Nucleosynthetic Assembly of the Third-Most-Dense Alkaline Earth, Calcium, a Group-2 Series Element

      One April morning in 2015 I saw a rainbow, and it was as if the act of seeing it had awakened me from a hypnotic trance. I began to see things with new eyes, and I realized something that should have been obvious all along. Our Universe is assembling itself! That revelation completely changed the way I think about everything, and it left me eager to learn details about our physical world that I had either glossed over or missed entirely due to my prior lack of interest or because those details had only recently been discovered. BTW, the latter excuse for my lack of knowledge is another reason I’m so excited. One can see from dozens of science related Twitter posts that those details are incredible and are arriving everyday at a quickening pace!

      Which brings me to the issue at hand. I recognized when I started  writing about calcium that there was an important something that I had missed, and it was a something that left me with a burning desire to get to an answer for the following question. 

      Just what process does Our Universe use to assemble 20 protons and 20-some neutrons into the amazing invention we call calcium? To be specific I’m talking about Ca-40 and it’s four other stable isotopes, Ca-42, Ca-43, Ca-44 & Ca-46.

      And what a great question that turns out to be because it has an amazing answer that becomes even more amazing when one realizes that Our Universe has gone to a lot of trouble to help us get to the bottom of it. How so, you might ask? 

      For one thing, Our Universe has given us specific barcodes so that we can identify each type of atom it has assembled! 

      Heat atoms up or agitate them in other ways and their electrons can get boosted into higher energy-level orbits. Such energized electrons throw-off photons of specific energies (wavelength frequencies) when they fall back to lower-energy orbits. The photons so released will show up in emission spectroscopic analyses, which are done with spectrometers of specific design, as energetic sets of sharp bands of visible light, UV, X-rays or other forms of electromagnetic radiation at specific wavelength frequencies. 

      Check it out.  Some examples of the “barcodes” for calcium and three other atoms are shown below in the following emission spectra. 

      Alternatively, the specific energies of light absorbed by the atoms can be seen in spectrometers as dark bands in an absorption spectrum.

      Neat isn’t it? But wait, I thought stars were mostly hydrogen/helium fusion furnaces. And, if that’s so, from whence comes calcium? Fortunarely, Our Universe has given us other ways to help us know that.

      I began my search by asking  “iPhone-Siri” a few questions. Siri’s knowledgable voice is new for me. She makes mistakes, especially when she doesn’t understand me, but she is way smarter than I am about some things, and her clouds of Apple servers are getting smarter by the day! 

      For example, I’ve known for a long time that light travels at 186,000 miles per second, but when I asked Siri about it she corrected me with her stilted, less than sultry attempt at femininity when I asked, “Siri, how fast does light travel in miles per second?” She replied, “Let me think about that, Frank.”–“Speed of light in vacuum converts to about 186,282 miles per second.” Needless to say, I was impressed and found myself talking to my phone, OMG! I replied, “Thank you Siri”, and next asked her how how far the Earth is from the sun and she said, “The distance from the Earth to the sun is about 0.984 astronomical units.” That also confirmed what I already knew but she did it with enhanced precision. An “au” is the comparative planetary distance from the Earth to the sun,  and I’ve also known that distance coverts to about 93 million miles. I didn’t bother to confirm that figure but I did confirm with her that light from the sun takes about 8.3 minutes to reach the Earth, and I verified that it takes light about 2.5 million years to arrive from the Andromeda galaxy. 

      It still feels weird but I think Siri and I may have a long and abiding relationship!

      All of the above brings up another question. Just how did humanity learn about these distances so they could pass the information along to Siri? Fortunately, Siri doesn’t keep her sources secret. It turns out that Hipparchus and Eratosthenes did some very clever geometric thinking about 2000 years ago to get some approximate distances relating to the sun, moon and planets. 

      However, nowadays, radar works better for such “short” distances and measurements based on the Doppler redshift, gravitational lensing, and light coming from the consistent energy of quasar galaxies and pulsar stars are needed to get at the distances of galaxies and other objects that are so far-distant they approach the beginning of time, and now we’re talking more than 13-billion years!

