Sunday, March 16, 2008

Burning Hydrogen

All stars, including our Sun, begin their lives burning Hydrogen. For stars the mass of our sun or less, the process of burning is the proton-proton chain. Two Hydrogen atoms fuse to form a deuterium atom. This is the longest step because it takes a while for a proton to release energy and become a neutron. Then another Hydrogen atom fuses in forming Helium 3, a light isotope. Then this atom fuses with another He3 atom and releases two protons to become Helium 4, the regular isotope. This process takes over 10^9 years to complete (over a billion years), and that's why the sun is still shining.

The most common production of Helium in stars heavier than our sun is done in a cycle called the Carbon-Nitrogen-Oxygen Cycle (CNO cycle for short). The most common CNO cycle begins with a Carbon 12 atom (this atom is given because today's stars have a small metallic content). Then a Hydrogen atom fuses in, creating Nitrogen 13. Then as a positron departs a C13 is left. Then another Hydrogen fuses in creating N14. Then another H creating Oxygen 15. Then another positron leaves forming N15 (note that in all the steps metioned so far about the CNO cycle energy is released). Then, as a last Hydrogen atom fuses in, a Helium 4 atom is ejected leaving a C12 atom to restart the process again. A star like our sun burns its Hydrogen in 10 billion years, while more massive stars only burn it for less than 10 million years.


Anonymous said...

How does a proton loose energy and become a neutron. Cannot a neutron decay and form into a proton?

Louis said...

In the proton-proton chain, energy comes in from the pressure and temperature in the Sun's core and fuses two protons (a.k.a. Hydrogen Nuclei). The extra energy becomes mass and turns one of the protons into a neutron. The proton loses its charge by the emission of a positron (or an anti-electron). Also, by a similar process (although not requiring any energy), a neutron emits a regular electron to become a proton.