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