In the begining, a star is nothing but a large cloud of hydrogen and some helium, floating around in space. After a while, the gravity caused by its great mass pulls it together. After a great deal of time, some of the mass is concentrated into large, warm clumps, called protostars or protosuns. The amount of mass in these protostars determines what kind of star this floating matter will become. In turn, the type of star they become determines what kind of nuclear reactions will take place within ("DavE". "Nuclear Reactions in Stars."www.oxford.net/~crdreyer/stars/: 1).
Though the stars and sun have been around since long before mankind, only since the 1920's have we had an inkling of what powers them. This power comes from nuclear reactions (why else do you think we would mention them on this site?). These reactions are possible because of the high temperatures inside stars (like, for example, 29 million degrees F in the Sun). At these incredibly high temperatures, electrons get pulled away from the nuclei of atoms, and the matter changes state. Suddenly, it is not gas, liquid, or solid, but plasma. (For further info on plasma, see this class' plasma site.) Inside the star, temperature, density, and pressure become overwhelmingly high, and atomic nuclei begin crashing into each other at enormous speeds. All this action creates a series of temperature-controlled thermonuclear reactions.
Which kind of nuclear reaction occurs depends on the size of the star, and the resulting heat inside ("Star (astronomy)." http://encarta.msn.com: 3.).
Hydrogen is the element that fuels stars. In the super-hot centers of stars, hydrogen nuclei get pulled away from electrons. The great pressure of the overlying material makes the protons from these same atoms collide so hard and so violently that the nuclei fuse together, in a process called nuclear fusion. (This is the opposite of the process used by nuclear power plants, which use nuclear fission.)These reactions fuse nuclei from four hydrogen atoms into a single helium nucleus. This process also releases a lot of energy, which makes a star's light and heat.
This is the method that occurs in Sun-sized stars, at a temperature of about 16 million degrees Celsius ("Star (astronomy)." http://encarta.msn.com: 3.).
In some stars (more specifically, ones with temperatures above 20 million degrees Celsius), reactions occur with carbon instead of helium. This process, called the carbon cycle, starts with carbon-12 and hydrogen and ends with carbon-12 and helium. The process is difficult to explain, but basically, the carbon-12 acts as a catalyst, speeding up the production of Helium from hydrogen. So stars undergoing this kind of reaction burn faster and more brightly than other stars ("Star (astronomy)." http://encarta.msn.com: 3.).
In some stars, (this time, ones with core temps of 100 million Celsius), helium goes through a different type of reaction. Two helium nuclei can collide to produce energy. When this happens, they form what is called a beryllium nucleus. Sometimes, before this unstable nucleus can disintegrate, another helium nucleus crashes into it, forming a carbon nucleus ("Star (astronomy)." http://encarta.msn.com: 3.).
AND AT EVEN HIGHER TEMPERATURES.....
When temperatures are even higher, carbon and helium may come together and make oxygen. Then, the process of fusion continues....elements with progressively higher atomic numbers are created, and the reaction's results climb the periodic table--as far as iron ("Star (astronomy)." http://encarta.msn.com: 3.)!
L C E E O I T R
OF FISSION AND FUSION