Online Astronomy eText: The Sun
The Structure of the Sun and the Nature of its Surface Link for sharing this page on Facebook
(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

Gross Properties of the Sun
      The Sun is an immense ball of gases nearly a million miles in diameter, with a mass of more than 300,000 times the mass of the Earth. This happens to be about a thousand times the mass and a thousand times the volume of Jupiter, so the Sun is, like Jupiter, about 30% denser than water. However, unlike Jupiter, which is made of liquid hydrogen and helium, the Sun is entirely gaseous. This difference is caused by the Sun's high temperature, which ranges from a low of 6000 Kelvins (about 10000 Fahrenheit) at the surface to a high of about 15 million Kelvins (almost 30 million Fahrenheit) in the center. These high temperatures tear any molecules or atoms of hydrogen and helium to pieces, and those pieces are much smaller than ordinary atoms, so even at very high densities the materials in the Sun are still gaseous.
      Because of the Sun's high temperature and large size it radiates a tremendous amount of light and to a lesser extent, other kinds of electromagnetic radiation. It gives off approximately 400 trillion trillion watts (4 x 1026 watts) of power, which is created by thermonuclear reactions in its deep interior.
      Despite its immense size, mass and brightness the Sun is actually a fairly average star. There are stars with more than a hundred times the mass of the Sun, and stars with as little as a few percent of its mass. Stars with up to a thousand times the diameter of the Sun, and as little as a hundredth of its diameter. Stars with as much as a few million times its brightness, and as little as one fifty-thousandth of its brightness. And in almost every other respect, whatever properties are discussed for the Sun there are stars with much larger or much smaller values than those which apply to the Sun.
      Despite being a very average star, the Sun is of immense importance to us, because our lives depend upon its light and heat. Without it life on Earth would soon end. In addition, the Sun is very important to astronomers, because being by far the closest star, we can study it in far more detail than any other star.

The Structure of the Sun
      The Sun consists of an atmosphere, which we can look through to the surface, and an interior which is beneath the surface and not directly visible. The surface is not an actual physical surface, but an optical surface similar to the apparent surface created when haze or smog makes it impossible to see distant objects. Just as if you were to drive to where a "haze surface" was you would see perfectly normal air and landscapes all around you, if were standing at the "surface" of the Sun you would be able to look for tens or hundreds of miles in all directions, and see no sign of any surface save for the hazy apparent surface created by the absorption of light by the gases surrounding you. Even if you were to go down into the Sun, although the gas would get thicker and thicker, so that the distance you can see would get smaller and smaller, there would never ever, even in the very center, be any hint of any actual physical surface.
      In the absence of any better way to define the surface, the surface of the Sun is simply defined as the place that you can just barely look down to, before the haziness of the surface prevents you from seeing any further. The region above that is the atmosphere, and the region below it the interior. Given this definition you can see all parts of the atmosphere and the surface, but you cannot see into the interior. It must be studied by techniques other than direct visual observation.
      The interior is divided into three regions. The nuclear core is the innermost 10% or so of the radius and mass: the region where extremely high temperatures and densities cause the thermonuclear fusion of hydrogen into helium, producing the energy that the Sun radiates into space. The energy flows diffuses through the radiative zone (sometimes called the radiative core because it is on the inside) by random motion of photons of light from one place to another. Because the gas is extremely dense in the central parts of the Sun, in some places the photons may only be able to go a fraction of an inch before being bounced in some random direction, but throughout the radiative zone the heat flow is entirely through the motion of photons. In the outer 35% or so of the solar interior, however, there is a convective zone (sometimes called the convective envelope because it is on the outside), in which vertical mixing motions of the gases help carry the heat outwards
      Even though we can't see into the interior, we can still study it in various ways. Theoretical calculations can reveal how the various layers of the interior must interact with each other in order to maintain a stable structure. And sound waves produced by the motions in the convection zone permeate the Sun, causing its surface to vibrate. Studies of the resulting vibrations (helioseismology) can reveal much about the Solar interior, and even rather remarkably, allow us to "see" large-scale disturbances on the side of the Sun which is turned away from us (the Sun rotates, so we can see all of it at one time or another, but of course at any given time half of it is turned away from us).

