burning shells The star will expand to a red giant once again, this time with shells of burning helium, and some shells still burning hydrogen. This path to red giant is called the to distinguish this evolution from the hydrogen shell burning path to stage 9.

Stage 11 – Asymptotic Giant Branch Because the carbon core is so compact and hot, the fusion rates increase in the outer shells, that the star increases its luminosity. The non-burning core continues to contract, its temperature increases, which increases the helium and hydrogen fusion rates in the shells, continuing to increase the luminosity and radius.

Stage 12 Radius can swell to 300 R sun (approximately the radius of Mars’ orbit)