Calculating the temperature of the Sun can be approached in several ways, but one of the most common methods is using the concept of blackbody radiation and the StefanBoltzmann law. Here's a stepbystep guide to understanding and calculating the Sun's temperature:
Method 1: Using the StefanBoltzmann Law
The StefanBoltzmann law relates the total energy radiated per unit surface area of a blackbody to the fourth power of its temperature. The formula is:
[ P = \sigma T^4 ]
where:
 ( P ) is the power per unit area (energy radiated per second per square meter),
 ( \sigma ) is the StefanBoltzmann constant (( \sigma \approx 5.67 \times 10^{8} , \text{W} , \text{m}^{2} , \text{K}^{4} )),
 ( T ) is the absolute temperature in Kelvin (K).
StepbyStep Calculation:

Determine the Solar Constant: The solar constant (( S )) is the amount of solar energy received per unit area at the top of Earth's atmosphere. It is approximately ( 1361 , \text{W/m}^2 ).

Calculate the Total Power Output of the Sun: The total power output (( L )) of the Sun can be calculated by considering the surface area of a sphere with a radius equal to the average distance from the Earth to the Sun (1 Astronomical Unit, AU, which is about ( 1.496 \times 10^{11} , \text{m} )).
[ L = S \times 4 \pi (1 , \text{AU})^2 ]
 Calculate the Surface Area of the Sun: The surface area (( A )) of the Sun can be calculated using its radius (( R_{\odot} \approx 6.96 \times 10^8 , \text{m} )).
[ A = 4 \pi R_{\odot}^2 ]
 Apply the StefanBoltzmann Law: The total power output of the Sun is also given by the StefanBoltzmann law:
[ L = \sigma A T^4 ]
 Solve for the Temperature: Rearrange the equation to solve for ( T ):
[ T = \left( \frac{L}{\sigma A} \right)^{1/4} ]
Example Calculation:

Solar Constant: ( S \approx 1361 , \text{W/m}^2 )

Total Power Output:
[ L = 1361 , \text{W/m}^2 \times 4 \pi (1.496 \times 10^{11} , \text{m})^2 ]
[ L \approx 3.846 \times 10^{26} , \text{W} ]
 Surface Area of the Sun:
[ A = 4 \pi (6.96 \times 10^8 , \text{m})^2 ]
[ A \approx 6.087 \times 10^{18} , \text{m}^2 ]
 Apply the StefanBoltzmann Law:
[ T = \left( \frac{3.846 \times 10^{26} , \text{W}}{5.67 \times 10^{8} , \text{W} , \text{m}^{2} , \text{K}^{4} \times 6.087 \times 10^{18} , \text{m}^2} \right)^{1/4} ]
 Solve for ( T ):
[ T \approx 5778 , \text{K} ]
So, the effective temperature of the Sun is approximately 5778 Kelvin.
Method 2: Using Wien's Displacement Law
Wien's Displacement Law relates the temperature of a blackbody to the wavelength at which it emits radiation most strongly. The formula is:
[ \lambda_{\text{max}} T = b ]
where:
 ( \lambda_{\text{max}} ) is the peak wavelength,
 ( T ) is the absolute temperature in Kelvin,
 ( b ) is Wien's displacement constant (( b \approx 2.897 \times 10^{3} , \text{m} , \text{K} )).
If you know the peak wavelength of the Sun's spectrum (which is around 500 nm or ( 5 \times 10^{7} , \text{m} )), you can calculate the temperature:
[ T = \frac{b}{\lambda_{\text{max}}} ]
[ T = \frac{2.897 \times 10^{3} , \text{m} , \text{K}}{5 \times 10^{7} , \text{m}} ]
[ T \approx 5794 , \text{K} ]
This method also gives a temperature close to the previously calculated value.
Conclusion
Both methods provide a good estimate of the Sun's effective temperature, which is around 5778 K. These calculations assume the Sun behaves like an ideal blackbody, which is a good approximation for many