The Sun

Properties M = 2 X kg = 300,000 M Earth R = 700,000 km > 100 R Earth 70% H, 28% He T = 5800 K surface, 15,000,000 K core

Structure

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 L Sun = 3.8 X 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=mc 2 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