ASTR 1102 Chapter : Class 3
Class 3 Monday, January 25, 2010
Light Carries Energy
Inverse Square Law of Brightness
b= L/4πd^2 ∞ 1/d^2
b=brightness
L= luminosity (wattage)
d= distance
Thermal Radiation
L= Area * SigmaT^4
L= luminosity
Sigma= 6*10^-8 w/m^2/k^2
Area- emitting area
Temperature Balance
• Temperature remains stable when energy-out equals energy-in
EnergyOut=EnergyIn
(4πRplanet^2)*SigmaT^4= (Lsun/4πd^2)*(πRplanet^2)
T^4= (1/d^2)*(16πLsun/Sigma)
T∞ (1/sqrtd)
Tplanet= 260degreesK/(sqrt(dAU)) (To get – C degrees subtract 276 from degrees K)
Must correct for reflectivity of planet’s surface
Planet Temperatures
• Venus is very hot from runaway Greenhouse Effect of CO2
• Earth would be freezing without small Greenhouse Effect
• Jupiter, Saturn, and Neptune are slightly warmer from residual heat of formation
4 Icy Planets (5 including Pluto)
4 Rocky Planets
