It is known that everywhere in the solar system - with the exception of what I will discuss below - the ratios between two isotopes, C-12, and C-13 (both of which are stable nuclei) is approximately 99 to 1.   The ratio can vary very slightly, on the order of a few parts per thousand, and precise measurements of these tiny variations can tell us something about the history of particular carbon species and give insight to questions about the temperatures of their formation, for instance.   But around here, and by "around here" I mean at least out to the orbit of Saturn (probably further) the ratio is 99:1.

However, in the universe as a whole, this ratio is now understood to vary widely, from ratios 1:1 to 5 orders of magnitude higher, 100,000 to 1, as I learned from a recent lecture given by Larry Nittler of the Carnegie Institution (who did a very credible job substituting for Ernst Zinner, the announced speaker at this symposium).    This has been learned as the outgrowth of a discovery some 24 years ago, that many ancient meteorites (as well as dust collected by spacecraft and by high altitude aircraft) contain tiny inclusions called "presolar grains" which are small fragments of cooled plasma that have formed in one of three ways, supernovae explosions, nova explosions or in AGB stars.   The latter type of stars - AGB stands for asymtotic giant branch - are not particularly exotic.   Your sun, after powering Amory Lovins cars for a few billion miles or a few billion years (which ever comes first) will ultimately become an AGB star.

It turns out that the distribution of these particles - about 100,000 such particles have been analyzed since their discovery - can offer many insights into stellar evolution, and can work to confirm theoretical nuclear physics calculations involved in said evolution that were hitherto purely mathematical abstractions and were long though to be beyond the realm of experimental laboratory confirmation.   These particles contain "nuclear fossils" represented by extinct nuclei like aluminum-26 (half life approximately 700,000 years) and titanium-44 (half-life about 60 years), as well as isotopic distribution of isotopes of oxygen (which has three stable isotopes, 16, 17, and 18) and nitrogen (which has two stable isotopes, 14 and 15) as well as the carbon already mentioned.

The fact that we exist at all depends very subtly on a number of factors for which we don't have very much appreciation.  

Two fellows, John Barrow and Frank Tipler wrote a provocative monograph on this topic entitled The Anthropic Cosmological Principle that argues that the value of a fundemental constant of the universe, the fine structure constant implies that humanity must exist.

(I wrote a diary here that was to some extent about the fine structure constant, Oh.  Oh.  Plutonium Contamination Suspected.   Blogging about science is a very, very, very, very bad idea.)

The book in any case is a fun read which posits a "theory" that is not testable and thus is more philosophy than science, although one could argue the same way about a lot of fairly sophisticated sciences.   At it's best this idea is stimulating, at its worst, it degenerates into pure mysticism and gobbleygook used to argue for dubious faith based stuff like "intelligent design."   From my limited knowledge, I suspect that Frank Tipler may have gone over the edge, but I'm not qualified to state so definitively.

Anyway, it happens that our existance also depends very sensitively on the isotopic distribution of the elements.   I had a nice chat today with a fellow at his poster - he works in one of those probably doomed pharmaceutical giants - about this stuff, isotopic distributions in common elements.    We discussed the fact that one (non-radioactive) isotope of hydrogen, deuterium, is actually toxic in large quantities because it slows down proton transfers in proteins.    (He was studying proteins containing another rare isotope, oxygen-18.)   If deuterium were not a trace isotope in the solar region (which is not every in the universe) it is very unlikely that life as we know it could exist.

One might have argued that theories of stellar evolution - although to some extent based clearly on constructs assembled by very detailed observation, and assembled by the world's greatest minds - have been speculative.   In particular, there is really no way to experimentally observe a stellar collapse or, um, is there?

At various times, if I recall correctly, I have discussed the origin of the chemical elements here, since the subject fascinates me because of my general interest in nuclear chemistry, despite the fact that people who know nothing at all about nuclear chemistry hate the science in a rote fashion and often express this hatred for this science here, often to general applause.   There are four processes that account for the elements, one being the "Big Bang" - the name "Big Bang" was viewed by the person who coined it, in a fashion similar to how the art movement called "Impressionism" was originally a derisive name that stuck.   "The Big Bang" accounts for the existance of hydrogen with impurities of helium and lithium.   None of these elements are actually stable, although two of them represent the bulk of the matter in the universe.  

The only truly stable element is iron, since its binding energy is such that lighter elements that fuse into it (or lower elements) will release energy, and heavier elements splitting to form it will absorb energy.   Stars that are roughly the size of our sun will fuse light elements like hydrogen into helium, and can carry out this process for periods as long as ten billion years.   Ultimately such stars run out of hydrogen.   The fusion of two helium atoms into beryllium-8 is actually endothermic and consumes significant energy, (helium-4 is actually more stable than any of the elements near it, including hydrogen, lithium, and beryllium - it is the only nuclei that has a neutron capture cross section of zero)  and thus the gravitational pressure inside a star must become great enough that it is statistically possible for three helium nuclei to collide simultaneously to form.

