Online Astronomy eText: The Sun The Structure of the Sun and the Nature of its Surface (supplement to regular textbook discussion of the Sun) This discussion presumes that you have already read the chapters about the Sun in your text. Topics Covered On This Page Gross Properties of the Sun The Structure of the Sun The Physical Nature of the Sun The Atmosphere of the Sun The Nature of the Solar Surface How the Brightness of the Sun's Layers Depends Upon Density and Temperature The "surface" of the SunLayers above the surface provide little light because they are faint.Layers below the surface provide little light because it can't escape.Only a very narrow region (the photosphere) provides all of the light. Two identical blobs of gas give off exactly equal amounts of light.The total light from both of them is twice as much as from either blob.       Now, let's consider the total light emitted by the blobs. Since they are equally bright, the total will be exactly twice the light emitted by one of them. And it doesn't make any difference whether you look at the blobs side by side, or look through one at the other one, or even put them both in the same space, PROVIDING THEY ARE OPTICALLY THIN, so when you look through one you can still clearly see the other one, and the light it is emitting.       Since, if you put two blobs in the same place, they will be twice as bright as a single blob, putting say ten blobs together would give you ten times as much light. And as a general result, making a hot gas denser will make it brighter in direct proportion to the density, so long as it remains optically thin. The more gas you stuff into the emitting region the brighter it will glow. But suppose you put so much gas into the region that you can just barely see through it. In other words, so that it is just barely optically thick. Then if you were to make twice as dense, you would only be able to see half as far, as shown below. A dense blob of gas, which is optically thick.Making it twice as dense, you can only see halfway through it.       The part of the blob which you can see is just as bright as the whole blob was before you made it denser, because there is as much gas in the front half of the blob as there used to be in the whole blob. Of course the back half also has just as much gas and gives off just as much light as the whole blob originally did, but the light from it can't get through the gas in front of it, so it doesn't count. As a result, once a gas is optically thick, making it denser doesn't change its brightness.       The resulting effect of density on brightness is shown below. For optically thin gases (on the left side of the diagram), the brightness increases uniformly with density (producing a straight, sloped line graph), but for optically thick gases (on the right side of the diagram) the brightness is constant, regardless of how dense the gas is. The variation of brightness with density for gases hot enough to glow.For optically thin gases, brightness increases uniformly with density.For optically thick gases, brightness is independent of density.Brightness per square foot is proportional to the fourth power of the temperature,or B / R2 = (constant) x T4.       To see how this works near the surface of the Sun, here is another diagram, similar to the one above, showing the brightness variations with density for three different temperatures: Dependence of brightness on density and temperature.In optically thin regions, density increases uniformly with brightness.In optically thick regions, brightness is independent of density.In all regions, brightness increases with temperature.       The three "curves" represent the brightness increase at three different temperatures: "warm", "hot" and "very hot". The five dots represent the combined effects of density and temperature at five different places -- two in the atmosphere, one at the surface, and two in the interior. Point 1 represents a situation similar to that in the corona -- extremely high temperatures but very low density, and as a result, very low brightness. Point 2 represents a situation similar to that in the chromosphere -- quite a bit cooler but substantially denser, and therefore considerably brighter. Point 3 represents the photosphere -- a region where the gas is relatively cool, but is becoming optically thick and is therefore much brighter than the higher atmospheric regions. Points 4 and 5 represent places deep in and deeper inside the interior, in which brightness increases because temperature increases, but is independent of density.