The sun is a hot sphere of gas whose internal temperatures reach over 20 million degrees kelvin due to nuclear fusion reactions at the sun's core which convert hydrogen to helium. The radiation from the inner core is not visible since it is strongly absorbed by a layer of hydrogen atoms closer to the sun's surface. Heat is transferred through this layer by convection. The surface of the sun, called the photosphere, is at a temperature of about 6000K or more accurately, 5762 ± 50 K and closely approximates a blackbody.
The total power emitted by the sun is calculated by multiplying the emitted power density by the surface area of the sun which gives 9.5 x 1025 W.
Animmation of convection currents inside the sun and the current image of the sun (updated daily) from SOHO.
The total power emitted from the sun is composed not of a single wavelength, but is composed of many wavelengths and therefore appears white or yellow to the human eye. These different wavelengths can be seen by passing light through a prism, or water droplets in the case of a rainbow. Different wavelengths show up as different colours, but not all the wavelengths can be seen since some are "invisible" to the human eye.