The Sun

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay

In many ways the Sun is an ideal star to have nearby. Its steady energy output creates a stable environment for life on earth. For the astronomer, the Sun is as typical an example of a star as one could hope for.

The Outer Layers of the Sun

The Sun has three visible layers. The bright surface we can see is the innermost layer, the photosphere. The photosphere looks sharply-defined, but actually the Sun has no solid surface. Deep in the Sun electrons are torn free from atoms and these electrons absorb photons of light before they can get very far. We see down into the Sun to the level where its gases become opaque, a distance of about 200 miles (300 km). This distance is negligible compared to the Sun's diameter of 864,000 miles (1.3 million km), so the Sun appears to have a very sharp outline. The surface temperature of the Sun is about 5,500 c (9000 f).

Above the photosphere is the chromosphere, about 5000 miles (8000 km) deep. This layer emits faint light of its own, mostly bright red light emitted by hydrogen atoms. During a total eclipse of the Sun, when the photosphere is covered by the moon, the chromosphere becomes briefly visible as a thin red rim of light. Thin and faint as the chromosphere is, in one way it is the most important layer of the Sun or any other star. As light from the photosphere shines through the chromosphere, the relatively cool gases of the chromosphere absorb some of the light. The absorption of light by atoms in the chromosphere makes spectroscopy possible and enables astronomers to determine the chemical composition of the Sun and other stars.

Also visible during a total eclipse is the outermost layer of the Sun, the corona, which appears as a ghostly white halo reaching out 500,000 or more miles (800,000 km) until it fades into invisibility. Although very thin, the coronal gases are also very hot, over 1,000,000 degrees. Nobody is quite sure yet why the corona is so hot. Acceleration of particles by the Sun's magnetic field, and compression of the coronal gases by shock waves from disturbances in the photosphere may play a role in heating the corona.

The Surface of the Sun

The Sun's visible surface shows a variety of features. Most conspicuous are the sunspots, great dark disturbances which can be many times larger than the earth. Observations of sunspots from day to day show that the Sun rotates. The Sun is gaseous, not solid, and does not rotate uniformly. The equatorial regions rotate in about 28 days, and middle latitudes in about 30 days. Small Sunspots persist for a week or so, but large ones may last through several rotations of the Sun.

In detail, Sunspots consist of a dark core, the umbra, surrounded by a lighter halo of radiating dark streaks, the penumbra. Sunspots appear dark only because they are cooler and less luminous than the average surface of the Sun. In reality, Sunspots are hotter than many stars and give off appreciable light of their own. Sunspots come and go in a cycle of about 11 years. The first spots in a cycle appear up to 45 degrees north or south of the Sun's equator (never in polar latitudes) and newer spots appear progressively closer to the equator. The maximum in the Sunspot cycle occurs when spots occur 20-30 degrees from the equator. The last spots appear about 10 degrees from the equator, never on the equator itself. Often the first spots of a new cycle appear before the last spots of an old cycle have disappeared. flares, or intense outbursts of charged particles, are associated with Sunspots.

Another feature of the photosphere is granulation, a polygonal pattern of light patches with dark boundaries. A typical granule is about 300 miles (500 km) across, roughly the size of Texas or France, and lasts a few minutes. Granules are almost certainly convection cells, where hot gases rise in the interior of the granule and cooler gases sink along the boundary. Granules are grouped into a larger pattern, called supergranulation. Supergranules are about 25,000 miles (40,000 km) across.

The picture at right shows typical granules. They are about the size of Texas and last a few seconds.

The picture above, looking more toward the edge of the sun, shows the granules obliquely and conveys a better idea of their three-dimensional structure.

The superb image above shows the granules as billowing humps. One arcsecond on the Sun corresponds to 700 kilometers. The area spanned by the picture is a bit wider than the Earth.

In the chromosphere, other features are visible. There are millions of jets of gas called spicules, each about 10,000 miles (16,000 km) high, and vast prominences, great surges of hot gas that can rise hundreds of thousands of miles from the Sun. Some prominences are erupted outward, others appear to condense in the corona and fall inward. Some take the form of loops, clearly driven by magnetic fields.

