THE EDGE OF THE SUN — MENZEL 221 



earth's atmosphere enhances convection because the condensation of 

 water vapor heats the gas, putting back into it the calories that went 

 to cool the air when the water evaporated. 



No water exists in the sun's atmosphere, of course. But Unsold 

 showed some time ago that a completely analogous process takes place 

 there — ionization of hydrogen. Since hydrogen is by far the most 

 abundant constituent of the sun's atmosphere, and since most of this 

 gas is completely ionized at the lower levels, a large bubble, cooling 

 as it rises, would eventually become neutral. As the electrons recom- 

 bine with their ions, they release a large quantity of energy which 

 can be used for heating the rising gas. The convection can be extreme. 



The shining surface of the sun, the part not affected by sunspots, is 

 by no means uniformly bright. It contains many bright flecks on a 

 slightly darker background, spots that look like foam-capped waves 

 on a stormy sea. We have generally referred to this structure as 

 "granulation." The granules are 300 miles or so across on the average, 

 with considerable variation in size. In the neighborhood of sunspots 

 they are much coarser, if indeed the bright structure we can see can 

 properly be called granules. Near the limb of the sun, where we are 

 looking tangentially down to the solar atmosphere and hence see the 

 higher layers, large brightened patches with a veined structure take 

 the place of the relatively simple granulation. These are the so-called 

 faculae. 



Astronomers were first inclined to regard this granular pattern as 

 arising in some sort of convection. But this theory gradually gave 

 way to the idea that the white spots were clouds of a solid material, 

 floating in the gas and appearing more luminous because of their 

 higher emissivity, like a gas mantle heated to incandescence. We 

 had to abandon this second hypothesis and revert to the earlier view 

 when we found that the solar temperature was too high to permit 

 the presence of either liquid or solid matter. 



Astronomers have long discussed the mode employed by the sun 

 for the escape of energy from the deep interior. Initially, they took 

 for granted that convection was the whole mechanism and the early 

 theories of stellar constitution were devised on that hypothesis. But 

 around the turn of the century, Schuster, K. Schwarzchild, and others 

 showed that radiative processes would dominate, with the atmosphere 

 in stable equilibrium. But they reckoned without knowledge of the 

 effect that hydrogen ionization would have on the equilibrium, and 

 it remained for Unsold to demonstrate the importance of the hydro- 

 gen convective zone in stellar structure. The turbulent convection 

 serves to bring the hotter layers closer to the surface than they would 

 have been in simple radiative equilibrium. We do not know pre- 

 cisely how deep the outer convective layer extends, because small 

 changes in the initial assumptions can lead to great differences far 



