The Solar Furnace

The spectacular nuclear furnace we call the Sun provides the energy that drives our planet's climate system. Let's take a look at the Sun (not literally... we don't want to go blind!). We'll examine the structure of our neighborhood star and investigate the means by which it generates energy and radiates it outward into space.


The Sun is a gigantic sphere of gas (or more precisely, plasma - a state of matter similar to a gas, but with an electrical charge due to separation of electrons from their parent atoms). The properties of the Sun's plasma are not uniform throughout its interior, so we recognize several distinct regions within (and around) the Sun. At the center of the Sun is its core, where nuclear fusion generates energy by converting hydrogen into helium. The core is surrounded by a layer called the radiation zone, which in turn is surrounded by the convection zone. We'll describe each of these regions in detail shortly. At the top of the convection zone is the photosphere, the visible "surface" of the Sun. You may be surprised to learn that the Sun also has an atmosphere! The layer of the Sun's atmosphere immediately above the "surface" is called the chromosphere, while our star's upper atmosphere is known as the corona.

Regions of the Sun

This illustration shows the major regions of the Sun. Moving outward from the center, the regions of the Sun's interior are the core, radiation zone, and convection zone. The photosphere, which is the visible surface of the Sun, is the boundary between the solar interior and the Sun's atmosphere. The atmosphere of the Sun includes the thin chromosphere and the extended corona.
Credit: Astronomical Illustrations and Space Art ( by Fahad Sulehria.

The Sun generates energy in its core via a type of nuclear reaction known as nuclear fusion. Basically, the tremendous heat and pressure at the heart of the Sun causes the nuclei of several hydrogen atoms to fuse together to form helium atoms. When this happens, a relatively small portion (less than 1%) of the mass of the atoms is converted into energy. Although very little mass is lost, a large amount of energy is generated; for the conversion of mass to energy proceeds in accordance with Einstein's famous equation, E = mc2, in which c, the speed of light, is a very, very big number!

It is important to realize that this nuclear fusion process is not "burning" in the conventional sense we are used to. Normal combustion, whether in a flame or a conventional explosion (such as when a stick of dynamite blows up), is a type of chemical reaction that breaks or creates molecular bonds between atoms, but does not alter the atoms themselves. Nuclear reactions (including nuclear fusion), by contrast, change the atoms themselves by altering the atomic nuclei, which are held together by forces that are much, much stronger than those involved in molecular bonds. The quantities of energy involved with nuclear reactions are therefore much, much larger than those involved in normal burning. We're talking about the forces associated with the most powerful types of nuclear bombs humanity has so far produced.

The core of the Sun, where this nuclear fusion process takes place, has a temperature around 15 million degrees Celsius! The energy thus created actually takes many years to work its way upward through the different layers of the Sun. Eventually it reaches the visible "surface" of the Sun (the photosphere), which is relatively cool (slightly less than 6,000° C). From there is hurtles off into space. Slightly more than 8 minutes later, a small portion of this light reaches Earth, where it heats and illuminates our planet.

An In-depth Look at Energy Generation in the Sun

The Sun generates energy via a type of nuclear reaction called nuclear fusion. There are two major classes of nuclear reactions: nuclear fission and nuclear fusion. Both have been used to produce weapons here on Earth; the bombs dropped on Japan by the United States near the end of World War II were fission bombs, while the more powerful "H-bombs" (or "Hydrogen bombs") which were first developed during the Cold War in the mid-1950s employ a nuclear fusion reaction. Nuclear power plants rely on fission to generate energy. Scientists have long sought to harness nuclear fusion for power generation, but as yet have been unable to do so.

Both types of nuclear reactions involve the creation of new atomic nuclei from existing atoms. During fission reactions, a heavy nucleus (for example, Uranium) splits to form two lighter nuclei, releasing energy in the process. During fusion reactions, on the other hand, two lighter nuclei (such as hydrogen) combine (or "fuse" together) to form a heavier nucleus (such as helium), again releasing energy in the process. In each type of nuclear reaction, some matter is converted into energy. Einstein's famous equation, E = mc2, can be used to calculate the amount of energy produced when matter is converted into energy. Since c, the speed of light, is a very, very large number, even a tiny amount of matter can generate huge quantities of energy. For example, if we were to convert a single gram of matter (about the mass of a penny) entirely into energy, we would produce 25 million kilowatts-hours of energy; enough to run a typical (500 gigawatt) modern electrical power plant for several days.

