Sun (Stellar Structure)
The Sun is the star about which Earth revolves. A typical star, Earth's sun is composed of gases and heavier elements compressed to enormous density and heated to levels that sustain nuclear fusion (the transformation of hydrogen into helium and heavier elements). The Sun consists of an inner core surrounded by a radiative zone and then a convective zone. The surface of the Sun is termed the photosphere. Surrounding the Sun is a solar corona—an atmosphere of hot plasma, gases, and outflowing particles.
Nuclear fusion take place in the Sun's core and it is in this region that the bulk of the Sun's production of energy, heat, and gamma rays takes place. The radiative zone surrounding the core is of such high density that photons generated in the
core can take millions of years to pass through to the surrounding radiative zone. Undergoing an enormous number of collisions, absorptions and regenerations, photons span a spectrum frequencies that correspond to gamma rays, x ray, ultraviolet light, visible light, infrared light, microwaves, and radio waves. Photon passage through the convective zone provides energy to drive massive convective currents of hot gas.
The photosphere is the visible outer or surface layer of Sun. At the photosphere, solar temperatures cool to about 5800 K. The photosphere often features sunspots (areas of surface relatively cooler due to differential temperatures in convective currents). Sunspots occur in cycles with maximum activity peaking every 11 years.
Largely composed of gas, the Sun exhibits differential rotation speeds that depend on solar latitude. The rotational period varies from approximately 25 days at the equator to 29 days near the polar regions.
The chromosphere surrounds the photosphere and extends thousands of miles. Temperatures increase in the chromosphere and range up to 1,000,000 K. The chromosphere is part of the solar corona that extends millions of miles into space. Influenced by turbulent magnetic fields coronal temperatures range up to 3,500,000 K. At these high temperatures, electrons are stripped from gases and plasma streams form a solar wind. The solar chromosphere and corona are usually visible only when an eclipse blocks the photosphere.
Solar flares and prominences, flame-like eruptions of hot gas, sometimes extend into the chromosphere and corona.
British astronomer Fred Hoyle once described the evolution of a star—including, of course, the Sun—as a continual war between nuclear physics and gravity.
The gravity of the stellar material pulls on all the other stellar material striving to bring about a collapse. However, the gravitational compression is opposed by the internal pressure of the stellar gas that normally results from heat produced by nuclear reactions. This balance between the forces of gravity and the pressure forces forms an equilibrium, and the balance must be exact or the star will quickly respond by expanding or contracting in size. So powerful are the separate forces of gravity and pressure that should such an imbalance occur in the Sun, it would be resolved within hours. That fact that Earth's sun is about 5 billion years old emphasizes just how exactly and continuously that balance is maintained.
In addition to its reliance on balance between gravity and pressure, the internal structure of a sun depends on the behavior of the stellar material itself. Most stars are made primarily of hydrogen, the dominant form of matter in the universe. However, the behavior of hydrogen will depend on the temperature, pressure, and density of the gas. Indeed, the quantities temperature, pressure, and density are known as state variables, because they describe the intrinsic state of the material. Any equation or expression that provides a relationship between these variables is called an equation of state.
Most of the energy that flows (i.e., undergoes a series of transformations) from a star originates at its center. The way in which this energy flows to the surface will also influence the internal structure of the star.
There are three ways by which energy flows outward through a star. They are conduction, convection, and radiation.
However, the more opaque the material is, the slower the convectional and radiative transfers of heat and energy (e.g. electromagnetic radiation or "light") flow of energy will be. In the Sun, where light flowing out from in the core will travel less than a centimeter before it is absorbed, it may take a million years for the light energy to make its way to the surface.
The mode of energy transport, equations of state, and equilibrium equations can be quantified and self-consistent solutions found numerically for stars of given mass, composition and age. Such solutions provide model stellar interiors, and supply the detailed internal structure of a particular star. For the vast majority of stars that derive their energy from the nuclear fusion of hydrogen into helium, the internal structure is quite similar. Such stars are termed main sequence stars and are located in a band on a Hertzsprung-Russell diagram (developed independently between 1911–13 by Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957).
The Sun is a main sequence star. The Sun's core is surrounded by a churning convective envelope that carries the energy to within a few thousand kilometers of the surface, where energy again flows primarily by radiation as it escapes into space. This structure is common to all main sequence stars with mass less than 1.5 times the mass of the Sun.
Changes to the stellar structure over time are described by the theory of stellar evolution.
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