The Physical Nature of the Sun
      The Sun is a gas all the way from the outer atmosphere to the central core. In the outer atmosphere the gas is extremely rarified (many trillions of times thinner than the air at the surface of the Earth), but as you go downwards the gas gradually becomes denser and denser under the weight of the gases above it. Near the apparent surface of the Sun the gas is still much thinner than the air at the surface of the Earth, but is thick enough that it is beginning to get hard to see through it. And of course by the time you reach the surface, you cannot see any further through the gas (since that is how we define the surface). However, even at the surface the gas is still tens of times thinner than the air at the surface of the Earth, so as described above, you could look in all directions for tens or hundreds of miles without seeing any real surface near you.
      As you proceed downwards into the interior, the gases become steadily denser. A few tens of thousands of miles down, they are as thick as the air you are breathing, and halfway into the interior (a couple of hundred thousand miles down), thousands of times thicker yet; in fact, even denser than water. They do not stop getting denser, however, because long before you reach that level the high temperatures inside the Sun have torn the atoms of which it is made into bare atomic nuclei and free electrons, and such particles are much smaller than ordinary atoms, so they can be squeezed closer and closer together, almost without limit, until the gas is amazingly dense. By the time you reach the center of the Sun, the gas is more than a hundred times denser than water, and yet it could be made nearly ten thousand times denser before it would stop acting like a gas, and begin to act like a liquid.
      By the time that you reach the center of the Sun the extremely high density makes it impossible to see more than a fraction of an inch. All around you the incredibly hot gases would be radiating unbelievable amounts of X- and gamma-radiation (several millions of trillions of watts per square foot), so any object not already completely gaseous would be instantly vaporized and its atoms torn to pieces by the intense radiation.

The Atmosphere of the Sun
      The atmosphere is divided into layers according to its appearance, and the physical conditions within those layers. The lowest portion of the atmosphere, the photosphere (the sphere of light), is the blazing disc that we see when we look at the Sun. It is one of the coolest parts of the Sun, with temperatures ranging from around 4500 Kelvins (inside sunspots) to a little less than 6000 Kelvins (in regions with more normal temperature and appearance). It is a fairly thin region compared to the rest of the Sun (just a few hundred miles thick) made of fairly low density gases (much thinner than the air at the surface of the Earth), but there is enough gas in the photosphere that it can completely block our view of the regions beneath it. As a result the light that we see coming from the Sun comes from the photosphere itself (as discussed below, the outer layers are much fainter than the photosphere, and the inner regions cannot be seen, no matter how bright they might really be). Because it is a fairly thin region and the Sun is quite far away, the photosphere appears to be a very sharp surface, and gives the illusion of an actual surface. For that reason, the photosphere is considered to be both the lowest part of the solar atmosphere, and the "surface" of the Sun.
      The region above the photosphere, the chromosphere (the sphere of color), is usually divided into two regions: a lower region where the temperatures are fairly low (about the same as in the photosphere), and an outer region called the transition zone where temperatures rapidly rise to nearly a million Kelvins. The chromosphere has a different appearance from the photosphere because it is a much thinner gas (in terms of density; in terms of miles, it is thicker than the photosphere). In the photosphere the gases are becoming so dense that we cannot see through them. In such circumstances the radiation coming from the solar interior is at least partially modified before passing outwards into space. This causes the radiation to be continuum radiation (radiation of all colors or wavelengths) with superimposed absorption lines created by the various atoms in the photosphere. In the chromosphere, however, the gases are much thinner, and the radiation consists of line emission. Since the photosphere emits and passes on a continuum radiation of all colors, it appears white (although its actual color is slightly yellowish, because the peak of the Sun's radiation is in the yellow-green portion of the spectrum), but since the chromosphere gives off light of only certain wavelengths it is colored according to the amount of light emitted at those wavelengths. The main radiation of the chromosphere is red and blue line radiation due to hydrogen atoms, which causes the chromosphere to appear reddish, hence it its name.
      The outermost layer if the atmospere, the corona, is most easily viewed from space or during a solar eclipse, because it is extremely rarified (in other words, consists of very thin gases), and despite being extremely hot (more than a million Kelvins) is much fainter than the lower layers and in fact fainter than the scattered sunlight which surrounds the disc of the Sun, under normal circumstances. The radiation of the corona consists of emission lines at specific wavelengths, many of which are in the ultraviolet and X-ray regions of the electromagnetic spectrum, because of its extremely high temperature. Among other radiations emitted, there are emissions by iron atoms which have lost nearly half of their electrons, therefore have a structure completely different from normal iron atoms, and as a result give off radiation in a way completely different from any other kind of atom. At one time it was thought that the odd radiations of these atoms were due to a previously unknown element called coronium, but we now know that the radiations are simply due to ordinary atoms under extraordinary conditions. However, there is an element, helium, which was discovered as a result of chromospheric and coronal emissions, and is named after the Greek Sun god Helios.

