Transcript Document
Annoucements • Next test is in one week • Review session will be next Wednesday – Please hand-in or e-mail questions • Practice test will be available by Monday How Stars Evolve • Pressure and temperature – Normal gases – Degenerate gases • Evolution of the Sun after the main sequence – – – – – – Red giant phase Helium flash Horizontal branch Asymptotic branch Planetary nebula White dwarf 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 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 Movement on HR diagram Helium Flash • He core – 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 White dwarf • Star burns up rest of hydrogen • Nothing remains but degenerate core of Oxygen and Carbon • “White dwarf” cools but does not contract because core is degenerate • No energy from fusion, no energy from gravitational contraction • White dwarf slowly fades away… Evolution on HR diagram Time line for Sun’s evolution Transport of energy through the radiative zone It takes about 200,000 years for photons made in the core to make it through the radiative zone Radiative zone game 1. Balloons start at center of room (in core of the Sun). 2. Everyone needs to randomly tap the balloons – gentle taps in random directions. 3. The balloons exit the Sun when they leave the center seats. 4. We’ll time how long it takes for half the balloons to leave the Sun. Radiative zone game 1. Guess how long it will take half the balloons to reach the edge of the room. 2. Do the experiment compare with guesses. 3. How does this game relate to how photons carry energy from the core to the surface of the sun?