Solar Disturbances and the Earth

Solar disturbances of all sorts ebb and flow with the Sunspot cycle, and all are related to disturbances on earth. Prominences and flares in particular emit streams of electrons and protons that reach the earth a day or so after eruption. These particles are trapped in the earth's magnetic field until they build up to a great enough density to break free. Then bursts of these particles strike the magnetic pole regions of the Earth, causing displays of the aurora and generating enormous electric currents in the upper atmosphere that disrupt communications and even damage equipment. Fortunately, since it takes a day for the particles to reach earth, it is possible to monitor the Sun and issue warnings when major disturbances occur.

Why Does the Sun Shine?

The Sun, average star though it is, puts out energy on a staggering scale. The largest energy unit most people are familiar with is the megaton, used in measuring the power of nuclear weapons. A megaton is the energy of a million tons of high explosive. The Sun's energy output is about equal to 90 billion megatons every second. The entire power-generating capacity of the earth equals about 60,000 megatons per year, so in one second the Sun produces over a million years' worth of energy for the earth. If the Sun derived its energy by burning coal, it would take only 18 hours to burn a mass of coal equal to the earth. And the Sun has been doing this for 4.6 billion years.

Where the Sun gets its energy was one of the great scientific problems of the 1800's, because geologists had found evidence the earth was very old, but astronomers and physicists could not find an energy source capable of powering the Sun for such great spans of time. The discovery of nuclear energy about 1900 solved the problem. The Sun derives its energy by nuclear fusion, in which four hydrogen atoms collide, in a fairly complex process, to form a helium nucleus. This process is much the same as takes place in a thermonuclear weapon, but on a vastly greater scale.

If we could tap the Sun's energy directly, our energy problems would be solved. Unfortunately, only a tiny part of this energy reaches the earth. Almost all the energy of the Sun escapes into space, as does the energy of other stars. We see some of this energy as starlight. What solar energy does strike the earth is spread out too broadly to be collectable in large amounts.

The Interior of the Sun

At first glance, it would seem that nothing could be harder to study than the interior of the Sun and other stars. Actually, it is possible to say a good deal about the interior of the Sun. Unlike the earth, the Sun is made of gas. In fact the Sun is very nearly an ideal gas, one in which particles do not interact appreciably with one another. The temperature of the Sun is so high that no molecules can form, and in the deep interior even atoms have been ripped apart into separate particles. An ideal gas follows a simple law in which

Pressure x volume = constant x temperature

The properties of ideal gases, together with a few other principles, allow astronomers to probe the Sun's interior. The pressure at any point must be equal to the weight of the overlying gases. The temperature at any point is governed by the energy produced there, the energy entering and leaving the area, and the ability of the gases to conduct heat. Because stars are nearly ideal gases, it is much easier to arrive at a good knowledge of the interior of the Sun and stars using theoretical calculations than it is for the earth. Ideal gases are much easier to deal with mathematically than solids or liquids are.

The Sun derives its energy by nuclear fusion, in a small central core. Temperatures in the core are estimated at about 15,000,000 C. The pressures necessary to contain such incredibly hot gases are enormous: 25 billion times atmospheric pressure, and this tremendous pressure compresses the Sun's gases until they weigh about 150 grams per cubic centimeter, or more than ten times denser than lead. Nevertheless, the gas is still gas (actually plasma). Deep in the Sun, energy escapes outward by radiation, but near the surface of the Sun convection occurs, and the Sun's heat is transported by rising currents of hot gas. It takes several thousand years for energy to get from the core to the surface of the Sun.

In the diagram above, the left side shows the visible surface of the sun in a wavelength of red light emitted by hydrogen. The right side shows a cross section of the sun, with a small central fusion core (white) and a zone where energy travels outward as high-energy radiation. Near the surface the temperature has dropped enough that convection takes over. There are at least two layers of convection cells, accounting for granules and supergranules.

Above is a diagram of the three visible layers together with the interior. The visible surface, the photosphere, looks sharp as seen from Earth but actually we can see a couple of hundred kilometers into it before free electrons in the gas make it opaque. Dense hot gases like the photosphere emit light of all wavelengths, a so called continuous spectrum, but thin gases like the chromosphere absorb specific wavelengths. Every chemical element absorbs its own specific wavelengths. When the light is spread out into a spectrum, the missing wavelengths show up as thin dark lines. This is how we know what the sun and other stars are made of.

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Created 26 March 1998, Last Update 14 December 2009

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