Sun's Surface

The upper half of the sun consists of three major areas: the core, the radiative zone and the convective zone.

Core
The core starts from the center and extends to 25 percent of the sun's radius. Here, gravity pulls all of the mass inward and creates an intense pressure. The pressure is high enough to force atoms of hydrogen to come together in nuclear fusion reactions. Two atoms of hydrogen are combined to create helium-4 and energy in several steps:

1. Two protons combine to form a deuterium (hydrogen atom with one neutron), a positron (similar to electron, but with a positive charge) and a neutrino
2. A proton and a deuterium atom combine to form a helium-3 atom (two protons with one neutron) and a gamma ray.
3. Two helium-3 atoms combine to form a helium-4 (two protons and two neutrons) and two protons.

These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes from the following reactions:

1. A helium-3 and a helium-4 combine to form a beryllium-7 (four protons and three neutrons) and a gamma ray.
2. A beryllium-7 captures an electron to become lithium-7 (three protons and four neutrons) and a neutrino.
3. The lithium-7 combines with a proton to form two helium-4 atoms.

The helium-4 atoms are less massive than the two hydrogen atoms that started the process, so the difference in mass was converted to energy as described by Einstein's theory of relativity (E=mc²). The energy is emitted in various forms of light (ultraviolet light, X-rays, visible light, infrared, microwaves and radio waves). The sun also emits energized particles (neutrinos, protons) that make up the solar wind. This energy strikes Earth, where it warms the planet, drives our weather and provides energy for life. We are not harmed by most of the radiation or solar wind because the Earth's atmosphere protects us. As shown in Figure 2, we can use special telescopes aboard the satellite SOHO to look at the various wavelengths of light the sun emits and get images that scientists can study.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 2. Composite image from all of SOHO's instruments. The interior image from Michelson Doppler Imager (MDI) illustrates the rivers of plasma underneath the surface. The surface was imaged with the extreme ultraviolet imaging telescope (EIT) at 304 angstroms. Both images were superimposed on a Large Angle Spectroscopic Coronograph (LASCO) C2 image, which blocks the sun so that it can view the corona. The image shows the range of SOHO's research from the solar interior out to the corona.

Radiative Zone
The radiative zone extends 55 percent of the sun's radius from the core. In this zone, the energy from the core is carried outward by photons. As one photon is made, it travels about 1 micron (1 millionth of a meter) before being absorbed by a gas molecule. Upon absorption, the gas molecule is heated and re-emits another photon of the same wavelength. The re-emitted photon travels another micron before being absorbed by another gas molecule and the cycle repeats itself; each interaction between photon and gas molecule takes time. Approximately 10²⁵ absorptions and re-emissions take place in this zone before a photon reaches the surface, so there is a significant time delay between a photon made in the core and one that reaches the surface.

Convective Zone
The convective zone, which is the final 30 percent of the sun's radius, is dominated by convection currents that carry the energy outward to the surface. These convection currents are rising movements of hot gas next to falling movements of cool gas, much like what you can see if you placed glitter in a simmering pot of water. The convection currents carry photons outward to the surface faster than the radiative transfer that occurs in the core and radiative zone. With so many interactions occurring between photons and gas molecules in the radiative and convection zones, it takes a photon approximately 100,000 to 200,000 years to reach the surface!