The Nature of the Solar Surface
      To understand the nature of the Sun's surface we need to take into account two factors. One is that as we go downwards the amount of light which can escape from any given region gets smaller and smaller until at the "surface", no light can escape at all. The other is that as we go downwards the layers get brighter and brighter at a fairly rapid rate (the reason for this is explained a bit later). As shown in the diagram below, this means that most of the light that escapes the surface of the Sun comes from a fairly narrow region. Layers which are above this region don't contribute much light because they aren't very bright. Layers which are below this region don't contribute much light because their light can't escape.

The "surface" of the Sun
Layers 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.

      If the thickness of the region that most of the light of the Sun comes from was several thousand miles thick it would look a bit fuzzy. But the actual thickness is only a few hundred miles, and although if you were very close to the Sun you could see that the layer is a bit fuzzy, at the Earth's distance of nearly a hundred million miles, the layer looks perfectly sharp, giving the impression of an actual physical surface.
Note that this "surface" is the same as the photosphere, because the photosphere is defined as the apparent surface of the Sun, or the region that the light of the Sun comes from (which is the same thing). The bottom of the light-emitting region is called the base of the photosphere, and the region above that, where the light actually comes from, is the photosphere itself. Once you get out into regions which are essentially transparent and because they are so rarified, give off very little light, you are in the chromosphere or corona.

How the Brightness of the Sun's Layers Depends Upon Density and Temperature
      To understand how the light of the Sun is emitted in a little more detail, we need to define two terms: optically thin and optically thick. A gas is optically thin if you can see right through it. It is optically thick if you can't see through it. So the air between your eyes and this text is optically thin, but if you look off in the distance and see an apparent "surface" caused by haze or smog, the air between you and that apparent surface is optically thick.
      Whether a gas is optically thin or thick depends on its density, and how many miles of it there are. The density of the air in the examples in the previous paragraph was exactly the same, but in one case, you were looking through very little air, and in the other case, miles and miles of air. A thin layer of gas can be optically thin, no matter how dense it is, if it is sufficiently thin (in millimeters or inches). A rarefied gas can be optically thick, no matter how rarefied it is, if there are enough miles of gas.
      The atmosphere of the Sun is the region that you can look right through. That means it is optically thin. But you cannot see into the interior, below the surface. Thatis means that the gas between you and the base of the photosphere is optically thick. And, of course if you were inside the Sun you couldn't see out of the Sun (if you can't see in, you can't see out either), so the gas there (or, at least some sufficiently thick layer of that gas) is optically thick.
      As you will see below, if a gas is optically thick, one thing and one thing only affects its brightness -- namely, its temperature. But if it is optically thin, then both the temperature and the density affect its brightness.
      To see how this works, suppose that we have a small blob of gas (blob #1 in the diagram below). The blob is presumed to be extremely hot, so that it is glowing brightly. Now, let's consider another blob (#2, in the diagram below). This blob is presumed to have exactly the same size and density and composition and temperature as the first blob. As a result, it would be giving off exactly the same amount of 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.