Birth out ofthe interstellarmedium
Contractionto a normalhydrogen-burningstar
Expansionto red giant
Contractiontowhite dwarfOR . . . . . .supernova explosion
Collapse toneutron starORblack hole
Stellar Insides
Stellar Insides: How are stars constructed?
Atmosphere
Envelope
Core
A few principles . . .
1. Stars tend to remainstably inflated, more orless.
Gravity balancespressure:
HydrostaticEquilibrium
Center of Sun:
3 billion atmospheres
2. Stars must be hotteron the inside.
Center of Sun:
16 million K
High temp, pressure gas completely ionized: Plasma
3. Stars require a powerful,stable energy source.
Thermonuclear Fusion
Two particles collide& fuse – form heavierparticle.
Net reaction in main sequence stars:
4 protons (hydrogens) helium + neutrinos + energy
1H + 1H 2H + positron + gamma ray + neutrino
1H + 2H 3He + gamma ray
3He + 3He 4He + 1H + 1H
Proton-proton (PP) chain
2H Deuterium
2
2
Alternative:CNO Cycle
* 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
* Important consequence: creation of elementsthat did not previously exist!
Nucleosynthesis
Sun’s luminosity = 3.9 1026 Watt
Sun converts 4 million tons hydrogen toenergy per second!
Stars lose mass over time!
Transporting energy from core to surface
1. Via photons (radiative transfer): ‘random walk”
Net heat flow
2. Via convection
Hot gasrises . . .cool gassinks.
Sun
Radiativezone
Convectivezone
Solargranules