Expert: Philip A. Stahl - 11/10/2007

Question
How is energy released in stars and how does this effect the composition of elements in the universe?

Answer
Hello,

Energy is released in stars by the process of nuclear fusion which can take different forms, with different reactions producing different amounts of energy depending on the reactants.

In the Sun, for example, "hydrogen fusion" (into helium) leads to the basic set of three reactions for the proton-proton cycle, viz.

H1   +   H1 + e ->  D2  + v  + energy

D2  +  H1  ->  He 3  + (gamma) + energy

He3  +  He 3  ->  He 4  + H1 + H1 + energy

In the first, H1 denotes the proton (hydrogen nucleus), e is an electron, D2 is deuterium - an isotope of hydrogen- and v is the neutrino, and energy given off.

This leads to the next reaction, with deuterium fusing with a proton to
yield the isotope helium 3, a gamma ray and more energy given off.

Finally, this leads to the end stage reaction, with the helium 3 combining
with another helium 3 nucleus to give helium 4, two more protons (to start the cycle anew) and energy given off.

Eventually, as these reactions progress, hydrogen is used up in the core, and a new set of nuclear fusion reactions has to enter - wherein helium is now fused to yield carbon.

The two basic reactions underlying this can be written:

1)  He4 + He4 ->  Be8  +  Gamma (-95 kev abs.)


2) Be 8 + He4 ->  C 12  + Gamma + 7.4 Mev

In (1) two helium nuclei fuse to give Beryllium 8 and a gamma ray with 95 keV of energy ABSORBED. (Thus, (1) is endothermic)

In (2) the beryllium fuses with a helium -4 nucleus to give carbon 12 and 7.4 MeV of energy released in each reaction.

(1 eV = 1.6 x 10^-19 J  so  1 MeV = 1.6 x 10^-13 J  e.g one million times more than 1 eV)

Obviously, if only stars the size of the Sun existed, you would not get elements much heavier than carbon in the universe.

This is why supernovae (or exploding stars) are so important. In the case of these much more massive stars (> 3 times the mass of the Sun) much heavier elements can be built up via fusion inside the cores.  The original work on this was done by Fowler, Hoyle et al in their paper 'Synthesis of Elements in Stars' in The Review of Modern Physics over four decades ago.

Basically, they theorized that in very massive stars
the core collapses after the nickel-iron stage is reached and the implosion leads to an enormous explosion called 'supernova'. (The implosion is essentially a result of the outward radiation-gas pressure of the star being unable to support the inward directed stellar gravity arising from the immense mass in the core and surrounding layers)

En route to this stage, the gravitational collapse is such that protons and electrons are squeezed together in the core to form countless neutrons which then become available for combination with existing elements formed(up to the Ni-Fe stage).

The authors developed their paper to show how two basic neutron capture paths could occur in type I supernovas, leading to elements of very high atomic number.

These neutron capture paths they referred to as "the r process" and the "s process". The essential feature in each is that a large flux of neutrons becomes available for addition to elements of the iron group. (The "r" in
r-process refers to "rapid", e.g. rapid neutron capture path, and the "s" in s-process, refers to "slow", e.g. slow neutron capture path)

Specifically, the r-process features neutron capture on a very short time scale (0.01 sec  <  10 sec) which is able to produce isotopes of elements in the range (70 < A < 270). The latter limit is in the neighborhood of
Rutherfordium.

The s-process features neutron capture with emission of gamma radiation and occurs over time scales in the range (100 yr. <  10^5 yrs.) and produces isotopes in the range: 23 < A < 46.

Another process identified in the supernova event is the p-process, which denotes proton capture with emission of gamma radiation - or alternatively, emission of a neturon after gamma ray *absorption*.

Of course, the details are much more complex than presented here - which serves merely to provide (in the absence of knowledge of your physics, math background) a basic answer to your questions.

For example, competing processes can arise - thus the p-process reactions can and do compete with positron emission until proton addition is no longer possible, and positron emission occurs alone.

Bear in mind the positron = e(+) is a *postively charged* electron, so that when each such emission occurs the effect is to *reduce* the atomic number by 1)

Hopefully this answer sheds some useful light on a difficult area. Which, I may add, still has numerous avenues of investigation and research!