 Every body that has a temperature radiates energy. How much energy does our sun radiate and how much of it reaches us on Earth? How much energy does the Earth actually radiate into space? And what is the surface temperature of our sun? These and similar questions can be answered by the Stefan-Boltzmann law. This law describes the relationship between the temperature T of a body and its radiant power P. The surface area A of the body also influences the radiant power. The Stefan-Boltzmann law is P is equal to sigma times A times T to the power of four. Sigma is the Stefan-Boltzmann constant with the value 5.67 times 10 to the power of minus eight joules per meter squared times Kelvin to the power of four times second. This is a constant that has an exact value that only depends on other physical constants, namely the Boltzmann constant, the Planck constant, and the speed of light. What exactly is the radiant power P? It indicates how much energy per second the surface of the body radiates. The unit of radiant power is watt or joule per second. In addition, the Stefan-Boltzmann law only applies to black bodies or bodies that come close to a black body. However, the term black has nothing to do with the color of the body. The sun, for example, is a nearly black body, although, as you know, it is not black. A black body absorbs all the radiation that hits it. It does not reflect any radiation and does not allow any radiation to pass through. The Stefan-Boltzmann law shows how much the radiated energy depends on the temperature. Doubling the surface temperature of the sun would result in 16 times more solar energy reaching the Earth. Let's take an example of how to determine the temperature of the sun using the Stefan-Boltzmann law. We can deduce the total amount of energy emitted by the sun from the energy we receive from the sun. The sun emits a power of 3.8 times 10 to the power of 26 joules per second. The radius of the sun is 6.96 times 10 to the power of eight meters. Let's assume that the sun is not flat but spherical. We can therefore use the formula for the surface area of a sphere to determine the surface area of the sun. A is equal to four pi times r squared. Let's rearrange the Stefan-Boltzmann law with respect to the temperature T and insert the surface area formula. If we now insert the radiant power, the Stefan-Boltzmann constant and the solar radius, we get 5,760 Kelvin as the surface temperature of the sun. This corresponds to 6,033 degrees Celsius.