There are many different types of possible nuclear fusion reactions. In our Sun, the predominant fusion reaction is called the proton-proton chain. In the proton-proton chain, through a series of steps, four protons (hydrogen nuclei) combine to form one helium nucleus (consisting of two protons and two neutrons). Each helium nucleus has a mass that is slightly less (by about 0.7%) than the combined masses of the four hydrogen nuclei. In other words, every kilogram of hydrogen becomes 993 grams of helium plus a whole lot of energy. This energy is mostly in the form of X-ray and gamma ray photons.

the proton-proton chain

The Sun generates energy via a type of nuclear fusion reaction called the "proton-proton chain". Through a series of steps, four hydrogen nuclei (protons) are converted into a single helium nucleus (which contains two protons and two neutrons). Neutrons are slightly less massive than protons, so the helium nucleus is slightly less massive than the four hydrogen nuclei. The "missing" mass is converted into energy in the form of gamma ray photons. This fusion process also produces two odd types of subatomic particles, neutrinos and positrons (the anti-particle of the electron).
Credit: Artwork by Randy Russell

Each second the Sun converts about 600 million tons of hydrogen (8.9 ×1037 protons) into helium, releasing 3.83 x 1023 kilowatts of energy. The fusion process transforms 4.26 million tons of matter (equivalent to the mass of about 60 million people) per second into energy. How much energy is generated? It's the equivalent of exploding 90 billion megatons of TNT or roughly 10 billion hydrogen bombs. In less violent terms, if we could somehow capture all of Sun's energy output for one second, it would supply Earth's energy needs (at the current rate of use) for 500,000 years.

The core is the only place in the Sun where this nuclear fusion process occurs. The incredibly energetic photons immediately begin working their way outwards towards the the Sun's surface. Given the radius of the Sun (696 thousand km) and the speed of light (almost 300 thousand km/sec), you might think that these photons reach the surface of the Sun in a matter of seconds. In actuality, scientists estimate that energy's trip from the core to the photosphere takes somewhere between 17 thousand and 50 million years! How can this be? Because the density at the Sun's core is so incredibly high, an average photon travels less than one millimeter before colliding with some matter. Since photons must undergo so many collisions (some of which deflect them back in the direction from which they came!) during their outward journey, their path to the surface is anything but straight. This ultra-zigzag path, often referred to as a random walk, significantly delays the flow of energy from a star's core to its surface.

An In-depth Look at the Parts of the Sun

The innermost, ultra-hot core of the Sun extends outward from the star's center to about 20% of its radius. The temperature at the center of the core is around 15 million kelvin (if you are unfamiliar with the Kelvin temperature scale, see the box at the bottom of this page), and gradually decreases further from the center. The core is the location within the Sun in which hydrogen fusion occurs.

Above the core is a region called the radiation zone. It extends from the top of the core outward to about 70% of the Sun's radius. Temperatures are still quite high here, somewhere on the order of 5 to 10 million kelvin. This region derives its name from the manner in which energy flows through it. Basically, energy is carried through the radiation zone via electromagnetic radiation, in the form of energetic photons. As you will soon see, this is not the only way in which energy is transported within the Sun.

Above the radiation zone, and extending all the way to the "surface" of the Sun, is the convection zone. This part of the Sun is relatively "cool", with temperatures ranging downward from a peak of around 2 million kelvin. Energy flows upward through this area in a different manner than in the underlying radiation zone. Gigantic blobs of matter, heated by the radiation zone below, rise to the Sun's surface, carrying heat with them. As these blobs of plasma emit their energy into space at the Sun's surface, they cool somewhat; enough so that their densities increase and they sink back down. This convective motion is akin to that seen in a lava lamp or in a boiling pot of water or oatmeal.

Granulation pattern of the Sun's photosphere (as viewed from above)
Granulation of the Sun's photosphere (oblique view)

These images show the granulation pattern on the "surface" of the Sun (the photosphere). The photosphere is the uppermost boundary of the Sun's convection zone. The granulation pattern is caused by the convective flow of heat rising to the photosphere from the Sun's interior. The granulation pattern is similar to what you see when you look at the top of a pot of boiling oatmeal. In the image on the left, in which we are looking straight down onto the solar surface, the centers of the granules (which are generally brighter) are areas where hot plasma is rising to the surface; while the darker boundaries of the granules are areas where plasma that has cooled is sinking back down. In the image on the right, our view is at an oblique angle, allowing us to see some of the three-dimensional structure of the granules. Note how the hotter centers of granules bulge upward, while the cooler edges are sinking downward. Granules are typically several hundred to a thousand kilometers across.
Credit: The Royal Swedish Academy of Sciences and Oddbjorn Engvold, Jun Elin Wiik, and Luc Rouppe van der Voort - University of Oslo (left). Goran Scharmer and Mats G. Löfdahl of the Royal Swedish Academy of Sciences (right).

At the topmost boundary of the convection zone lies the photosphere. The photosphere is often referred to as the "surface" of the Sun. How can a giant ball of gas-like plasma have a surface? The photosphere is not a solid surface like the ground beneath our feet that makes up Earth' surface. Instead, the photosphere marks an abrupt transition in the optical properties of the material that makes up the Sun. Below the level of the photosphere, the photons bounce around so much that they don't travel direct paths to viewers on Earth. We cannot see deeper into the Sun than the photosphere, so the photosphere is the "visible surface" of the Sun. As the images above clearly show, the turbulent motions of the underlying convection zone are clearly evident at the "visible surface" of the Sun. The temperature at the photosphere is around 5,800 kelvin.

