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The Interior of a Star

Non-degenerate stars have a simple structure. The core is the hottest and most dense region of a star. As one moves away from the star's center, both the density and the temperature of the star drop, until one reaches the star's photosphere, where the star's radiation freely escapes into space. The core of the Sun, for instance, is about 15 million degrees, while the surface temperature is effectively 5778°.

Stars evolve over extremely long times. Main sequence stars evolving on times longer than 2 million years, and in the case of stars smaller that the Sun, on times longer than 10 billion years. A three solar mass red giant burns its core hydrogen in about 250 million years, and it burns its core helium in about 100 million years. The shortest time scales are for stars transiting from the core burning of one type of element to the core burning of the next heavier element. For instance, a 3 solar mass star transits from the core burning of hydrogen to the core burning of helium in about 10 million years. On these time scales, the structure of most stars is static, and the pressure at any radius within the star is equal to the pressure exerted by the weight of the overlying layers. This means that the pressure in a star is greatest at its core, and it drops as one moves out from the core to the star's surface.

A star is composed of plasma, which is a gas containing free electrons. The plasma at the core of a star is fully ionized, so that no electrons in the plasma are bound to atomic nuclei. The pressure in a fully ionized plasma is related to the temperature and density by the ideal gas law: P = n k T, where P is the pressure, n is the number density of the plasma (the number of electrons and nuclei per unit volume), k is the Boltzmann constant, and T is the temperature. Away from the stellar core, where the temperature is lower, some of the electrons are bound to nuclei. The plasma in this case departs from the ideal gas law because of the energy states of the bound electrons.

Gas, however, is not the only source of pressure within a star; the radiation trapped within the star also exerts a pressure. In parts of very massive stars, the radiation pressure exceeds the gas pressure.

Energy in a star is transported to the surface through one of two mechanisms: radiative diffusion and convection. A third mechanism, thermal conduction, is unimportant in stars. For most stars, both radiative diffusion and convection are at work. For instance, in a star smaller than the Sun, energy is transporting through the core by radiative diffusion, but energy is transported through the outer by convection. In contrast, in a very massive star, energy is transported through the core by convection, and it is transported through the outer layers by radiative transport.

These two transport mechanisms have a second important impact on stellar structure: when convection is present, the elements in that layer are uniformly mixed, but when radiative diffusion is present, the elements in a layer become stratified, particularly if nuclear fusion is occurring in this layer.

So where does the nuclear fusion occur? For most of a star's life, nuclear fusion occurs at a star's core. This is particularly true of main sequence stars; at the cores of these stars nuclear fusion is converting hydrogen into helium. In later stages, when a star is a red giant, core nuclear fusion converts helium into carbon and oxygen, and later in life, core fusion converts carbon and oxygen into a variety of heavier elements.

But nuclear fusion also occurs in one or more shells away from the core, and for short intervals during the evolution of a star, nuclear fusion only occurs in shell. The shells of nuclear fusion lie at the radii where the composition of a star changes. Once a star burns all of its core hydrogen, it has a core of pure helium surrounded by a mixture of hydrogen and helium; the fusion of hydrogen into helium occurs at the transition between these two regions. If there is a transition from pure helium to carbon and heavier elements, then a shell where helium fusion occurs can arise. The nuclear fusion in these shell produce a substantial amount of energy, and they continue the chemical transformation of the star from light elements to heavier elements.

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