Lecture 10
March 6, 2008


Properties and Structure of the Sun

Key Concepts:

  1. What are the key characteristics of our Sun?
  2. How does the energy/light get out from the center to the photosphere?
  3. What is ideal gas law and hydrostatic equilibrium (i.e., why doesn't the Sun blow up)?
  4. What is nuclear fusion and how does it work?
  5. What is ideal gas law and hydrostatic equilibrium (i.e., why doesn't the Sun blow up)?

Basic Properties

Radius 700,000 km (about 100 Earth Radius)
Mass 2 x 1030 kg (about 300,000 Earth Mass)
Distance 1.5 x 108 km (1 AU or about 8 light minutes)
Surface Temperature 5800 K on average; 4000 K near sunspots
Core Temperature 15 million K
Composition 70% hydrogen 28% helium, 2% heavier elements
Luminosity 4 x 1026 watts
Rotation Rate 27 days (equator) to 31 days (poles)

Sun's Structure (Sun, Today)

A movie of the large filament flare from March 6, 1999. The whole event took about 4.5 hrs.

Energy Transport: heat flows from hot to cold

Quiz 10A

  • Radiative Zone:
    • inner 2/3 of the Sun
    • gas fully ionized in plasma form
    • energy carried outwards by photons
    • very dense, and photon takes 100,000 years to exit the Sun

  • Convection Zone:
    • outer 1/3 of the Sun
    • gas is cooler, and hydrogen atoms begin to form and absorbed photons more efficiently
    • energy transported by bulk motion (i.e. lava lamp)
    • granulation (convection cell) = white (hot) in the middle, narrow darker (colder) area around it

Ideal Gas Law and Pressure

The strength of the pressure is proportional to the density times the temperature of the gas.

Quiz 10B: Ideal Gas Law in Action

Hydrostatic Equilibrium

  • structure of the Sun depends on a balance between its internal forces

  • Unless balanced by some force or pressure, the Sun will collapse on itself under its own gravity

  • The Sun has not, thus a source of pressure exists

  • higher pressure inside ==> higher T inside

Reading assignment for next lecture: Unit 50-51