How stars produce energy?

Fusion: The Energy of the Sun
Discovery Enterprises, LLC

For many years astronomers were puzzled about how the Sun provided energy. While Earth’s fossil record indicates that the Sun has been shining for hundreds of millions of years, efficient chemical reactions known to early scientists—such as burning coal—could only provide energy from a similar mass for a few thousand years. Not until the 1920s did astronomers discover that nuclear reactions (energy released by the fusing of atomic nuclei) were a star’s principal source of energy.
 
Nuclear Fusion
Nuclear fusion powers the Sun. Fusion reactions release energy when two light nuclei combine to make a heavier atom. Here two hydrogen nuclei combine to produce a helium nucleus, releasing energy and a neutron in the the process.

Nuclear reactions can occur inside stars, because the interior temperatures of stars are in the millions of degrees. For example, the temperature of the core of the Sun reaches 16 million degrees C (29 million degrees F). At such high temperatures the electrons are completely stripped away from the nuclei of atoms, and the matter is neither solid, liquid, nor gaseous but exists in a fourth state called plasma (a gaslike state in which the [[KW]] atoms [[/KW]] lose their electrons and become ions). At the high temperature, pressure, and density of star interiors, atomic nuclei crash into one another at tremendous speeds, creating temperature-controlled thermonuclear reactions.

A.Hydrogen Burning
Hydrogen, the simplest of the elements and chief constituent of most stars, furnishes the fuel for stars like the Sun. Because the core of a typical star is so violent and hot, hydrogen nuclei are separated from their electrons. In the star’s core, the great pressure of overlying material forces the protons to collide so violently that the nuclei fuse together. The nuclear reactions fuse the nuclei of four hydrogen atoms into a single helium nucleus, liberating energy in the process and producing a star’s light and heat. In this fashion, more than 4 million tons of the Sun’s mass are destroyed and turned into energy every second. For a more detailed description of the hydrogen-burning process that occurs in stars like the Sun, see Sun: Nuclear Fusion in the Core.

B.Carbon Cycle
A more complex sequence of reactions, involving the nuclei of carbon atoms, produces the same net effect in some stars as that of the fusion of hydrogen nuclei. The carbon cycle starts with carbon-12 and hydrogen and ends with carbon-12 and helium. Carbon-12 acts as a catalyst (a substance that speeds a chemical reaction) in the production of helium from hydrogen. Because the carbon cycle is only triggered by temperatures that exceed 20 million degrees C (40 million degrees F), these reactions only occur in more massive stars. Carbon speeds the fusing of hydrogen nuclei into helium in the carbon cycle, so these stars burn faster and more brightly than do other hydrogen-burning stars.

C.Helium Burning
If a star’s core temperature rises to about 100 million degrees C (about 200 million degrees F), helium, which is inert (unreactive) in cooler stellar interiors, will participate in certain nuclear reactions. The collision of two helium nuclei can release energy and form a beryllium nucleus. The beryllium nucleus is unstable, but sometimes another helium nucleus collides with the beryllium nucleus before it can disintegrate, forming a carbon nucleus. Bluish-white supergiants and yellow giants use helium as fuel. Stars with insufficient mass never achieve high enough internal temperatures to fuse helium atoms.
At still higher temperatures, carbon and helium can combine to form oxygen, and this fusion process can continue, forming elements of successively higher atomic number (number of protons) up to iron. Compared to the long, stable hydrogen-burning stage in most stars, these processes predominate for relatively short periods of time and only in fairly massive stars. Astronomers believe that most of the elements found on [[KW]] Earth [[/KW]] could have been formed in this way deep within stars that existed long before the Sun formed.