Question

What is the triple-alpha process? List the sequence of events that occur in this process. Does this effect occur in all stars, or only high mass stars?

Answer 1

The combination or fusion of three alpha particles (helium nuclei ⁴He) to form a carbon nucleus (¹²C) is known as the triple alpha process.

• The fusion process is not at all simple since there does not exist a stable configuration with an atomic mass of 8 (⁸Be) that is formed by the fusion of two ⁴He nuclei. The lifetime of ⁸Be is a very short 3 × 10^-16 seconds.

• However if this unstable ⁸Be nucleus is struck by another ⁴He nuclei it is possible to form ¹²C (and a gamma ray). This can occur because the lifetime of ⁸Be is actually longer than the mean collision or scattering time of helium nuclei at temperatures around 10⁸K which are needed to make this fusion process proceed. In 1952 Edwin Salpeter proposed this process as the way to form ¹²C.

• Not long afterwards Fred Hoyle suggested that the fusion of ⁸Be and ⁴He would be greatly enhanced if the ¹²C nucleus possessed an energy level near to the combined energies of the two nuclei involved. The subsequent reaction would then be a resonant reaction. This resonant energy level was found by experiments at the Kellogg Radiation Laboratory at CalTech.

The triple alpha process will occur in red giant stars that have left the main sequence (and have consumed their core hydrogen) and have core temperatures of 10⁸K and higher. Once ¹²C has been formed it is possible with temperatures around 6 × 10⁸ K to continue forming heavier nuclei by the combination of two ¹²C nuclei to make ¹⁶O , ²⁰Ne, ²⁴Mg and with temperatures around 10⁹K the combination of two ¹⁶O nuclei can make ²⁸Si, ³¹P, ³¹S and ³²S

High-Mass Post-Main Sequence Evolution.

Evolution of high-mass stars off the main sequence is an involved process and one still not fully understood. Such stars are rare and have very short lifespans relative to lower-mass stars. Supergiants such as Betelgeuse, Deneb, Rigel and Antares are some of the most prominent stars in our sky and visible over vast distances due to their extreme luminosities. This section provides a basic outline of the stages.

High-mass stars consume their core hydrogen at prodigious rates so may only survive on the main sequence for millions rather than billions of years. Once this fuel is used up, the core contracts due to gravity and heats up. This triggers helium-burning in the core. Unlike lower-mass stars, this helium fusion (triple-alpha process) starts gradually rather than in a helium flash. In moving off the main sequence, the effective temperature of the star drops as its outer layers expand. The decrease in temperature balances the increased radius so that the overall luminosity remains essentially constant. The energy liberated by helium fusion in the core raises the temperature of the surrounding hydrogen shell so that it too begins fusing.

In stars of 5 solar masses or higher, radiation pressure rather than gas pressure is the dominant force in withstanding collapse. The mass is large enough that the gravity acting on the core after helium-burning is sufficient to produce temperatures of 3 × 10⁸ K where fusion of carbon with helium to produce oxygen dominates. A star of 8 solar masses or more can go on to synthesise even heavier elements in the core.

Gravitational core contraction after all the core helium is used up generates a temperature of about 5 × 10⁸ K at which point carbon nuclei fuse together to produce sodium, neon and magnesium. Production of magnesium releases a gamma photon, that of sodium releases a proton and neon produces a helium nucleus. Once all the core carbon is consumed, further collapse pushes temperatures to about 10⁹ K. At this temperature, reactions that release gamma photons, such as ¹⁶O + ⁴He ? ²⁰Ne + ?, may be reversed by a process called photodisintegration. Helium nuclei released via this process can fuse with other neon nuclei to produce magnesium.

Once the neon is used up, core contraction increases the temperature to 2 × 10⁹ K where two oxygen nuclei fuse to form silicon. This in turn may undergo photodisintegration to form magnesium and helium nuclei that then fuse with other silicon nuclei to produce sulfur. Similar stages of reactions see sulfur produce argon and argon synthesise calcium. Eventually elements such as chromium, manganese, iron, cobalt and nickel may be produced. Ultimately the silicon in the core is converted, into iron with final core temperature reaching about 7 × 10⁹ K. The core region of a supergiant thus resembles the layers of an onion with a dense iron core surrounding by shells of silicon and sulfur, oxygen and carbon

The onion-like layers inside a supergiant in the final stages of its life. Successive layers correspond to the different elements produced by fusion, with a dense core of iron at the centre.

Nucleosynthesis of elements above helium is less efficient so that each successive reaction produces less energy per unit mass of fuel. This means that the reactions occur at greater rates so that radiation pressure balances gravity. Whilst a massive star may spend a few million years on the main sequence, its helium core-burning phase may be a few hundred thousand years. The carbon burning phase lasts a few hundred years, neon-burning phase a year, oxygen-burning half a year and the silicon-burning only a day.

These massive stars evolve extremely rapidly once they move off the main sequence. Statistically they are very low in numbers as they are less likely to form than lower-mass stars and their lifetimes are so short anyway. As we shall see in a later section, they also make dramatic exits.