Above the photosphere the Sun's vast atmosphere extends outward into interplanetary space. Once again, you may wonder what we mean when we speak of a sphere of gas-like plasma having an atmosphere. Indeed, the term "atmosphere" is not perfectly analogous to the sense in which we use it to refer to the air around our home planet. The fact that we say that the giant gas planets, including Jupiter and Saturn, have "atmospheres" illustrates the different situations in which astronomers use this term. In the case of the gas giant planets, the interiors are denser and behave as liquids, while the atmospheres are more tenuous and behave as gases. The Earth's atmosphere over the oceans (as opposed to over solid ground) is somewhat similar, though the analogy is far from perfect. In the case of the Sun, the density of material in the solar atmosphere is much less than is the case below the photosphere within the Sun's "interior". Also, the physical properties that control motions of material and the temperatures encountered are far different in the Sun's atmosphere than in the layers of the Sun beneath the photosphere.

There are two major regions within the Sun's atmosphere: the lower and much smaller chromosphere, and the upper and much larger corona. The relatively thin chromosphere is just a few thousand kilometers deep, less than Earth's diameter. Although temperatures within the Sun gradually decrease as one moves outward, from 15 million kelvin in the core to 5,800 kelvin at the photosphere, they begin to climb once again as we rise through the Sun's atmosphere. The temperature of the chromosphere increases from 4,300 kelvin (slightly above the photosphere) to around 50,000 kelvin (near the corona). Powerful magnetic fields in the Sun's atmosphere accelerate the plasma as they transfer energy to it, heating the material in ways that scientists still don't fully understand. Until relatively recent times, when special filters and space-based telescopes became available, the Sun's atmosphere was only visible during total solar eclipses. During an eclipse, the chromosphere could be seen as a colorful reddish zone around the edge of the occluded solar disk, thus earning the region its name (Greek "chromos" = "color"). The Sun's much larger upper atmosphere, the corona, extends unevenly for millions of kilometers into space. The the temperature of the solar atmosphere climbs sharply in a narrow transition region between the chromosphere and the corona. The temperatures in the corona range from around 800,0000 to an astonishing 3 million kelvin! The corona (Latin for "crown") can be seen as a tenuous, fuzzy zone around the Sun during solar eclipses. The size and shape of the corona varies substantially over time.

The Sun's chromosphere
The Sun's corona

The Sun's atmosphere, as seen during two different total solar eclipses. On the left, the narrow, red chromosphere is visible in this 1999 eclipse photo. On the right, the irregular, extended corona can be seen in this 1980 eclipse image.
Credit: Chromosphere image courtesy Luc Viatour (left). 1980 eclipse image courtesy Rhodes College, Memphis, Tennessee, and High Altitude Observatory (HAO), University Corporation for Atmospheric Research (UCAR), Boulder, Colorado. UCAR is sponsored by the National Science Foundation (right).

Energy, in the form of photons of light and other types of electromagnetic radiation, constantly streams outward from the Sun. You may not realize, however, that matter is also continuously flung outward by the Sun. An electrically charged "soup" of protons, electrons, and lesser numbers of heavier atomic nuclei flows outward into space. This extremely tenuous plasma is called the solar wind. In a sense, the solar wind is a vast extension of the Sun's atmosphere. How vast? The solar wind flows past Earth and beyond, so in one way of looking at it our home planet orbits within the atmosphere of the Sun! In fact, all of the planets (including Pluto, if it is still on your list of planets!) are within the gigantic "bubble" of the solar wind. Eventually, on the far edge of our solar system, the solar wind merges with the outpourings of other stars, and the extended solar atmosphere ends. The gigantic region within this solar wind "bubble" is called the heliosphere. The boundary of the heliosphere, where the extended atmosphere of the Sun finally gives way to interstellar space, is called the heliopause. The exact location of the heliopause varies over time and is probably different in different directions; however, it is certainly at least 10 billion km from the Sun's surface, or more than 70 times the distance between the Sun and Earth. The Sun, you see, is much larger than you may have ever realized!

The Kelvin scale of temperature

You are undoubtedly aware of the Celsius and Fahrenheit temperature scales. However, the Kelvin temperature scale may be unfamiliar to you. The Kelvin scale is used primarily by astronomers. It is closely related to the Celsius scale; one degree Celsius is the same as one kelvin. However, the zero points for the two systems are different. Zero degrees Celsius is defined as the freezing point of water. Zero on the Kelvin scale is at "absolute zero", the coldest possible temperature, which is around –273° C (or –460° F). Thus water freezes at +273 kelvin, and boils at +373 kelvin (100° C or 212 ° F).

Note that, by convention, we say "kelvin" as opposed to "degrees Kelvin" (e.g. water boils at 100 degrees Celsius or 373 kelvin). Here are a few important temperatures in each of the three scales, for comparison:

Fahrenheit Celsius Kelvin
Absolute zero
–459.67° F
–273.15° C
0 K
Freezing point of water
32° F
0° C
273.15 K
Surface of the Sun
9,980° F
5,527° C
5,800 K
Center of the Sun
26,999,540° F
14,999,727° C
15,000,000 K
Last modified September 2, 2010 by Randy Russell.

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