Transcript mission
1 Review: Evolution of Sun As usual, PowerPoint slides available at the web site www.clifford.org/drbill/mission/ Dr. Bill Pezzaglia Stars Born in Galaxy Arms 2 This shows stellar formation In in the spiral arms (where Density waves bunch up matter (Whirlpool Galaxy M51) Nebulae are Stellar Nurseries 3 Dark “Bok Globules” are where stars are forming right now (Eagle Nebula M16) Sun formed in 50 million years Dark nebula material contracts due to gravity, heats up Spins faster and flattens out into a disk Center becomes the sun when it gets hot enough to ignite Disk becomes the planets 4 Sun produces Energy in Core 5 Only in core is it hot enough (and enough pressure) for nuclear fusion to occur Hydrogen is combined into Helium 0.7% of mass converted to energy Sun will last for 10 billion years It is 5 billion years old Details Proton Proton Chain 6 Hydrogen burning at the center of the Sun usually takes place in a three-step process. Step 1: Two protons (hydrogen nuclei, shown in red) combine to form a deuterium (proton+neutron), plus a massless neutrino and positron (anti electron). The positron annihilates an electron producing gamma rays (photons). Step 2: The deuteron combines with a third proton, forming an isotope of helium (3He) and releasing another gamma-ray photon. Step 3: Two 3He nuclei formed via the first and second steps collide, forming a different helium isotope with two protons and two neutrons (4He) and releasing two protons. The gamma-ray photons released in these steps, as well as the kinetic energy (energy of motion) of the released protons, are the source of the Sun's energy. Year 5 billion AD, sun runs out of fuel •Helium is building up in core of sun •Eventually core reactions will stop •Hydrogen fusion continues in shell around core 7 Evolution to Red Giant 8 7. Core reactions stop, no outward radiation pressure Core gravitationally contracts, and heats up 8. Subgiant Branch: Causes increased shell burning, Causes outer star to expand to giant size. Surface will be cooler as heat is spread over greater area 9. Helium Flash: the helium in core ignites in triple alpha Triple Alpha Reaction •The net reaction is 3 Heliums combine to make Carbon •But this reaction takes place in 2 steps •10x less efficient than Hydrogen fusion •Sun will last maybe 500 million years as a red giant 9 Horizontal Branch Star 10 9. Helium Flash: the helium in core ignites in triple alpha process Causes core expand and cool a bit, shell burning decreases 10. Horizontal branch Outer star hence contracts in size, but gets hotter, stabilizes AGB: Asymptotic Giant Branch •Carbon is building up in core of giant •Eventually core reactions will stop •Hydrogen and Helium fusion continues in shell around core 11 AGB: Asymptotic Giant Branch 12 10. Core reactions stop, no outward radiation pressure. Core gravitationally contracts, and heats up. 11. Asymptotic Branch: Causes increased shell burning, Causes outer star to expand to giant size. Surface will be cooler as heat is spread over greater area Carbon Flash: More massive stars will ignite in carbon fusion. Sun probably won’t. 25 The Death of the Sun A lightweight star (like the sun) will not be able to ignite the carbon core, instead it will blow up, forming a Planetary Nebula 14 15 Thermal Pulses •Late AGB stars have thermal pulses every 100,000 years, sheds matter (perhaps 10% of mass each time!) •Theory: Helium shell burning runs out, helium shell contracts, heats up outer hydrogen shell, causes increased hydrogen shell burning (creating more helium) The helium rains down onto the dormant helium shell, with increased mass it contracts, eventually it reaches critical temperature and we get a helium shell flash, creating the thermal pulse Planetary Nebula •Thermal pulses can blow off outer star, leaving the carbon-oxygen core (which will become a dense “white dwarf” star) 16 M57 Ring Nebula •Ejected material travels at 20 to 30 km/sec, and hence will disperse after about 50,000 years. Summary 17