The Elements up to Silicon

The elements up to iron are produced in the sequence of fusion reactions that can take place in stars after they leave the main sequence. However, the nature of the fusion reactions is different for elements above and below silicon (Si). We first discuss the production of elements up to silicon, and then discuss the conversion of silicon into elements up to iron.

Helium Burning

We have seen that carbon (C) is produced from helium (He) by the triple-alpha process in red giant stars (recall that an alpha particle is another name for a helium-4 nucleus),

⁴He + ⁴He + ⁴He --> ¹²C

and that part of that carbon can be converted to oxygen (O) if the temperature is high enough by the capture of another helium-4 nucleus:

⁴He + ¹²C --> ¹⁶O + gamma ray

The rate for this reaction is not very well known, but it is very slow. In principle the oxygen produced in the preceding step can be converted to neon by capturing another alpha particle:

⁴He + ¹⁶O --> ²⁰Ne + gamma ray

However, the rate for this reaction under red giant helium burning conditions is very slow so little neon is produced during helium burning. Thus, helium burning produces a core of carbon and oxygen.

Carbon Burning

At temperatures of ~ 5 x 10⁸ K and densities of ~ 3 x 10⁶ g/cm³, carbon may fuse into heavier elements. The primary reactions are the fusion of a carbon nucleus with another carbon nucleus to produce isotopes of neon (Ne), sodium (Na), or magnesium (Mg). Typical reactions are

¹²C + ¹²C --> ²⁰Ne + ⁴He

¹²C + ¹²C --> ²³Na + proton

Generally, such reactions are possible only at the temperatures reached in stars having 4 or more solar masses. The additional burning stages discussed below require higher temperatures still, and even more massive stars.

Neon and Oxygen Burning

At temperatures around 1 x 10⁹ K, both oxygen and neon can burn. Oxygen can fuse with another oxygen to produce silicon plus an alpha particle. The neon burns in a different way, however. First, neon is dissociated (broken into pieces) by a high energy photon, which are very plentiful at these high temperatures (remember the Wien Law).

gamma ray + ²⁰Ne --> ¹⁶O + alpha particle

Then, the alpha particle produced in the dissociation step can be captured on another Ne nucleus to produce magnesium:

alpha particle + ²⁰Ne --> ²⁴Mg + gamma ray

This two-step process is representative of the kinds of reactions that will burn silicon to iron, which we consider next.