At the point that a star begins to rely on helium burning it actually expands to a very large extent and becomes a red giant - the AGB star referred to in the introduction - when our sun reaches this point its surface will actually extend beyond the orbit of the earth which, of course, will be vaporized.    Elements up to iron are synthesized in this phase, as mentioned above, but actually some heavier elements result from what is called the "s process" or slow process.   Even though hydrogen is largely depleted at this phase, traces of it still exist, and when residual hydrogen (or helium or other elements) fuse with iron (or when other other elements close in weight to iron fuse) the resulting nuclei can be heavier than iron.   This process also takes place because of the many neutrons that are available from nuclear reactions in stars.  The new nuclei will decay by beta (or positron) decay and may absorb energy - since in a star reams of energy are available) and overshoot the iron "limit."   Thus elements like cobalt, copper, zinc and beyond will be formed even in ordinary stars, albeit in smaller quantities than elements that occur before iron in the periodic table.   Theoretically (although probably not practically) this process can result in elements as heavy as lead, but none heavier, because of the extremely short half life of elements like francium which are too unstable to wait around for a "slow" process.

Because of nuclear physics quirks that are beyond the scope of this very simplistic diary, it happens that the distribution of isotopes of elements will be different when formed in this process than in other processes.   In the talk I attended there were many plots of ratios of carbon, nitrogen and oxygen isotope ratios found in these stardust inclusions, and the plots all had very distinct regions of concentration.

The r-process (or rapid purpose) takes place when a star that is at least 10 times more massive than our sun depletes all of the light elements up to iron and begins to collapse gravitationally.    This process takes place catastrophically and the rebound of this collapse accounts for supernova.    As a supernova happens, enormous amounts of energy are generated, and a tremendous flux of neutrons and heavy nuclei are available.   It can be shown that the r process produces elements as heavy as californium (element 98) which can persist for millions of years.    One plutonium isotope, the 244 isotope is so long lived that it is known to have been present in significant quantities during the formation of the earth.   The "nuclear fossil" of this plutonium is present in our atmosphere (and many very old rocks) in the form of certain xenon isotopes.

A star the size of our sun that collapses will become a white dwarf, consisting largely of what is called "electron degenerate" matter.   A discussion of this type of matter is also beyond the scope of this diary, but suffice it to say that white dwarfs have the mass (approximately) of our sun in the volume of our earth.   Sometimes white dwarfs will approach another star (as part of a binary system) that they will actually gravitationally rip matter off the surface of the star.   This event which is very bright, but long lasting is called a nova, but is not, um, "super."    This is also an element forming process, and nuclear fossils of this process are largely represented by silicon and magnesium isotope distributions.

The clearest nuclear fossil of a supernova is the isotope calcium-44.   This nuclear fossil can only be formed in a supernova type event and initially the element that is formed is not calcium, but is the neutron deficient unstable isotope titanium-44, which has a half-life of 63 years.   Titanium decays by the release of an anti-electron, a positron, to scandium-44 which as a half-life of 3.927 hours and itself decays by positron emission to give calcium-44.

In the solar system, this is a relatively rare isotope of calcium:   Only 2.086% of the calcium in your bones is this isotope.  The vast majority of calcium on earth is calcium-40, which is still being synthesized on this planet because of the decay of naturally occuring radioactive potassium-40, along with the nuclei of argon-40, which represents about 1% of earth's atmosphere.   (The nuclear stability rules predict that actually calcium-40 is metastable and will prove - although its not been observed - to be very slightly radioactive, with a half-life much lonnger than the universe as a whole.)

However there are presolar grains in which the calcium is pure calcium-44, meaning conclusively that this isotope must have started out as titanium-44, undergone chemical fractionation not available to calcium formed or expelled in supernova, generally as titanium carbide, and subsequently decayed to calcium.

Dr. Nittler closed his presentation with a photograph of a young and very beautiful Joni Mitchell and her famous song "Woodstock" about a concert (as Dr. Nittler wryly commented) that she didn't attend.   (Maybe in the back of his mind was discussions by lay people of nuclear science about which they know nothing.)  

Her lines, which appeal to all scientists interested in stellar evolution where

We are stardust
We are golden,
billion year old carbon.

(OK, we'll leave out the part about being golden.)

Because of his work and the work of his peers like Dr. Zimmer, we can now do nuclear fossil astronomy in earthbound laboratories and make a fairly good estimation how the carbon in our bodies formed, basically what kind of carbon we are.

It turns out that the carbon in our flesh mostly formed - about 90% of it - in AGB stars.  The bulk of the rest formed in supernova, with trace amounts formed in ordinary nova.

So, if you think you're a star, you're right, but very little of you is actually the brilliant star you think you are.

Have a nice evening and a nice day tomorrow.


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