. The Sunís luminosity, temperature, and composition all vary with depth in its interior, from the Sunís visible disk (left) to the center of the Sun (right). The nuclei are squeezed into a smaller volume by the pressure of the material above, becoming hotter and more densely concentrated at greater depths. At the Sunís center, the temperature has reached 15.6 million degrees Kelvin, and the pressure is 233 billion times that of the Earthís atmosphere at sea level. Nuclear fusion reactions in the Sunís energy-generating core synthesize helium from hydrogen, so this region contains more helium and less hydrogen than the primordial amounts detected in the light of the visible Sun.
. The Sun gets its energy when hydrogen nuclei are fused together to form helium nuclei within the solar core. This hydrogen burning is described by a sequence of nuclear fusion reactions called the proton-proton chain. It begins when two protons, here designated by the letter 1H, combine to form the nucleus of a deuterium atom, the deuteron that is denoted by D, together with the emission of a positron, e+, and an electron neutrino, νe. Another proton collides with the deuteron to make a nuclear isotope of helium, 3He, and then a nucleus of helium, 4He, is formed by the fusion of two 3He nuclei, returning two protons to the gas. Overall, this chain successively fuses four protons together to make one helium nucleus. Even in the hot, dense core of the Sun, only rare, fast-moving particles are able to take advantage of the tunnel effect and fuse in this way.
. The Sun is powered by the nuclear fusion of hydrogen in its core. Energy produced by fusion reactions is transported outward, first by countless absorptions and emissions within the radiative zone, and then by convection. The visible disk of the Sun, called the photosphere, contains dark, magnetic sunspots. It is enveloped by a million-degree corona. A high-speed solar wind gushes out of coronal holes.
. At high magnification, the photosphere appears completely covered with an irregular, strongly-textured pattern of cells called granules. The hot granules, each 1.5 million meters or smaller across, rise at speeds of 500 meters per second, like supersonic bubbles in a immense, boiling cauldron. The rising granules radiate their energy, and cool material then sinks down along the dark, inter-granular lanes. This photograph was taken at 430.8 nanometers with an 11-nanometer interference filer.; it has an exceptional angular resolution of 0.2 arc-seconds, or 150 thousand meters at the Sun . (Courtesy of Richard Muller and Thierry Roudier, Observatoire du Pic-du-Midi et de Toulouse.)
. This velocity image, or Dopplergram, of the Sunís photosphere, with rotational motion removed, shows motion toward and away from the observer as light and dark patches. The velocity field is made up of about 2,500 supergranules, each about 35 million meters across. Near the disk center they are hardly visible at all, thus indicating that the velocities are predominantly horizontal. (Courtesy of the SOHO MDI/SOI consortium. SOHO is a project of international cooperation between ESA and NASA.)
. In about 8 billion years the Sun will become much brighter (top) and larger (bottom). Note the different time scales, expanded near the end of the Sunís life to show relatively rapid changes. (Courtesy of I-Juliana Sackmann and Arnold I. Boothroyd ).