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Contraction
to a normal
hydrogen-burning
star
Birth out of
the
interstellar
medium
Expansion
to
red giant
Contraction
to
white dwarf
OR . . .
Collapse to
neutron star
OR
black hole
. . .supernova explosion
Stellar Insides
Stellar Insides: How are stars constructed?
Atmosphere
Core
Envelope
A few principles . . .
1. Stars tend to remain
stably inflated, more or
less.
Gravity
balances
pressure:
Hydrostatic
Equilibrium
Center of Sun:
3 billion atmospheres
2. Stars must be hotter
on 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 heavier
particle.
Net reaction in main sequence stars:
4 protons (hydrogens)  helium + neutrinos + energy
Proton-proton (PP) chain
+ 1H  2H + positron + gamma ray + neutrino
2
1H
2
1H
+ 2H  3He + gamma ray
3He
+ 3He  4He + 1H + 1H
2H
 Deuterium
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 elements
that did not previously exist!
Nucleosynthesis
Sun’s luminosity = 3.9  1026 Watt

Sun converts 4 million tons hydrogen to
energy 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 gas
rises . . .
cool gas
sinks.
Sun
Convective
zone
Radiative
zone
Solar
granules