Solar surface convection zone

Fig. 5.7. Evolutionary tracks for Z = 0.02 (near solar metallicity) stars with different masses in the HR diagram. (Luminosities are in solar units.) Points labelled 1 define the ZAMS and points labelled 2 the terminal main sequence (TAMS), the point where central hydrogen is exhausted. The Schonberg-Chandrasekhar limit may be reached either before or after this (for M > 1.4 Af0). Points marked 3 show the onset of shell hydrogen-burning. Few stars are found in the Hertzsprung gap between point 4 and point 5 , where the surface convection zone has grown deep enough to bring nuclear processed material to the surface in the first dredge-up. Adapted from Iben (1967).

Relative to silicon, the total elemental lithium is even 140 times less abundantin the solar photosphere than on Earth or inmeteorites, as recorded by the solar spectrum. Since the Li/Si element abundance ratio in the meteorites should also have entered the Sun when it formed, one concludes that the Sun must be destroying its initial lithium supply as it ages. This occurs at the base of the surface convection zone of the Sun. The bottom of that zone lies at a depth that is about 1/4 of the Sun s radius. Here the high temperatures (a few million degrees kelvin, MK) that solar-surface nuclei experience is hot enough to destroy lithium, especially 6Li, by nuclear interactions with protons (6Li + p —3He + 4He). Those proton-induced nuclear reactions destroy 6Li much more readily than they do 7Li because the quantum probabilities of the reaction are greater than for 7Li. As a result, to deplete elemental lithium by a factor of 140 in the Sun... 

A subclass of the carbonaceous chondrites that uniquely stands out among all others is the Cl (or Cl) carbonaceous chondrite. These chondrites possess the highest proportional abundances of the highly volatile and moderately volatile elements, are chondrule free, and they possess compositions that match that of the solar photosphere when compared on a silicon-based scale (see also Chapter 2.01). The photosphere is the top of the Sun s outer convection zone, which can be thought of as an analogue to the Sun s surface. The Sun s photospheric layer emits visible light and hence its composition can be measured spectroscopically. This, plus the fact that the Sun contains >99.9% of the solar system s mass, makes the compositional match with Cl carbonaceous chondrites seem all that more significant. 

Solar beating of tropical oceans warms the surface water, promoting evaporation. Where the equatorial surface waters are warmest and the northeast and southeast trade winds meet, a band of cirrostratus and cirrus clouds spreads out from convective precipitation regions. This area is known as the Intertropical Convergence Zone. 

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