How Stars Evolve Pressure and temperature The fate of the Sun Normal gases Degenerate gases The fate of the Sun Red giant phase Horizontal branch Asymptotic branch Planetary nebula White dwarf Reading: sections 21.1-21.3, 22.1-22.3

Pressure and Temperature Pressure is the force exerted by atoms in a gas Temperature is a measure of how fast the atoms in a gas move Hotter  atoms move faster  higher pressure Cooler  atoms move slower  lower pressure Do cold balloon demo

Degenerate gas Very high density Motion of atoms is not due to kinetic energy, but instead due to quantum mechanical motions Pressure no longer depends on temperature This type of gas is sometimes found in the cores of stars

The Fate of the Sun How will the Sun evolve over time? What will be its eventual fate?

Sun’s Structure Core Envelope Where nuclear fusion occurs Supplies gravity to keep core hot and dense

Main Sequence Evolution Core starts with same fraction of hydrogen as whole star Fusion changes H  He Core gradually shrinks and Sun gets hotter and more luminous

Gradual change in size of Sun Now 40% brighter, 6% larger, 5% hotter

Main Sequence Evolution Fusion changes H  He Core depletes of H Eventually there is not enough H to maintain energy generation in the core Core starts to collapse

Red Giant Phase He core H layer Envelope No nuclear fusion Gravitational contraction produces energy H layer Nuclear fusion Envelope Expands because of increased energy production Cools because of increased surface area

Sun’s Red Giant Phase

HR diagram Giant phase is when core has been fully converted to Helium

Helium Flash He core H layer Envelope Eventually the core gets hot enough to fuse Helium into Carbon. This causes the temperature to increase rapidly to 300 million K and there’s a sudden flash when a large part of the Helium gets burned all at once. We don’t see this flash because it’s buried inside the Sun. H layer Envelope

Movement on HR diagram

Movement on HR diagram

Helium Flash He core H layer Envelope Eventually the core gets hot enough to fuse Helium into Carbon. The Helium in the core is so dense that it becomes a degenerate gas. H layer Envelope

Red Giant after Helium Ignition He burning core Fusion burns He into C, O He rich core No fusion H burning shell Fusion burns H into He Envelope Expands because of increased energy production

Sun moves onto horizontal branch Sun burns He into Carbon and Oxygen Sun becomes hotter and smaller What happens next?

Helium burning in the core stops H burning is continuous He burning happens in “thermal pulses” Core is degenerate

Sun moves onto Asymptotic Giant Branch (AGB)

Sun looses mass via winds Creates a “planetary nebula” Leaves behind core of carbon and oxygen surrounded by thin shell of hydrogen Hydrogen continues to burn

Planetary nebula

Planetary nebula

Planetary nebula

Hourglass nebula