Sun Core and Fusion Reactions in It :


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Deep in the heart of our sun is its core, which is where the fusion reactions that power our star take place. That means there's no adjective quite strong enough to describe just how hot and dense the core is, where temperatures reach over 27 million degrees Fahrenheit (15 million degrees Celsius) and material is packed together more than 10 times more densely than in lead. [Read more about fusion in the sun]

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.