The Sun is not on fire: Chemical Energy / Total Luminosity= 10,000 years
The Sun is not contracting: Gravitational Potential Energy / Luminosity = 25 million years
The Sun is powered by nuclear energy (E=mc^2)
Nuclear potential energy (core) / luminosity = 10 billion years
Pressure and Gravitational Equilibrium:
● Weight of upper layers compresses lower layers
● Energy supplied by fusion maintains the pressure that balances the inward crush of
● the rate at which energy radiates from the surface of the Sun must be the same as the rate
at which it is released by fusion in the core.
● Contraction stopped when fusion began.
The Sun’s Dimension
Radius: 6.9 x 10^8 m (109 times the earth)
Mass: 2 x 10^30 kg (30,000 earths)
Luminosity: 3.8 x 10^26 watts
The Sun’s Structure
Core: energy generated by nuclear fusion = 1 million K
Radiation Zone: energy transported upward by photons
Convection Zone: energy transported upward by hot rising gas
Photosphere: visible surface of the sun = 6000 K
Chromosphere: middle layer of the solar atmosphere = 10^4-10^5 K
Corona: Outermost layer of the solar atmosphere = 1 million K
Solar Wind: a flow of charged particles from the surface of the Sun
● The Sun shines because gravitational equilibrium keeps its core hot and dense enough to
release energy through nuclear fusion.
● From inside out, the layers of the Sun are core, radiation zone, convection zone,
photosphere, chromosphere, and corona.
Fission: Big nucleus into smaller pieces (ie. nuclear power plants)
Fusion: Small nuclei stick together to make a bigger one (ie. the Sun, stars)
● High temperatures enable nuclear fusion to happen in the core.
● The Sun releases energy by fusing four hydrogen nuclei into one helium nucleus.
● The proton-proton chain is how hydrogen fuses into helium in the Sun.
● Releases positron and neutrino (ionized plasma particle) binds with another.
IN - 4 protons OUT - He nucleus, 2 gamma rays, 2 positrons, 2 neutrinos
Total mass is lower than 0.7 %
The solar thermostat keeps burning rate steady.
● Decline in core temp causes fusion rate to drop so core contracts and heats up
● Rise in core temp causes fusion rate to rise, so core expands and cools down
● Energy gradually leaks out of the radiation zone in form of randomly bouncing photons,
which is how energy from fusion gets out of the Sun.
Convection (rising hot gas) takes energy to the surface.
Bright blobs on the photosphere show where hot gas is reaching the surface.