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 Fusion

In going from hydrogen to iron, energy is released as nuclei fuse to make bigger ones. Why?

The protons and neutrons are held together through a type of energy called nuclear binding energy. The nuclear binding energy for H-1, a proton, is zero because there is only one particle in the nucleus.

As the number of particles in the nucleus increases, energy is released. This is the same amount of energy that is required to break them apart.

Energy in this table is reported in units of MeV or mega-electron volts. A MeV is equal to 1.602 x 10-13 joules.

From Hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

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?

Adding additional protons and neutrons to iron doesn't release energy because the binding energy peaks at this element. For element heavier than iron, fusion requires energy.

How did the heavier elements form?

It was from the energy of other explosions. A large, exploding star or supernova releases the energy needed to fuse all of the heavier elements.

The Earth and all of the material on it were formed from stardust!

Professor Patricia Shapley, University of Illinois, 2011