The triple alpha process

7.2.2 The triple alpha process

Eventually, either by the proton–proton chain or the CNO cycle, the core of the star will be converted into 4 He. At this point nuclear fusion stops so that the pres- sure in the core that prevents gravitational collapse drops. The core thus reduces in size but, as it does so, its temperature will rise. Finally, when the tempera-

ture reaches ∼100 million K, a new reaction, known as the triple alpha process

(3α), occurs. It is so-called because it involves three helium nuclei which are also known as alpha particles (Figure 7.2). This is an extremely subtle process. The fi rst obvious nuclear reaction that would occur in a core composed of helium is

that two 4 He nuclei fuse to form 8 Be. However, 8 Be is very unstable – it has a life- time of only 10 ⫺19 s – and virtually instantly decays into two 4 He nuclei again. Only when the core temperature has increased to 100 million K, does it become likely that a further 4 He nucleus can fuse with 8 Be before it decays. The result is

Stellar Evolution – The Life and Death of Stars

a 12 C nucleus. It is highly signifi cant to our existence here on Earth that there is such a difference in temperature between that (∼15 million K) at which the hydrogen fuses to helium and that (∼100 million K) at which 12 C can be formed. If this were not the case, and the process could happen at the core temperatures close to that at which the proton–proton or CNO cycles operate, there would be no long period of stability whilst the star remains on the main sequence with

a relatively constant luminosity. This, of course, has allowed stable tempera- tures to exist on Earth for billions of years and so has enabled intelligent life to evolve.

There is a further real problem in attempting to form 12 C. A temperature of 100 million K is required to give the 4 He nuclei a reasonable chance to fuse with a

8 Be nucleus before it has a chance to decay. The 4 He nuclei are thus moving very fast and so have appreciable kinetic energy. It would be expected that this energy

would prevent a stable 12 C nucleus arising as it would be suffi cient to split the newly formed nucleus apart. [If a white billiard ball ( 4 He) approached a red ball ( 8 Be) very slowly they might just ‘kiss’ and remain touching, but if it came in at high speed the energy of impact would split them apart.] So why is 12 C so common? This problem was pursued with great vigour by the British astrophysicist, Fred Hoyle, in the early 1950s. As he then stated: ‘Since we are surrounded by carbon in the natural world and we ourselves are carbon- based life, the stars must have discovered a highly effective way of making it, and

I am going to look for it.’ He realised that the excess energy that was present in the reaction (and thus

expected to break up the newly formed 12 C nucleus) could be contained if there happened to be an excited state (called a ‘resonance’ by particle physicists) of the carbon nucleus at just the right energy above its ground state. This is because, due to the quantum nature of matter, though atomic nuclei usually exist in their ground state, it is possible for them to absorb energy (such as an interaction with

a gamma-ray photon) and jump into an excited state. This will later decay back to the ground state with the emission of a gamma ray of the same energy. This is analogous to an atom absorbing a photon of energy which lifts an electron to a higher energy level. The electron will then, in one or more steps, drop back down the energy levels emitting photons as it does so.

Hoyle realised that a stable carbon nucleus could only result if it had an excited

state that was very close in energy to that of the excess energy of the three 4 He that came together in its formation. This would thus lift the resulting 12 C nucleus into an excited state from which it could drop back to the ground state by the emission of a gamma-ray photon and so reach a stable state.

Some experiments in the late 1940s had suggested that such an excited state might exist, but Hoyle had been told that these were in error. Hoyle argued that there must be an appropriate excited state otherwise we could not exist and

Introduction to Astronomy and Cosmology

pestered the particle physicists at the California Institute of Technology (Caltech), led by William Fowler, to repeat the experiments. Fowler did so (it is said, only so that Hoyle would go away) and found that there was indeed an excited state within 5% of the energy predicted by Hoyle! Hoyle was essentially using the ‘anthropic principle’, which states that our existence as observers puts constraints on the universe in which we live. William Fowler received the Nobel Prize in part for this work. Many believe that Hoyle should also have won the Nobel Prize for his incisive observation and his following work in showing how the elements are

synthesized in stars.