Simply put, the Sun is a great ball of gas, hot enough to glow in every tier. In the very innermost part of the Sun, called its core, the temperature is about 15 million Kelvins, the density is 150 times that of water, and the pressure is over 200 billion times greater than atmospheric pressure here on Earth. This heavy, sweltering place is where the Sun's energy is produced via a process known as thermonuclear fusion. Those darned blemishes.
Sunspots appear dark in contrast to the hot, surrounding photosphere.
While fusion is difficult to mimic on Earth, the scorching belly of the Sun and other stars is a perfect environment for it. Here, the temperatures are high enough for hydrogen nuclei to smash together and form helium nuclei, releasing tremendous amounts of energy in various forms. Energy produced in the form of light keeps bouncing around inside the Sun, as though the Sun were made entirely of mirrors. Energy in the form of small particles called neutrinos, however, can travel directly out of the Sun and into the Solar System. Neutrino observatories on Earth measure the continual wash of these tiny, fast-moving particles.