Stars begin their life as large spheres of almost pure hydrogen. Throughout their lifetime they generate energy through nuclear fusion. This begins in solar-mass stars with a process called the proton-proton chain, which converts hydrogen into helium. Two hydrogen atoms combine to form one helium atom, but the mass of the helium atom is slightly less than the combined mass of the two hydrogen atoms that created it. This mass loss is released as energy and the energy released can be calculated using the formula E=mc2 where E is the energy released, m is the mass loss and c is the speed of light in a vacuum.
The fusion process leaves a core of helium surrounded by an envelope of hydrogen. As the fusion process continues, the helium core becomes larger and more dense. Whilst the star is “burning” hydrogen it is said to be on the Main Sequence.
In stars like our Sun, this is where the process ends and the star will retain this structure until the end of its lifetime. In more massive stars, however, the density of the helium core becomes so great that the temperature becomes high enough to start a new fusion process.
This new process 'burns' helium to create carbon. This leaves a carbon core, surrounded by helium, which is then surrounded by a burning hydrogen shell and a hydrogen envelope.
If the star is massive enough, a similar process will happen again and carbon will begin to fuse into oxygen. The process will continue as long as the core of the star becomes hot enough to begin each new fusion process, or until iron is created.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001