The main body of our solar system is the sun. The other objects we have discussed so far, put together, would fit within the sun and not even be noticed. We know the sun to be a star much like many of the other stars in the night sky. The exterior of the sun has been studied since Galileo viewed the sun through his telescope.

The exterior portions of the sun can be separated into three atmospheres. The lowest atmosphere is referred to as the photosphere. The photosphere of the sun is about 300 kilometers thick. It is the layer of the sun considered to be the surface of the sun. It is not a solid surface, but a gaseous surface. It is the portion of the sun that we see when we view the sun in the visible portion of the spectrum. The temperature of the photosphere is roughly 5800 Kelvin. The deepest portion of the photosphere is the source of a continuous spectrum. (Recall Kirchoff's laws dictate that a hot opaque object will give off a continuous spectrum.)

The second layer of the sun is called the chromosphere. This layer is on the order of 2000 kilometers thick with a temperature ranging from 4000 Kelvin at the bottom to around 25,000 Kelvin at the top. Portions of the chromosphere shoot to as much as 10,000 kilometers above the photosphere in spikes of gas called spicules. The chromosphere is thinner in density than the photosphere.

The outermost layer of the sun is the corona. The corona is a very tenuous atmosphere which extends a very large distance from the sun. The temperature of the corona can exceed one million Kelvin. It would seem an object with this temperature should yield a large intensity of light (recall the Stephan-Boltzmann law). However, since the corona is such a thin gas, the amount of light is greatly reduced. Therefore, the corona is only visible when the rest of the sun is eclipsed by the moon or artificially eclipsed by instrumentation. The corona may also be viewed using film sensitive to x-rays. (Recall Wien's law says the higher the temperature, the shorter the peak wavelength of the light given off.)

The surface of the sun is often spotted. These "sunspots" are regions on the surface of the sun with a lower temperature than the rest of the sun. The temperature of sunspots is on the order of 4000 Kelvin. Since their temperature is lower than the rest of the photosphere, they give off less light than the photosphere and appear dark.

Sunspots have been studied for many years (in fact, the Chinese reported sunspots in the fifth century B.C.). Astronomers have discovered that there is a pattern to the number and position of sunspots. The number of sunspots seems to cycle over a time period of roughly 11 years. The number of spots slowly rises at the beginning of the cycle until reaching a maximum and then decreases for the rest of the cycle. This process then repeats itself. In addition, the sunspots tend to begin at larger latitudes from the solar equator. As the 11 year cycle progresses the placement of sunspots becomes closer and closer to the equator. It should be pointed out however, that any single sunspot group only lasts a couple of months.

The source of sunspots is not well understood. However sunspots form in groups of two or larger, and this has given us a clue as to their source. Studying the magnetic field of sunspots has indicated that these pairs are formed because sunspots come in magnetic north and magnetic south pairs. In fact, sunspots above the equator of the sun may show magnetic polarity with the north sunspot on the right and the south sunspot on the left. In this case, the polarity below the equator will be exactly the opposite. Astronomers have also found that both these polarities reverse themselves when a new 11 years cycle begins. This makes the 11 year cycle a 22 year cycle. After 22 years, the magnetic polarity has switched to the polarity that existed 22 years earlier.

This magnetic trend indicates that the sunspots are somehow linked to the magnetic field of the sun. Astronomers believe that the magnetic field of the sun becomes twisted by the sun's differential rotation. (The equator of the sun rotates faster than the polar regions of the sun.) These twisted, or knotted regions of the magnetic field hinder solar convection and cause these solar regions to be cooler.

The characteristics of the interior of the sun are more difficult to determine. Astronomers use the laws of physics to model the sun's characteristics from the surface to the center. If the observed characteristics of the sun match the characteristics predicted by the model, then we consider the model to be good. Let us begin at the very center of the sun.

We've known that the sun was our main source of energy for centuries. But what is the source of energy for the sun? It was not until the beginning of this century that a reasonable answer to this question became known. It was the discovery of x-rays combined with Einstein's theory of relativity that led us to the conclusion that the sun was powered by nuclear fusion. By this model, hydrogen nuclei at the core of the sun fuse together to form a helium nucleus. The mass of the helium nucleus is less than the mass of the four hydrogen nuclei required to make helium. Therefore, the missing mass is converted to energy.

Once produced, how does this energy reach Earth? There are three methods by which energy is transported from one place to another. One of these methods is conduction. Conduction is not an efficient energy transport method in fluids (gases and liquids). An example of conduction would be to place an iron skillet onto a stove. Although the bottom of the skillet is in contact with the heating element, the handle of the skillet will eventually get hot. Conduction does not take place in the sun.

A second form of energy transport is radiation. With radiation, energy is transported from one place to another by photons (particles of light). It should be apparent that this is the method by which energy goes from the surface of the sun, through the vacuum of space, to Earth. In addition, radiation is the transport method from the core of the sun through the first 80 percent of its radius. However, even though this radiation is traveling at the speed of light, it takes about a million years to reach the outer regions of the sun. This is because the light particle (photon) is bouncing off of electrons in the interior of the sun until it finally makes its way to the last 20 percent of the solar radius.

The last 20 percent of the solar radius uses the final form of energy transport, convection. This region is referred to as the convective zone. Convection is restricted to transport through fluids (liquids and gases). You may have experienced this form of energy transport when you noticed the basement of a house was cooler than the upstairs. This was because the hotter gases rose while the cooler gases sank. Another example is boiling water. The water at the bottom of a pan is heated by the bottom of the pan. This heated water then rises to the top and is replaced by cooler water flowing from the top to the bottom, the process then repeats itself.

This system of rising and falling gases is taking place in the convective zone of the sun. The surface of the sun shows signs of this convection in the form of granulation. Granulation is seen by the mottled surface of the sun. The granules can be seen in the figure above. The darker edges of these "cells" are cooler gases sinking away from the sun's surface. The brighter centers of these "cells" are the hot gases rising from the interior. Granulation takes place on a larger scale through the photosphere. These regions are referred to as super granules.

This page was last updated on 10/18/04.