Armed with knowledge of the Sun's size, composition and energy source, astronomers can work out a model of the physical conditions at every point within the Sun. This is often called the standard solar model. As a result of gravitational contraction, the temperature at the Sun's center reached 15 million Kelvin as the proton-proton reactions became established. This remains the current temperature in the solar core, where nuclear energy is generated. Approximate conditions in the region between the core and the surface can be calculated by using equations that describe the interior pressure and density and the energy generation rates inside the Sun. The relationship between temperature and pressure and volume is the same gas law that applies to the atmospheres of giant planets.
Schematic of the parts of the Sun. Click here for original source URL
These calculations indicate that the gas pressure at the Sun's core is about 250 billion times the air pressure at the Earth's surface. This high pressure compresses the gas in the core to a density about 158 times denser than water and about 20 times denser than iron. One cubic inch of this gas would weigh 2.5 kilograms or nearly 6 pounds! Although it may seem strange that material this dense can be a gas, it is inevitable given the very high temperatures. The particles are moving too rapidly to stick together in the kind of lattice that we find in a solid. Such a high temperature gas, where all the electrons have been stripped from the atomic nuclei, is called a plasma.
Despite the violent reactions going on in its core, the Sun itself is stable. Even though we associate fusion on Earth with nuclear weapons, the Sun is not a bomb and it is not exploding. At every point within the Sun, the inward pull of gravity is balanced by outward pressure due to energy released from nuclear reactions. The Sun is in what is called hydrostatic equilibrium. Very early in its history, the Sun must have collapsed from a giant gas cloud. However, the collapse phase took only 50 million years and it finished over 4.5 billion years ago. Ever since then, the Sun has been converting hydrogen into helium in a steady nuclear reaction.
The fusion core of the Sun occupies about the inner quarter of the Sun's radius. This 1/64 of the Sun's volume contains about half the solar mass and generates 99% of the solar energy. The Sun is opaque. We see an other layer that has a temperature of about 5700 K, very much cooler than the core. Photons diffuse out from the core, which means we can't can't see into the Sun, just as we can't see into a cloud because photons are bouncing off small water droplets. Since can not see the Sun's core directly, these statements about the interior are based on models of the chemical composition and energy generation.