Pique the Geek 20100314: Nuclear Fusion, Star Power

( – promoted by buhdydharma )

The first installment of this series may be found here, and it gives the basics as to how nuclear energy works.  The way that the stars generate their energy is interesting, and we shall consider it in greater detail this time.

Young stars almost always fuse hydrogen into helium.  There are several reasons for this, amongst them 1) hydrogen (protium, see the previous installment) is the most common nucleide in the cosmos, 2) more energy is released by fusing protium into helium nuclei than any other known process, and 3) there are multiple processes to accomplish it.

Before I continue, I should catch everyone up on some personal things.  First, it has been over a full year since I have bought “ready rolled” cigarettes.  I still smoke cigarettes, but have been rolling my own from Prince Albert tobacco and Top cigarette papers since the first of March last year.

Second, I am getting my garden ready to go.  As a matter of fact, I have several tomato plants in the peat pots, with more of them, peppers, and okra to germinate soon.  I shall directly plant the purple hull peas, cushaws, squashes, and melons when the ground warms a bit.  I shall keep you posted here and on What’s for Dinner? as things progress.

Third, my brother is resting uncomfortably at home after his wounds.  My nephew is still in hospital, but they removed the trachea tube yesterday, so he is in no danger of an obscured airway.  He is still on a feeding tube to reduce the risk of infection to his very horribly injured mouth and neck.  For those of you who are not aware of what happened, here is a link to my essay about it from Tuesday.  It made national news.  Update:  my brother was returned to hospital early this morning because of shortness of breath.  It was feared that he had a pulmonary embolism, but it turned out to be pneumonia.  Whilst pneumonia is not good, it is better than and embolism.

Now, to how the stars produce energy.  Fusing protium nuclei into helium nuclei releases the greatest amount of energy, gram for gram, than anything except antimatter-matter annihilation.  (Actually, there is antimatter involved in those reactions, but it is sort of incidental).  In their youth, and up past middle age (past middle age for a star like Sol is around 7 thousand millions of years, give or take), the primary source of energy is the fusion of four protons (protium nuclei) into one helium-4 nucleus amounts for by far the bulk of the energy released.  Thee are a couple of reasons for that, as hinted at in the opening.

The two main reasons are that there is more protium than anything else in those stars, and that the energy released by such a fusion releases a horrific amount of energy, both kinetic energy from collisions and gamma rays (photons), along with two positrons.  The positrons immediately encounter electrons in the core, and these pairs are converted into energy in an antimatter/matter reaction.  

That does not mean that those photons are emitted as soon as they are formed.  Because of the gravitational and electromagnetic confinement withing the core of a star, and also because of the extremely high density of the core, it might take many millions of years for a particular photon to reach the surface and be radiated.  We well may be sunbathing in photons generated when dinosaurs thrived, just now reaching us.  Once those photons reach the surface of our sun, it takes only about eight minutes for them to reach us, but they have a very tortuous path to the surface.

Interestingly, the rate of reaction to fuse four protons into one helium nucleus is extremely slow.  As I said in the first installment, that has to do with a “quark flip” that converts protons to neutrons.  Since this involves the weak nuclear force, it is typically very slow.  It is viable only because typical stars are so incredibly large that even a slow process pumps out a lot of energy.  To bring it into perspective, if your or my body were composed of stellar matter, and it had the tremendous density and temperature that exists there, the fusion process would be too slow even for us to maintain human body temperature (after correcting for the many millions of Kelvins required to maintain the reaction).  It is just the huge mass of stars that make them hot.

Now, after a while, stars fuse most of their protons in the core to helium nuclei.  In stars similar in size to our sun, hydrogen fusion continues in a layer surrounding the core, and the star begins to cool and expand.  Our sun will eventually do this, expanding in size to that comparable to earth’s orbit, becoming a red giant.  Finally, the core gets compressed if the star is big enough and helium begins to fuse into heavier elements.  For stars the size of our sun, this is about as far as the process can go, because as elements get progressively heavier, the energy released is smaller.

In very large stars, fusion continues up to the production of iron-56, one of the most stable nuclei (actually, nickel-62 is the most stable nucleus known, but there is not a viable mechanism to get to this nucleide in a star, so iron-56 is the end of the line).  After that, the star eventually begins to cool and fade, finally becoming cold and dark, UNLESS the star is heavy enough for gravity to cause the core to collapse catastrophically when the outward pressure caused by fusion diminishes.  In this case, a particular type of supernova is formed.

This is important, because all of the elements heavier than iron are formed this way.  Some of these elements are essential for life as we are familiar, so without these very massive stars going supernova, we would not exist.  The elements heavier than iron can not form by normal fusion, because they require more energy than fusion can produce, but the extreme gravitational potential energy of these very massive stars drives these energy-requiring reactions.  Basically, the gravitational potential energy is converted to kinetic energy as collapse occurs, and nuclei are rammed together at energies that are sufficient to form the heavy nuclei.  Then the star explodes, spewing out the heavy elements at relativistic velocities.  They finally slow down and form debris clouds, some of which coalesce and form new stars and planetary systems.

Our sun uses a process called the proton chain reaction to fuse hydrogen to helium, which takes first two protons to form deuterium, a positron, and a neutrino (this is the rate determining step, because of the quark flip to for a neutron involves the weak force and is slow).  Next, a deuterium fuses with another proton to form a helium-3 nucleus and a gamma photon.  Finally, two helium-3 nuclei fuse to form a helium-4 nucleus and two protons.  The other two reactions are fast, but since the concentration of both deuterium and helium-3 are low, do not happen very often due to rarity of encounters.  All three reactions release energy, and lots of it.

Large stars tend to use a very complicated cycle involving carbon to produce helium, but the overall energy release is the same because of conservation of mass/energy.

Well, you have done it again.  You have wasted a very nice, and obedient, set of photons reading this nonsense.  And even though Ann Coulter becomes less reactionary when she reads me say it, I always learn much more than I could possibly hope to teach by writing these essays.  Thus, please keep comments, corrections, and questions coming.  Obviously, tips and recs are also cherished.  Next time we shall talk about nuclear fusion on earth.

Warmest regards,

Doc

Crossposted at Daily Kos