Once Before Time - Martin Bojowald [93]
This force, fully independent of the electromagnetic force and gravity, acts only over small distances, at least in comparison with common length scales; compared to the Planck length, even these distances are huge. It becomes active only when two protons approach sufficiently closely, as it happened in the early universe—either by random central hits of protons rapidly flying about in the hot phase of big bang nucleosynthesis, or systematically in early post–big bang compressions due to gravitational attraction. One of the protons is then first transformed into a neutron and a positron, the electron’s antiparticle, and the other proton binds with the neutron to form a deuteron. Here one of the extremely weakly interacting neutrinos also arises, immediately fleeing the scene. The positron and one of the orbit electrons soon annihilates, and so from two light hydrogen atoms, with their two protons and electrons, a single deuterium atom with one proton, one neutron, and a remaining electron is formed—along with an amount of free energy, as the basis for the star’s shining.
Thus hydrogen can be condensed more than would be possible for simple protons, for a single nucleus takes up less space than two separate ones. But gravity has no mercy. Deuterons must approach each other, as well as the remaining common hydrogen nuclei, more and more, until at last further reactions happen. For instance, two deuterons have exactly the same combination of particles as one helium nucleus. And indeed, when they come close they can combine to form helium, again gaining energy thanks to the binding nuclear force. Like two protons to a deuteron, so two deuterons fuse to a helium nucleus—a process currently raising high technological hopes as a possible source of energy on Earth, as the fuel, hydrogen, would be nearly undepletable, and the process itself would be very clean; the “waste” would, after all, be helium.
In stars, fusion is easily possible because gravity compresses hydrogen without further ado, and the high pressure makes the nuclei approach each other closely enough to make the reaction happen. Much more energy is released than in the formation of deuterons, exactly the bulk of energy radiated away by stars. In the stars’ interiors, this leads to an increase of temperature, and the heated matter is subject to a strong pressure able to withstand gravitational collapse. Thus stars like our sun are stable for long periods of time—a lucky break for our survival.
When they grow old, stars slowly use up their hydrogen, including the deuterium. At some point, additional nuclear reactions of helium occur, since an approaching hydrogen or another helium nucleus can add protons and neutrons. Heavier elements form in this way, such as carbon and oxygen, important for life on Earth, but also silicon and iron, which form the largest part of the planet. Energy is released in these reactions, contributing to the radiation of stars; but it is no longer as much energy as in the star’s youth. The most stable nuclei turn out to be iron and nickel, where fusion stops; elements heavier than those are produced in stars only in minor quantities. (Most heavy elements as they can naturally be found on Earth form in violent stellar explosions rather than in normal fusion burning, adding protons and neutrons to iron in rapid fire.)
By these long-lasting processes repeated uncountable times, keeping the oven hot for billions of years, the first generation of stars cooked up a large variety of elements, much larger than was possible during the first fiery minutes of big bang nucleosynthesis. Some stars—exhausted by their mass enrichments—exploded in supernovae when they had used up all their nuclear fuel, releasing the elements into space. Long afterward, the stellar dust again gathered in clouds, just like the initial hydrogen, and started once more to form density centers. From one of these dense regions, in a later generation after the first stars, our own solar system and its planets, including Earth, were formed.
WHITE DWARFS: COLD