Fusion in Stars
Swirls of hydrogen and helium gas condensed into huge clouds. The gravitational force at the core brought the matter closer and closer together until some of the nuclei coalesced. This produced energy, the heat and light of the stars.
What is Fusion?Fusion a type of nuclear reaction where two nuclei come together to form the nucleus of a different element. Each element has a particular number of protons in the nucleus. Isotopes of an element all have the same number of protons but different numbers of neutrons.
In the core of a star, gravity produces high density and high temperature. The density of gas in the core of our sun is 160 g/cm3, much higher than the densest metal, and the temperature is 15,000,000 K (27 million degrees Fahrenheit). At this temperature, the hydrogen and helium gases become a plasma. That is, the electrons separate from the nuclei to give a mix of positively charged ions and electrons.
Under these conditions protons (H-1) react with other protons to make deuterium nuclei (H-2) and positrons. The deuterium nuclei can merge to form a helium nuclei (He-4), or they can interact with other protons to make another isotope of helium (He-3). Two He-3 nuclei can fuse to make a nucleus of an unstable beryllium nucleus (Be-6) that breaks apart to give He-4 and two protons. Energy is released at each step.
The fusion of hydrogen nuclei uses up hydrogen to produce helium and energy. Hydrogen is the fuel for the process. As the hydrogen is used up, the core of the star condenses and heats up even more. This promotes the fusion of heavier and heavier elements, ultimately forming all the elements up to iron.
Energy from Hydrogen FusionIn going from hydrogen to iron, energy is released as nuclei fuse to make bigger ones. Why?
Another way to think of the energy released by fusion is to look at the change in mass. The total mass of the helium nucleus is less than the sum of the mass of the 4 particles that make it up.
Why stop at Iron?