The Sun
Properties
• M = 2 X 1030 kg
= 300,000 MEarth
• R = 700,000 km
> 100 REarth
• 70% H, 28% He
• T = 5800 K surface,
15,000,000 K core
Structure
Sun’s Atmosphere
Sun’s Atmosphere
• Solar Wind – stream of particles blown out from
the sun
• Corona
– Extends several million km above surface
– T = 1,000,000 K
– Emits X-Rays
• Chromosphere
– T = 10,000 K
– Emits UV
• Photosphere
– T = 5,800 K
– Emits visible light
Sun’s Interior
• Convection Zone
– Energy travels up
toward the surface via
rising hot gas
• Radiation Zone
– Photons travel through
plasma 1/3 down from
surface
• Core
– Energy is produced
Sun’s Interior
• Convection Zone
– Energy travels up
toward the surface via
rising hot gas
• Radiation Zone
– Photons travel through
plasma 1/3 down from
surface
• Core
– Energy is produced
Energy
•
•
•
•
Measured in joules or calories
Power is the rate energy is used
1 Watt = 1 joule/s
We call the power output of a star it
Luminosity
• LSun = 3.8 X 1026 watts
Hydrostatic Equilibrium
• Why doesn’t gravity
just make the whole
star collapse?
– Gravity pulls in
– Pressure from inside
the sun pushes out
– These two things
balance perfectly
How does the Sun shine?
Historical Views
• Burning ball of coal or wood
• Energy release due to gravitational
contraction
– Kelvin-Helmholtz contraction
Nuclear fusion
• E=mc2
• Cloud of gas
collapsed, releasing
potential energy
• Gas heats
• Gravity creates high
pressure
• High pressure and
temperature allow
nuclear fusion
• Need high
temperature so
particles move fast
enough to collide
• Nuclear strong force
keeps particles stuck
together
Proton-Proton Chain
Solar Neutrinos
Solar Thermostat
Random Walk
• Radiation has to
make it out of the sun
• Photon bounces
randomly off of
electrons
– Density of Sun’s core
means lots of bounces
• Photons take a few
hundred thousand
years to get out
Solar Activity
Magnetic Fields
• Material can travel
along magnetic field
lines
• It is difficult to travel
across field lines
Sunspots
Solar Prominence
Solar Prominence Video
Solar Flare
• Sometimes magnetic fields get too
stretched and can’t handle the stress
anymore
• They break, releasing trapped material
• Video
Coronal Mass Ejections
CME Video
Sunspot Cycle
• Cycle lasts
~11 years
– 7-15 yrs
Sunspot Cycle