Models of how quickly stars evolve may have to be modified after astrophysicists found that the rate at which three helium nuclei transform into carbon-12 is very different from earlier estimates. This so-called triple-alpha process occurs in the interiors of certain stars and is important for determining how abundant different elements are in the universe. The researchers found that at temperatures below about 5 x 107 K the rate is significantly higher than previously thought, but that it is much lower at temperatures above 109 K (H Fynbo et al. 2005 Nature 433 136).
The triple-alpha reaction involves two helium-4 nuclei fusing together to form highly unstable beryllium-8 at a very fast rate. The beryllium-8 then reacts with a third helium-4 nucleus to produce carbon-12 in an excited state. The rate at which the triple-alpha process occurs is critically important for determining the abundances of elements throughout the universe. These elements are ejected when stars explode as supernovae at the ends of their lives.
Hans Fynbo and colleagues at the University of Aarhus in Denmark, along with co-workers in Switzerland, Spain, the Netherlands, the UK, Finland and Sweden, actually studied the reverse of the triple-alpha reaction. Working at the ISOLDE facility at CERN and the JYFL Accelerator Lab at Jyväskylä University, the physicists first produced short-lived radioisotopes of boron-12 and nitrogen-12. They then quickly extracted these isotopes as low-energy beams and stopped them in a thin foil, where they decayed into carbon-12. The foil was surrounded by detectors that could detect all three alpha particles emitted by the carbon-12 with high precision.
Fynbo and co-workers calculated that, for temperatures below about 5 x 107 K, the triple-alpha reaction rate was significantly higher than the standard rate — as published by NACRE (Nuclear Astrophysics Compilation of Reaction rates). The result suggests that the critical amounts of carbon that catalysed hydrogen burning in the first stars might have been produced twice as fast as previously believed. This means that the stars could have evolved twice as quickly.
At temperatures above 109 K, on the other hand, the reaction rate was much lower than NACRE’s. This could be important for models of nucleosynthesis — the process that forges all heavier elements — in supernovae.
“The triple-alpha rate depends heavily on temperature so when you go from 107 K to 1010 K it changes by 80 orders of magnitude,” says Fynbo.
