THE ANATOMY OF STARS
Stars are much more than uniform balls of gas. They have a complex and dynamic interior structure that is constantly changing and evolving. The Sun is the only star that is near the Earth, and thus it's the only star for which we have the kind of detailed knowledge that allows us to talk about how various parts function. In the sense, the Sun is not only the giver of life on our planet, but also the giver of knowledge. Without such a star close by to study, we would probably not know much about the billions of stars that are father away.
The Structure of the Sun:
We believe that the Sun is rather an ordinary star. At it center is the stellar core, comprising about 10% of the Sun's total volume. This core is the Sun's furnace. where nuclear fires rage, and energy generated in the core streams out from the center. Deep within the Sun, this energy transfer takes place largely through collisions of high energy-particles---protons and positrons, for example---that are generated by the core's nuclear reactions. About four-fifths of the way out, however, the energy-transfer mechanism changes, and the hydrogen-rich material in the Sun begins to undergo large-scale convection. This outer region, comprising the upper 200,000 kilometers (about 125,000 miles) of the Sun, is the convection zone. Thus energy is brought from the core to the surface in a stepwise process, first by collisions, then by convection.
The only part of the Sun that we actually see is a thin outer layer. We can peer perhaps 150 kilometers (about 100 miles) into the Sun; any deeper and the stellar materials becomes too dense to be transparent. The outer part of the Sun--the part that actually emits most of the light we see--is the photosphere. The sun does not have a sharp outer boundary, but gradually becomes thinner and thinner father away from the surface. These gaseous layers are not usually visible from the Earth. However, during a total eclipse of the Sun, when the Moon passes in front of the Sun, the Sun's spectacular halo--called the chromosphere and the corona--- may become visible for a few minutes.
The Sun constantly emits a stream of particles--maily ions (electrically charged atoms) of hydrogen and helium--into space around it. This stream of particles, called the solar wind, blows by Earth and the other planets all the time. Because the particles are charged, they affect the magnetic stream fields of the planets, compressing the fields on the “upstream” side and dragging them out on the “downstream” side. This interaction of the solar wind with the outer reaches of the Earth's atmosphere also gives rises to the aurora borealis, or northern lights.
The flow of energy from the Sun is a complex affair. Beginning with the conversion of mass in fusion reactions, the energy slowly percolates outward, first collisions and later in great convection cells under the solar surface. It takes a few tens of thousands of years for the energy to work its way to the photosphere, but only eight minutes to cover the distance between the Sun and the Earth.
Once the sunlight reaches our planet, a tiny fraction of it is converted by the process of photosynthesis in plants into chemical energy stored in large molecules. This is the primary source of energy for all living things on the planet. Another fraction of the energy in sunlight's heats the air and equator and, drive s the Earth weather patterns.
