Helium Capturing Reactions in Stars

Abstract

507The generation of 12C, 16() and 20Ne by successive a-capturing proc•sses in the helium core of stars is calculated under the condition at temperature-10°°K. The rat<s of forming i~C and 20Ne are found to be much larger than the previous calculations by Salpeter, Opik and Poyle, due to the resonance levels recently discovered. The production rates of 24Mg to 4°Ca are also <stimated. The formation of these nuclei is found to be ne~ligible at temperatur<s below 4 X 10S 0 K. The luminosity of such s:ars of globular clusters belonging to the horizontal branch in the H-R diagram that are supposed to be burning helium is accounted for in terms of the above thermonuclear reactions at the central temperature of (1.40±0.10) X10S°K and the density of about 10~gcm-~. The lifetime of a star in this evolutionary stage is estimated roughly as 107 years. In this course of evolution 20Ne is formed more abundant than others, in disagreement with the average cosmic abundances of 12C, Ill() and 2 0Ne. The modification of the abundances after this stage is suggested as nec<ssary.

Who Discovered the Hoyle Level?

“…The synthesis of helium into carbon in stars proceeds via resonant reaction, namely the three α particles fuse into an excited energy level in the 12 C nucleus. The rate of the reaction was calculated before the existence of this level was known and was found to be very low compared to the rate of destruction of 12 C by collisions with α particles.…”

“…The rate of the reaction was calculated before the existence of this level was known and was found to be very low compared to the rate of destruction of 12 C by collisions with α particles. As a consequence, it was impossible to predict the evolution off the main sequence towards the Red-Giant branch and the calculation implied that all the 12 C is converted into 16 O.…”

“…But stars do evolve and we know that somehow carbon is synthesized. In view of the impass, Hoyle predicted therefore, that 12 C has an energy level just at the right place and the reaction of carbon synthesis proceeds via this resonance level. The level was then discovered in the laboratory.…”

“…Carter argued as well, that humans do not occupy a privileged position in the Universe. The trigger to the idea that life as we know it, and the cosmos around us, "are tuned", emerged from Hoyle's prediction of the existence of a special energy level in the nucleus of 12 C. If such a level did not exist, argued Hoyle, life could not have develop in the cosmos, more accurately, 12 C based life could not emerge. Was it really so amazing?…”

“…The basic astrophysical problem emerged when Hoyle and Schwarzschild calculated, in the early nineteen-fifties, the evolution of stars off the main sequence into the red-giant using reaction rates known at the beginning of the nineteen fifties and got that as soon as 12 C is synthesized from helium, it absorbs another α particle and becomes 16 O leaving no carbon. The reaction forming 12 C was much slower than the reaction that destroys it.…”