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