A Primer on How the Sun Works

The sun produces energy as a by-product of nuclear fusion. The forces of fusion, with its massive release of energy, are balanced by the massive gravity to hold together our neighborhood star.

Our neighborhood star continues to be in a "first stage" where it is burning hydrogen and making it into helium. As hydrogen runs out, it will begin burning helium and then will go through heavier and heavier elements until it reaches an "iron stage" where it cannot continue fusion. Consider that as the sun ages, it will increase in radius in its so-called red giant phase, and it will eventually burn earth to a crisp. At the end of life, it will shed its outer layers and emerge as a white dwarf to finish its life. Fusion, fusion and more fusion will continue over billions of years.

Where Do Photons Come From?

Solar energy is produced when photons (produced by the sun) strike semiconductor materials and release electricity. But how exactly are photons produced?

The solar nuclear process creates immense heat, and it's this heat that causes atoms to discharge photons. Temperatures at the core are about 15 million degrees Kelvin (15 million degrees C or 27 million degrees F). Each photon created at the core travels about one micrometer before being absorbed by an adjacent gas molecule. This absorption then causes the heating of the neighboring atom and it re-emits another photon that again travels a short distance before being absorbed by another atom. This process then repeats itself many times over before the photon can finally be emitted to outer space at the sun’s surface. The last 20% of the journey to the surface the energy is transported more by convection than by radiation. It takes a photon approximately 100,000 years or about 1025 absorptions and re-emissions to make the journey from the core to the sun’s surface.

From the surface – the photons then travel through the sun's atmosphere.

The Sun's Atmosphere

Just like Earth, the sun boasts an atmosphere, and like the atmosphere of the earth, there are different layers with different characteristics. In the case of the sun, there are three principal layers: the photosphere, the chromosphere and the corona.

The photosphere is the lowest region of the sun's atmosphere and is the region that we can see. It is 300-400 kilometers wide and has an average temperature of 5,800 degrees Kelvin. It appears granulated or bubbly, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath; each granulation can be 1,000 kilometers wide. As we pass up through the photosphere, the temperature drops and the gases, because they are cooler, do not emit as much light energy. This makes them less opaque to the human eye. Therefore, the outer edge of the photosphere looks dark, an effect called limb darkening that accounts for the clear crisp edge of the sun's surface.

The chromosphere extends above the photosphere to about 2,000 kilometers. The temperature rises across the chromosphere from 4,500 degrees Kelvin to about 10,000 degrees Kelvin. The chromosphere is thought to be heated by convection within the underlying photosphere. As gases churn in the photosphere, they produce shock waves that heat the surrounding gas and send it piercing through the chromosphere in millions of tiny spikes of hot gas called spicules. Each spicule rises to approximately 5,000 kilometers above the photosphere and lasts only a few minutes. Spicules may also follow along magnetic field lines of the sun, which are made by the movements of gases inside the sun.

The corona is the top layer of the sun and extends several million kilometers outward from the other spheres. It can be seen best during a solar eclipse and in X-ray images of the sun. The temperature of the corona averages 2 million degrees Kelvin. Although no one is sure why the corona is so hot, it is thought to be caused by the sun's magnetism. The corona has bright areas (hot) and dark areas called coronal holes. Coronal holes are relatively cool and are thought to be areas where particles of the solar wind escape.

Solar Luminosity

How much solar energy is released by the sun?The total radiant energy emitted from the Sun is a quantity known as solar luminosity. The total luminosity of the Sun, which is emitted in all directions, has been estimated to be 3.8478 x 1026 watts.

Some scientists believe that long-term variations in the solar luminosity may be a better correlate to environmental conditions on Earth such as global warming. Variations in solar luminosity are also of interest to scientists who wish to gain a better understanding of stellar rotation, convection, and magnetism.

Solar Irradiation

From Earth, it is only possible to observe the radiant energy emitted by the Sun in the direction of our planet; this quantity is referred to as the solar irradiance.

Although referred to as the solar constant, this quantity actually has been found to vary since careful measurements started being made in 1978. Variations in the solar irradiance are at a level that can be detected by ground-based astronomical measurements of light. Such variations have been found to be about 0.1% of the average solar irradiance. Starting in 1978, space-based instruments aboard the Nimbus 7 Solar Maximum Mission, and other satellites began making the sort of measurements (reproducible to within a few parts per million each year) that allowed scientists to acquire a better understanding of variations in the total solar irradiance.

Oscillations, which cause variations in the solar irradiance lasting about five minutes, arise from the action of resonant waves trapped in the Sun's interior. At any given time, there are tens of millions of frequencies represented by the resonant waves, but only certain oscillations contribute to variations in the solar constant.

Sunspots

Sunspots appear as dark regions on the Sun's surface to observers on Earth. They are formed when the magnetic field lines just below the Sun's surface become twisted, and then poke though the solar photosphere. Sunspots give rise to variations that may last for several days, and sometimes as long as 200 days. They actually correspond to regions of intense magnetic activity where the solar atmosphere is slightly cooler than the surroundings.

Another key observation has been that the largest decreases in total solar irradiance frequently coincide with the formation of newly formed active regions associated with large sunspots, or with rapidly evolving, complex sunspots. Sunspots are especially noteworthy for their 11-year activity cycle.

Solar Cycles

The solar cycle is responsible for variations in the solar irradiance that have a period of about 11 years. This 11-year activity cycle of sunspot frequency is actually half of a 22-year magnetic cycle, which arises from the reversal of the poles of the Sun's magnetic field. From one activity cycle to the next, the north magnetic pole becomes the south magnetic pole, and vice versa.

Solar luminosity has been found to achieve a maximum value at the very time that sunspot activity is highest during the 11-year sunspot cycle. Scientists have confirmed the length of the solar cycle by examining tree rings for variations in deuterium-to-hydrogen ratios.

How the Sun Affects Global Warming

While humanity has been burning fossil fuels at an increasing rate and increasing the amount of greenhouse gases … some scientists believe that variations in the output of energy produced by the sun, which produces vastly more energy, are a primary cause of global warming. Some scientists have even achieved success in predicting weather patterns – weeks in advance based on solar observations.

Specifically, Scientists have speculated that long-term solar irradiance variations might contribute to global warming over decades or hundreds of years. More recently, there has been speculation that changes in total solar irradiation have amplified the greenhouse effect, i.e., the retention of solar radiation and gradual warming of Earth's atmosphere. Some of these changes, particularly small shifts in the length of the activity cycle, seem to correlate rather closely with climatic conditions in pre- and post industrial times. Whether variations in solar irradiance can account for a substantial fraction of global warming over the past 150 years, however, remains a highly controversial point of discussion.

Back from this page on Sun Information to our lead page on Solar Energy