Structure Of The Sun

Different layers of the sun are described by those which we cannot directly view and those which comprise the atmosphere. On a cursory examination of the Sun, one would find it to be a pretty, yellow ball hanging in the sky. Since we cannot (and I really mean CANNOT) look directly at the Sun without professional protection lest we permanently damage our eyes in a fraction of a second, we rely on photographs to discern some of the Sun's secrets. On the surface, it really looks like a yellow ball. What is surprising is the discovery that the Sun is made entirely of gas, all the way from the most exterior solar corona to the exotic and super-dense core. We know that the Sun is made of gas based on how we study it. All gases increase in temperature with an increase in the crowding of the gas particles. The more spread out the gas particles are from each other, the more cool the temperature will be. Any photograph of the Sun will reveal that the edges (termed "limbs" by astronomers) are darker than the middle of the picture.

The photograph to your right demonstrates this limb-darkening. As we look at the edge of the solar disk in the picture, it is obvious that the edges are darker in color than the rest of the picture. We are seeing the gas layer of the Sun that exists outside the visible photosphere layer. These gases are more spread out, cooler in temperature, and therefore darker in color. From this knowledge alone, it can be inferred that the Sun will get more hot as one proceeds on an imaginary journey from the outer regions in toward the center. Although the material is always gaseous, the gravitational pressure upon the gases farther and farther into the interior brings about a steady increase in temperature until you reach a region inside where that temperature exceeds 10,000,000 K.

Additionally, you can see that this picture shows the presence of dark spots known as sunspots. You will learn more about these interesting features later in this unit. For now, the sunspot is a region of unusual magnetic activity that causes the area to be cooler in temperature by as much as 2000 K, and yet far more energetic that the surrounding regions.

Finally, the Sun is not quite the smooth yellow ball it appears to be to the untrained eye. Close examination, even of this simple picture, reveals a pattern of circle called granulation. This granulation is the result of convection of the solar material, much like the convection of a pot of boiling water. The Sun literally is boiling upward from its interior and spewing out at the "surface." Sometimes, material may be ejected from the Sun, as solar flares, while other gases remain trapped in the beautiful corona. Unfortunately, the corona is only visible during a total solar eclipse, and very people have ever seen it.

The image to the left depicts the layers of the Sun, and a description of each is given below:

Core = The site of thermonuclear fusion, which is the engine of the sun. Temperatures here approach 15,000,000 K and pressures exceed 250,000,000,000 atmospheres. The core makes up 50% of the sun's mass but only 1/64th of the Sun's volume. All of the energy that emits from the Sun is produced in the core.

Radiative Zone = The region where energy from the core begins its journey outward, but the material is too dense and hot for heat transfer, and therefore the energy radiates out by creating alternating parallel magnetic and electrical fields, thus moving outward as electromagnetic radiation.

Convective Zone = A region of less dense material, the energy is primarily carried toward the surface by heat convection currrents which carry hot gases toward the surface before they cool and fall back inward. A really cool picture of convection at the Sun's surface is seen below. Hot gases from the interior have welled up and bulged to the surface where the heated gas encounters the cool region of space above. The cooled gas sinks back into the Sun, and the appearance of the cooler gaas changes in color from the hot white gas in the center of each little (size of the USA on this scale) convection cell to a brown color that outlines each convection cell.

Photosphere = A shallow region of 200 km thickness where the sun's energy escapes as visible light. This is the area we directly see, and it is always the coolest region of any star. Sunspots are visible on this layer, and these objects and their cyclical appearance are areas of much intense study.

Chromosphere = A region where spicules of hot gas erupt above the photosphere and where temperatures climb back above 10,000 K. This area demonstrates the strongest hydrogen emission spectra.

Corona = The site of solar wind blowing out from the sun, where the out-flowing energy energizes the thin gases to 2,000,000 K, by unknown mechanisms.

I have included this drawing of the Sun's layers simply because I thought it looked cool when I was developing this course.







This concludes your examination of the structure of the Sun. I am not even attempting to teach some of the myriad details surrounding our nearest star, but only want to help you become acquainted a little better with the Sun.

From here, please move forward to Thermonuclear Fusion to learn how the Sun generates its life-giving light and heat, or you may return to the Sun Introduction, or to the Syllabus.

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