Some research teams are developing simplified modelling of turbulence in 3D calculations. The most difficult challenge remains the numerical treatment of turbulence. In forming the stellar structure equations (exploiting the assumed spherical symmetry), one considers the matter density ρ ( r ), the temperature at the surface is the effective temperature of the star.Īlthough nowadays stellar evolution models describe the main features of color–magnitude diagrams, important improvements have to be made in order to remove uncertainties which are linked to the limited knowledge of transport phenomena. It contains four basic first-order differential equations: two represent how matter and pressure vary with radius two represent how temperature and luminosity vary with radius. The simplest commonly used model of stellar structure is the spherically symmetric quasi-static model, which assumes that a star is in a steady state and that it is spherically symmetric.
The lowest mass main sequence stars have no radiation zone the dominant energy transport mechanism throughout the star is convection. Thus, massive stars have a radiative envelope. In the outer portion of the star, the temperature gradient is shallower but the temperature is high enough that the hydrogen is nearly fully ionized, so the star remains transparent to ultraviolet radiation. Due to the strong temperature sensitivity of the CNO cycle, the temperature gradient in the inner portion of the star is steep enough to make the core convective. In the CNO cycle, the energy generation rate scales as the temperature to the 15th power, whereas the rate scales as the temperature to the 4th power in the proton-proton chains. In massive stars (greater than about 1.5 M ☉), the core temperature is above about 1.8×10 7 K, so hydrogen-to- helium fusion occurs primarily via the CNO cycle. Therefore, solar mass stars have radiative cores with convective envelopes in the outer portion of the star. The outer portion of solar mass stars is cool enough that hydrogen is neutral and thus opaque to ultraviolet photons, so convection dominates.
Thus, radiation dominates in the inner portion of solar mass stars. In stars with masses of 0.3–1.5 solar masses ( M ☉), including the Sun, hydrogen-to-helium fusion occurs primarily via proton–proton chains, which do not establish a steep temperature gradient. The internal structure of a main sequence star depends upon the mass of the star. In regions with a low temperature gradient and a low enough opacity to allow energy transport via radiation, radiation is the dominant mode of energy transport. In this case, the rising parcel is buoyant and continues to rise if it is warmer than the surrounding gas if the rising parcel is cooler than the surrounding gas, it will fall back to its original height. The different transport mechanisms of high-mass, intermediate-mass and low-mass starsĭifferent layers of the stars transport heat up and outwards in different ways, primarily convection and radiative transfer, but thermal conduction is important in white dwarfs.Ĭonvection is the dominant mode of energy transport when the temperature gradient is steep enough so that a given parcel of gas within the star will continue to rise if it rises slightly via an adiabatic process.
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