Birth out of

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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
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Stellar Insides
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Stellar Insides How are stars constructed?
Atmosphere
Core
Envelope
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A few principles . . .

• Stars tend to remain
• stably inflated, more or
• less.

Gravity balances pressure Hydrostatic Equilibri
um
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Center of Sun 3 billion atmospheres
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• Stars must be hotter
• on the inside.

Center of Sun 16 million K
High temp, pressure ? gas completely ionized
Plasma
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• Stars require a powerful,
• stable energy source.

Two particles collide fuse form
heavier particle.
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Net reaction in main sequence stars
4 protons (hydrogens) ? helium neutrinos
energy
Proton-proton (PP) chain
1H 1H ? 2H positron gamma ray
neutrino 1H 2H ? 3He gamma ray 3He 3He
? 4He 1H 1H
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2
2H ? Deuterium
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Alternative CNO Cycle
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Energy comes from conversion of mass into
energy
mass (41H) ? mass(4He) ? m (gt 0)
E mc2
gamma rays, neutrinos, positrons energy of
motion
Fusion requires high temp density collisio
ns must be frequent violent
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Important consequence creation of
elements that did not previously exist!
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Suns luminosity 3.9 ? 1026 Watt ?
Sun converts 4 million tons hydrogen to energy
per second!
Stars lose mass over time!
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Transporting energy from core to surface
1. Via photons (radiative transfer) random
walk
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2. Via convection
Hot gas rises . . . cool gas sinks.
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Sun
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