How the Sun Works

The Sun Is Also a Star

The sun is a star, just like the other stars we see at night. The difference is distance -- the other stars we see are light-years away, while our sun is only about 8 light minutes away -- many thousands of times closer.

­Officially, the sun is classified as a G2 type star, based on its temperature and the wavelengths or spectrum of light that it emits. There are lots of G2s out there, and Earth's sun is merely one of billions of st­ars that orbit the center of our galaxy, made up of the same substance and components.

The sun is composed of gas. It has no solid surface. However, it still has a defined structure. The three major structural areas of the sun are shown in the upper half of Figure 1. They include:

Above the surface of the sun is its atmosphere, which consists of three parts, shown in the lower half of Figure 1:

­All of the major features of the sun can be explained by the nuclear reactions that produce its energy, by the suns magnetic fields resulting from the movements of the gas and by its immense gravity.

It begins at the core.

How Hot is the Sun?

The core starts from the center and extends outward to encompass 25 percent of the sun's radius. Its temperature is greater than 15 million degrees Kelvin [source: Montana]. At the core, gravity pulls all of the mass inward and creates an intense pressure. The pressure is high enough to force atoms of hydrogen to come together in nuclear fusion reactions -- something we try to emulate here on Earth. Two atoms of hydrogen are combined to create helium-4 and energy in several steps:

These reactions account for 85 percent of the sun's energy. The remaining 15 percent comes from the following reactions:

­The helium-4 atoms are less massive than the two hydrogen atoms that started the process, so the difference in mass is converted to energy as described by Einstein's theory of relativity (E=mc²). The energy is emitted in various forms of light: ultraviolet light, X-rays, visible light, infrared, microwaves and radio waves.

The sun also emits energized particles (neutrinos, protons) that make up the solar wind. This energy strikes Earth, where it warms the planet, drives our weather and provides energy for life. We aren't harmed by most of the UV radiation or solar wind because the Earth's atmosphere protects us.

After covering the core, it's time to extend outward in the sun's structure. Next up are the radiative and convective zones.

The radiative zone extends outward from the core, accounting for 45 percent of the sun's radius. In this zone, the energy from the core is carried outward by photons, or light units. As one photon is made, it travels about 1 micron (1 millionth of a meter) before being absorbed by a gas molecule. Upon absorption, the gas molecule is heated and re-emits another photon of the same wavelength. The re-emitted photon travels another micron before being absorbed by another gas molecule and the cycle repeats itself; each interaction between photon and gas molecule takes time. Approximately 10²⁵ absorptions and re-emissions take place in this zone before a photon reaches the surface, so there is a significant time delay between a photon made in the core and one that reaches the surface.

The convective zone, which is the final 30 percent of the sun's radius, is dominated by convection currents that carry the energy outward to the surface. These convection currents are rising movements of hot gas next to falling movements of cool gas, and it looks kind of like glitter in a simmering pot of water. The convection currents carry photons outward to the surface faster than the radiative transfer that occurs in the core and radiative zone. With so many interactions occurring between photons and gas molecules in the radiative and convection zones, it takes a photon approximately 100,000 to 200,000 years to reach the surface.

We've finally made our way to the surface. Let's trace through the atmosphere next. Just like Earth, the sun boasts an atmosphere. However, the sun's is composed of 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. "The surface of the sun" typically refers to the photosphere, at least in lay terms. It is 180-240 miles (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 600 miles (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 1,200 miles (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 3,000 miles (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 final layer of the sun and extends several million miles or 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.

T­hrough telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's take a look at three of them: sunspots, solar prominences and solar flares.

Of course, the spheres are graced with interesting features and activity. We'll take a look at them here.

Dark, cool areas called sunspots appear on the photosphere. Sunspots always appear in pairs and are intense magnetic fields (about 5,000 times greater than the Earth's magnetic field) that break through the surface. Field lines leave through one sunspot and re-enter through the other one. The magnetic field is caused by movements of gases in the sun's interior.

Sunspot activity occurs as part of an 11-year cycle called the solar cycle where there are periods of maximum and minimum activity.

It is not known what causes this 11-year cycle, but two hypotheses have been proposed:

­Occasionally, clouds of gases from the chromosphere will rise and orient themselves along the magnetic lines from sunspot pairs. These arches of gas are called solar prominences.

Prominences can last two to three months and can extend 30,000 miles (50,000 kilometers) or more above the sun's surface. Up­on reaching this height, they can erupt for a few minutes to hours and send large amounts of material racing through the corona and outward into space at 600 miles per second (1,000 kilometers per second); these eruptions are called coronal mass ejections.

Sometimes in complex sunspot groups, abrupt, violent explosions from the sun occur. These are called solar flares.

Solar flares are thought to be caused by sudden magnetic field changes in areas where the sun's magnetic field is concentrated. They're accompanied by the release of gas, electrons, visible light, ultraviolet light and X-rays. When this radiation and these particles reach the Earth's magnetic field, they interact with it at the poles to produce the auroras (borealis and australis). Solar flares can also disrupt communications, satellites, navigation systems and even power grids. The radiation and particles ionize the atmosphere and prevent the movement of radio waves between satellites and the ground or between the ground and the ground. The ionized particles in the atmosphere can induce electric currents in power lines and cause power surges. These power surges can overload a power grid and cause blackouts. You can learn more about solar flares by reading Could an extremely powerful solar flare destroy all the electronics on Earth?

All of this activity requires energy, which is in limited supply. Eventually, the sun will run out of fuel.

­T­he sun has been shining for about 4.5 billion years [source: Berkeley]. The size of the sun is a balance between the outward pressure made by the release of energy from nuclear fusion and the inward pull of gravity. Over its 4.5 billion years of life, the sun's radius has gotten about 6 percent bigger [source: Berkeley]. It has enough hydrogen fuel to "burn" for about 10 billion years, meaning it has a bit over 5 billion years left, and during this time it will continue to expand at the same rate [source: Berkeley].

When the core runs out of hydrogen fuel, it will contract under the weight of gravity; however, some hydrogen fusion will occur in the upper layers. As the core contracts, it heats up and this heats the upper layers causing them to expand. As the outer layers expand, the radius of the sun will increase and it will become a red giant, an elderly star.

The radius of the red giant sun will be 100 times what it is now, lying just beyond the Earth's orbit, so the Earth will plunge into the core of the red giant sun and be vaporized. At some point after this, the core will become hot enough to cause the helium to fuse into carbon.

When the helium fuel has exhausted, the core will expand and cool. The upper layers will expand and eject material.

Finally, the core will cool into a white dwarf.

Eventually, it will further cool into a nearly invisible black dwarf. This entire process will take a few billion years.

So for the next several billion years, humanity is safe -- in terms of the sun's existence, at least. Other debacles are anybody's guess.

For more information about the sun and related topics, look over the links on the next page.