9.2 The Solar Interior
Lacking any direct measurements of the Suns interior, astronomers must use indirect means to probe the inner workings of our parent star. To accomplish this, they construct mathematical models of the Sun, combining all available observations with theoretical insight into solar physics. The result is the Standard Solar Model, which has gained widespread acceptance among astronomers.
An important technique for probing the Sun beneath the photosphere emerged in the 1960s, when it was discovered that the surface of the Sun vibrates like a complex set of bells. These vibrations, illustrated in Figure 9.4, are the result of internal pressure ("sound") waves that reflect off the photosphere and repeatedly cross the solar interior. These waves penetrate deep inside the Sun, and analysis of their surface patterns allows scientists to study conditions far below the Suns surface. This process is similar to the way in which seismologists study Earths interior by observing seismic waves produced by earthquakes. (Sec. 5.4) For this reason, study of solar surface patterns is usually called helioseismology, even though solar pressure waves have nothing whatever to do with solar seismic activity (which doesnt exist).
The most extensive study of solar vibrations is the ongoing GONG (short for Global Oscillations Network Group) project. By making continuous observations of the Sun from many clear sites around Earth, solar astronomers have obtained uninterrupted high-quality solar data spanning weeks at a time. The Solar and Heliospheric Observatory (SOHO), launched by the European Space Agency in 1995 and now permanently stationed between Earth and the Sun some 1.5 million km from our planet, provides continuous monitoring of the Suns surface and atmosphere. Analysis of the data from all these sources provides important detailed information about the temperature, density, rotation, and convective state of the solar interior.
As shown in Figure 9.5, the solar temperature also decreases with increasing radius, but not as rapidly as the density. Computer models indicate a central temperature of about 15 million K. The temperature decreases steadily, reaching the observed value of 5800 K at the photosphere.
The very hot solar interior ensures violent and frequent collisions among gas particles. In and near the core, the extremely high temperatures guarantee that the gas is completely ionized. With no electrons left on atoms to capture photons and move into more excited states, the deep solar interior is quite transparent to radiation. (Sec. 2.6) Only occasionally does a photon scatter off a free electron. The energy produced by nuclear reactions in the core travels outward toward the surface in the form of radiation with relative ease.
As we move outward from the core the temperature falls, and eventually some electrons can remain bound to nuclei. With more and more atoms retaining electrons that can absorb the outgoing radiation, the gas in the interior changes from being relatively transparent to being almost totally opaque. By the outer edge of the radiation zone, 500,000 km from the center, all of the photons produced in the Suns core have been absorbed. Not one of them reaches the surface. But what happens to the energy they carry?
That energy is carried to the solar surface by convectionthe same basic physical process we saw in our study of Earths atmosphere. (Sec. 5.3) Convection can occur whenever cooler material overlies warmer material, and this is just what happens in the outer part of the Suns interior. Hot solar gas physically moves outward, while cooler gas above it sinks, creating a characteristic pattern of convection cells. All through the convection zone, energy is transported to the surface by physical motion of the solar gas. Remember that there is no physical movement of material when radiation is the energy-transport mechanism; convection and radiation are fundamentally different ways in which energy can be transported from one place to another.
Convection does not proceed into the solar atmosphere. In and above the photosphere, the density is so low that the gas is transparent and radiation once again becomes the mechanism of energy transport. Photons reaching the photosphere escape freely into space.
Evidence for Solar Convection
Each granule forms the topmost part of a solar convection cell. Doppler measurements indicate that the bright granules are moving outward with speeds of about 1 km/s, while the dark granules are moving down into the solar interior, exactly as we would expect for the topmost tier of convection in Figure 9.6. (More Precisely 2-3) The brightness variations of the granules result from differences in temperature. The upwelling gas is hotter and therefore (by Stefans law) emits more radiation than the cooler downwelling gas. (Sec. 2.4) The adjacent bright and dark gases appear to contrast considerably, but in reality their temperature difference is less than about 500 K.
Careful measurements also reveal a much larger-scale flow beneath the solar surface. Supergranulation is a flow pattern quite similar to granulation except that supergranulation cells measure some 30,000 km across. As with granulation, material upwells at the center of the cells, flows across the surface, then sinks down again at the edges. Scientists believe that supergranules are the imprint on the photosphere of the deepest tier of large convective cells depicted in Figure 9.6.
Describe the two distinct ways in which energy moves outward from the solar core to the photosphere.