The sun has many layers, from the inner core to the outer corona. Each layer has its own characteristics, such as its temperature, its density, and the different processes which occur there. The sun is incredibly complex and we do not know all of the details about what happens at each level. Despite this, we have been able to piece together many bits of information to get a relatively accurate picture of how the different parts of the sun interact.
The interior of the sun includes the core, the radiative zone, and the convective zone.
The core is the center of the thermonuclear fusion reactions which power the sun. The matter in the core consists of plasma at temperatures of about 15,000,000 K. The density of the core is about 160,000 kg/m^3, which is 160 times as dense as water, or over 130,000 times as dense as Earth's atmosphere at sea level. Nuclear fusion of protons into helium nuclei in the core provides the vast amounts of energy needed to sustain the sun. Each second, the sun converts 5 million tons of mass into energy according to E=mc^2 (where E=energy, m=mass, and c=the speed of light), which generates energy on the order of 4x10^26 watts! If we could see the core of the sun, it would appear to be about 10^13 times brighter than the surface we now see.
The radiative zone surrounds the core. It acts as a sort of insulator for the core, helping it to maintain the high temperatures needed to sustain the nuclear fusion. The gamma photons produced in the core's fusion reactions are repeatedly absorbed and re-emitted by nuclei in the radiative zone, gaining lower energies and longer wavelengths with each successive re-emission. A photon can take as long as 50 million years to work its way from the core to the convective zone!
The convective zone surrounds the radiative zone and is the outermost layer of the interior. It is cooler and less dense than the radiative zone. It derives its name from the convection processes which bring the heat to the surface of the sun. Radiation is not as prevalent in this layer, so the matter heated by contact with the radiative zone forms giant convective cells which appear on the photosphere as granules. The plasma is made up of charged particles, which create large, prominent magnetic fields when they move with the convective cells. This plays a large part in the creation of sunspots and flares.
The photosphere produces the visible light we see (hence the name) and is generally considered to be the surface of the sun. It is one of the coolest layers of the sun at a meager 6000 K, and it has a density of about 10^-6 kg/m^3. It is comprised of about 73.5% hydrogen and 25% helium, with the other 1.5% primarily consisting of oxygen and carbon. The photosphere shines about 398,000 times as brightly as the full moon. At the photosphere, the diameter of the sun is 1.39x10^6 km, or about 109 times the diameter of the Earth. Large magnetic disturbances sometimes break through the photosphere and cause sunspots, which are cooler, darker regions.
The chromosphere is considered to be part of the solar atmosphere, along with the photosphere. It takes its name from the red light it shines, though this light is much dimmer than that of the photosphere. At 4300 K, it is cooler than the photosphere at its base, but it can get up to 50,000 K in its outer reaches. It extends about 2100 km past the photosphere, and its average density is 10^-9 kg/m^3. The chromosphere also is characterized by cellular convection patterns, with cells much larger than the photosphere's granules. The chromosphere is alive with activity, mainly due to the various magnetic fields which are dominant in the sun. This causes various events, such as filaments (long, dark areas), plage (bright areas surrounding sunspot regions), spicules (relatively small vertical jets of material which can be as large as the Earth), and flares (larger, explosive eruptions of solar material).
The corona is the outermost layer of the sun. It is divided into the inner corona and the outer corona. The inner corona is a wispy halo surrounding the sun. It cannot be seen except during a total solar eclipse (see picture below). The inner corona stretches out more than 1 million km into space. It has a temperature of about 2,000,000 K, but we do not really understand why it is this high. The corona emits many x-rays, and magnetic fields dominate the matter in it. These fields are the cause of the arches and wisps. At times, open field lines can develop. These are lines which extend so far out before returning that from the local area, they appear to be actually open. These open lines cause coronal holes, which are sources of solar wind, or high-speed, low-density streams of particles from the inner corona. Just outside of the inner corona lies the outer corona. The outer corona extends well beyond Earth, and interacts with Earth's magnetic field. It is not as easy to find as the inner corona, and in fact its existence was not postulated until the 1950s. The outer corona is the outermost atmosphere of the sun, and it includes much of the solar system.