Birth out of the interstellar medium

1. Birth out of the interstellar medium Contraction to a normal hydrogen-burning star Expansion to red giant Collapse to neutron star OR black hole Contraction to white dwarf OR . . . . . .supernova explosion

2. Stellar Insides

3. Stellar Insides: How are stars constructed? Atmosphere Core Envelope

5. A few principles . . . • Stars tend to remain • stably inflated, more or • less. Gravity balances pressure: Hydrostatic Equilibrium

6. Center of Sun: 3 billion atmospheres

7. Stars must be hotter • on the inside. Center of Sun: 16 million K High temp, pressure  gas completely ionized: Plasma

8. Thermonuclear Fusion • Stars require a powerful, • stable energy source. Two particles collide & fuse – form heavier particle.

9. Net reaction in main sequence stars: 4 protons (hydrogens)  helium + neutrinos + energy Proton-proton (PP) chain 2 1H + 1H  2H + positron + gamma ray + neutrino 1H + 2H  3He + gamma ray 3He + 3He  4He + 1H + 1H 2 2H  Deuterium

10. Alternative: CNO Cycle

11. * Energy comes from conversion of mass into energy: mass (41H)  mass(4He)  m (> 0) E = mc2 gamma rays, neutrinos, positrons + energy of motion * Fusion requires high temp & density: collisions must be frequent & violent

13. Nucleosynthesis * Important consequence: creation of elements that did not previously exist!

14. Sun’s luminosity = 3.9  1026 Watt  Sun converts 4 million tons hydrogen to energy per second! Stars lose mass over time!

15. Net heat flow Transporting energy from core to surface 1. Via photons (radiative transfer): ‘random walk”

17. 2. Via convection Hot gas rises . . . cool gas sinks.

18. Convective zone Radiative zone Sun

19. Solar granules