Photo courtesy SOHO consortium. SOHO is a project of international cooperation between the European Space Agency (ESA) and the U.S. National Aeronautics and Space Administration (NASA).

The sun warms our planet every day, provides the light by which we see and is absolutely necessary for life on Earth. In this article, we will examine the fascinating world of our nearest star. We will look at the parts of the sun, the amazing way it makes light and heat, and its major features.

Because we see the sun everyday, we tend to take it completely for granted. But if you think about it, you come up with lots of questions such as:

• If the sun is in the vacuum of space, how does it burn?
• What keeps all that gas from leaking into space?
• How big is the sun?
• Why does it send out solar flares?
• When will it stop burning?
• Is the sun like other stars?

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. The sun is an "average" star, merely one of billions of stars that orbit the center of our galaxy.

The sun has "burned" for more than 4.5 billion years and will continue to do so for several billion more. It is a massive collection of gas, mostly hydrogen and helium. Because it is so massive, it has immense gravity, enough gravitational force to hold all of hydrogen and helium together (and to hold all of the planets in their orbits around the sun!).

The sun does not "burn" like wood burns. Instead the sun is a gigantic nuclear reactor, as you will learn on the following pages...

Parts of the Sun
The sun is made of gas and has no solid surface as Earth does. However, it still has a defined structure. The three major surface areas of the sun are shown in the upper half of Figure 1:

• Core
• Radiative Zone
• Convective Zone

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between the European Space Agency (ESA) and the U.S. National Aeronautics and Space Administration (NASA). Figure 1. Basic overview of the parts of the sun. The flare, sunspots and the prominence are all clipped from actual SOHO images.

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

• Photosphere
• Chromosphere
• Corona - extremely hot outermost layer extending outward from the chromosphere several million miles or kilometers

Sun's Surface
The upper half of the sun consists of three major areas: the core, the radiative zone and the convective zone.

Core
The core starts from the center and extends to 25 percent of the sun's radius. Here, 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. Two atoms of hydrogen are combined to create helium-4 and energy in several steps:

1. Two protons combine to form a deuterium (hydrogen atom with one neutron), a positron (similar to electron, but with a positive charge) and a neutrino
2. A proton and a deuterium atom combine to form a helium-3 atom (two protons with one neutron) and a gamma ray.
3. Two helium-3 atoms combine to form a helium-4 (two protons and two neutrons) and two protons.

1. A helium-3 and a helium-4 combine to form a beryllium-7 (four protons and three neutrons) and a gamma ray.
2. A beryllium-7 captures an electron to become lithium-7 (three protons and four neutrons) and a neutrino.
3. The lithium-7 combines with a proton to form two helium-4 atoms.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 2. Composite image from all of SOHO's instruments. The interior image from Michelson Doppler Imager (MDI) illustrates the rivers of plasma underneath the surface. The surface was imaged with the extreme ultraviolet imaging telescope (EIT) at 304 angstroms. Both images were superimposed on a Large Angle Spectroscopic Coronograph (LASCO) C2 image, which blocks the sun so that it can view the corona. The image shows the range of SOHO's research from the solar interior out to the corona.

Radiative Zone
The radiative zone extends 55 percent of the sun's radius from the core. In this zone, the energy from the core is carried outward by photons. 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.

Convective Zone
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, much like what you can see if you placed 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!

Sun's Atmosphere

Sun Facts Average distance from Earth = 93 million miles (~150 million km) Radius = 418,000 miles (696,000 km) Mass = 1.99 x 1030 kg (330,000 Earth masses) Makeup (by mass) = 74 percent hydrogen, 25 percent helium, 1 percent other elements Average temperature = 5,800 degrees Kelvin (surface), 15.5 million degrees Kelvin (core) Average density = 1.41 grams per cm3 Rotational period = 25 days (center) to 35 days (poles) Magnitude = -26.8 (apparent), +4.8 (absolute) Apparent magnitude refers to a star's brightness in the sky from our vantage point on Earth. Absolute magnitude is the star's true brightness if all of the stars were the same distance from Earth. The lower the number, the brighter the star. Distance from center of Milky Way = 25,000 light-years Orbital speed and period = 138 miles per second (230 kilometers per second) and 200 million years

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

• Photosphere
• Chromosphere
• Corona - extremely hot outermost layer extending outward from the chromosphere several million miles or kilometers

Photosphere
The photosphere is the lowest region of the sun's atmosphere and is the region that can be seen from Earth. It is 180-240 miles or 300-400 km wide and has an average temperature of 5,800 degrees Kelvin. It appears bubbly or granulated, much like the surface of a simmering pot of water. The bumps are the upper surfaces of the convection current cells beneath and each granulation can be 600 miles (1,000 km) 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. 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.

Chromosphere
The chromosphere lies above the photosphere to about 1,200 miles or 2,000 km. 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 or 5,000 km 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.

Corona
The corona is the final layer of the sun and extends several million miles or kilometers outward from the photosphere. 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.

Sunspots, Solar Prominences and Solar Flares
Through telescope images we can see several interesting features on the sun that can have effects here on Earth. Let's look at sunspots, solar prominences and solar flares.

Sunspots
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; we are currently in a solar maximum time (Figure 3).

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 3. The sun's 11-year solar cycle as reflected by the number of sunspots recorded to date and projected (dotted line). Selected EIT 195 angstrom (green) and MDI magnetogram (gray) images are shown. In this cycle, the sun undergoes a period of activity (solar maximum) followed by a period of quiet (solar minimum). The rising level can be clearly seen in the comparison of EIT and MDI images.

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

• Uneven rotation of the sun distorts and twists magnetic field lines in the interior. The twisted field lines break through the surface forming sunspot pairs. Eventually, the field lines break apart and sunspot activity decreases. The cycle starts again.
• Huge tubes of gas circle the sun's interior at high latitudes and begin to move toward the equator. When they roll against each other, they form spots. When they reach the equator, they break up and sunspots decline.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 4. Large eruptive solar prominence in helium-2 image at 304 angstroms with an image of Earth added for size comparison. This prominence on July 24, 1999 was particularly large and looping, extending over 35 Earths out from the sun. Erupting prominences, when directed toward Earth, can affect communications, navigation systems,and even power grids, while producing auroras visible in the night sky.

Solar Flares
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. Solar flares are accompanied by the release of gas, electrons, visible light, ultraviolet light and X-rays. When this radiation and particles reach the Earth's magnetic field, they interact with it at the poles to produce the auroras (borealis, australis) as shown below (Figure 5). 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.

Photo courtesy SOHO consortium. SOHO is a project of international cooperation between ESA and NASA. Figure 5. The sun's magnetic field and releases of plasma directly affect the Earth and the rest of the solar system. Solar wind shapes the Earth's magnetosphere, and magnetic storms are illustrated here as approaching Earth. These storms, which occur frequently, can disrupt communications and navigational equipment, damage satellites and even cause blackouts. The white lines represent the solar wind; the purple line is the bow shock line; and the blue lines surrounding Earth represent its protective magnetosphere. The magnetic cloud of plasma can extend to 30 million miles or 50 million km wide by the time it reaches Earth.

Fate of the Sun
The sun has been shining for about 4.5 billion years. It has enough hydrogen fuel to "burn" for about 10 billion years. 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. 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. The radius of the red giant sun will be 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 and then eventually into a black dwarf. This entire process will take a few billion years.

As you can see, our sun is quite complex and interesting, and now you know more about how it produces the light and heat that all life on Earth depends on.