Sunrise over the Earth viewed from the Space Shuttle.
THE INTERIOR OF THE SUN
The Sun can be differentiated into three major parts - the core, the radiative zone, and the convective zone.
The core
The core occupies the central 40% of the Sun's diameter, and is the source of the Sun's energy. The temperature in the core is in the region of 15 million K, and the pressure in the region of 2.5 x 1011bar. The pressure is 250,000 times greater than Earth's atmospheric pressure. The density is over 15 times greater than the density of lead. The core is less than 2% the Sun's volume, but contains half the mass of the Sun.
The source of the Sun's power is thermonuclear fusion. Hydrogen atoms are converted to helium, and in the process there is a mass discrepancy. For every one helium atom produced, four hydrogen atoms are consumed. However, one helium atom contains less mass than the four hydrogen atoms combined. The missing mass has been converted to energy, in accordance with Einstein's equation E=mc2 , where m is the missing mass, and c is the speed of light. As c is a large number, only a small mass needs to be converted in order to gain a large energy output.
The Sun burns over 700 million tons of hydrogen per second, producing 695 million tons of helium. The missing 5 tons is converted into energy. The energy, in the form of light particles, or photons, takes about 170,000 years to travel through the Sun's outer layers and escape the Sun. A further eight minutes is needed for the photons to travel across the Solar System and enter your eyes.
The radiative zone
Surrounding the core can be found the radiative zone. This zone extends to just over 70% of the Sun's radius, and energy transport within this region is via radiation. The gas in the radiative zone is relatively calm.
The convective zone
The convective zone surrounds the radiative zone and accounts for most of the rest of the Sun's radius. Energy transport is through the mechanism of convection. The material in the convective zone is opaque to radiation - the radiation therefore heats up the bottom of the zone. The heated material rises, loses energy to space, and then sinks.
The internal structure of the Sun.
The photosphere, chromosphere, and corona
Above the convective zone is a very thin layer (thin by solar standards) of gas at a temperature of 5,780 K. This layer is the photosphere, 500 kilometres deep, and the gas is transparent to radiation. The radiation therefore escapes from the photosphere, and it is this escaping radiation that enables us to see the Sun.
The chromosphere is a layer of very thin gas surrounding the photosphere. It is transparent, and about 1000 kilometres deep. The corona (corona is Latin for crown) completes the Sun's structure. The bottom of the corona has a temperature of about 10,000 K but there is a huge increase in the temperature with increasing altitude. Only 100 kilometres into the corona and the temperature climbs to 1 million K. In other words, it appears as though heat flows from the cooler chromosphere underneath to the hotter corona above. As heat does not do this, there must be another mechanism at work that raises the coronal temperature. Such a mechanism is most likely the powerful interactions of the Sun's magnetic fields.
The corona is very faint indeed, and although it produces its own light, it has less than 0.0001% the luminosity of the lower solar layers. Consequently, the corona can only be seen by the naked eye when the rest of the Sun is obscured by the Moon during a total solar eclipse.
The solar corona and prominences are visible during a total eclipse.	The corona consists of rays and streamers.	A total solar eclipse is needed in order to view the Sun's corona.
The Sun as a bell
Sound waves travel through the Sun, in much the same way as earthquake shock waves travel through the earth. The sound waves are trapped inside the Sun - they cannot radiate through the vacuum of space. However, even if the sounds could travel to Earth, the frequencies are below the level of human hearing. The frequency, or pitch, of the notes depends on the pressure and temperature of the Sun's material - the Sun is an orchestra of over a million notes. The sounds, which are pressure waves, cause the photosphere to oscillate, with photospheric gas rising and falling by up to 10 kilometres.
Recent studies in this new field of helioseismology are leading to a greater understanding of the structure of the Sun and the complex convection and flow processes at work below the surface