      So, by knowing how fast light travels and how long it takes light to get from place to place we can time-travel and, thereby, we find out that much of what we see in the heavens occurred millions and even billions of years ago. And amongst the far-distant stars we can see light coming from some very old stars. Some of which the Hubble telescope and other such devices show have already died to leave behind their nebular remains as evidence of ancient supernova explosions.

      And that’s where most of it is! That’s where our telescopes and spectrometers find the bulk of calcium as well as other elements with masses larger than oxygen and silicon!

      The fact that we find Ca-40 in the nebular aftermaths of supernovae is interesting enough but that’s not the most interesting part of this story. 

      Most stars start off their existence in what astronomers call “The Main Sequence”. And almost all stars we see, both large and small, are members of that sequence.  All main sequence stars do just one primary thing. They all burn protons via proton-proton (helium-2) exothermic fusion reactions that throw off energy in the form of neutrinos and positrons, which is accompanied by the instantaneous transformation of proton-proton pairs into proton-neutron pairs, otherwise known as hydrogen-2 nuclei, aka deuterons. Those deuterons then fuse to form helium-4 nuclei with the release of vast amounts of additional energy in the form of heat and light. Happily, our sun, being such a main-sequence star, will continue to shed its heat and light on Earth by the slow process of proton-proton fusion for another 6-billion years or so. 

      However, eventually, as our star’s protons get depleted, our star will resort to the triple-alpha process, assembling carbon-12, and then the nitrogen-14 plus oxygen-16 fusion processes of the CNO cycle where carbon cycles through nitrogen and oxygen in exothermic fusion-assembly reactions. 

      Even these latter fusion reactions are just ordinary, semi-unspectacular steps in star maturation, but here comes the exciting part. 

      As  stars like our sun get more desperate in their search for something to fuse, they begin to poop out. Oxygen-16 nuclei burn stepwise through less and less energetic proton capture and alpha-particle fusions that involve the increasingly massive elements of silicon and up, even as far as nickel-56 and iron. But once any star gets to iron, all find themselves to be in serious trouble. 

      Fusion beyond iron is no longer exothermic. That is, fusion of elements more massive than iron consume energy. As a result, such stars begin to lose energy as iron and other heavier elements sink with iron into dense, central, solar cores. And now gravity begins to dominate over the pressure of radiation, causing all stars at this stage to begin to collapse.  

      But stars less than eight solar masses in size are not massive enough to create total collapse and devistation. Instead, the collapsing star experiences an increase in heat and pressure that has it reignite fusions with enough energy to cause smallish stars like our sun to bulge outward to a size that goes way beyond their original dimensions. 

      And that’s our sun’s fate. Six billion years or so from now our sun will rapidly bulge out and consume planet Earth! Our sun, no longer in the main sequence, will have become what astronomers call a “red giant”. Eventually, our sun’s giant sized atmosphere will drift into space to leave a star called a white dwarf, a star that fruitlessly piddles away at fusion, unable to fuse iron and ions of heavier elements. Ultimately white dwarfs just fizzle out to  energy deficient black dwarfs.

      A star transforming itself  into a red giant is certainly dramatic, but it’s nowhere near as dramatic as what happens late in the life of supergiant stars as they blast their way out of the main sequence!

      Stars much larger than eight solar masses are called supergiants. Why, because they are super huge! They may be huge and, as a result, they might not last long, but boy, do they ever die spectacularly! 

      Super giants rapidly burn through their proton source to assemble He-4, C-12, N-14, and  O-16. Then, doing anything they can to save themselves, they start a brief but magical “quasiequilibrium” assembly that includes calcium and a whole string of other elements by an alpha-particle bombardment/photon-release sequence. Did I mention the process was brief? Well, they don’t call the r-process “rapid” for nothing. All of the reactions shown below happen and come to the afore mentioned semi-equilibrium within a few short hours.              

      Si-28+He-4<=>S-32+photon  S-32+He-4<=>Ar-36+photon            Ar-36+He-4<=>Ca-40+photon   Ca-40+He-4<=>Ti-44+photon       Ti-44+He-4<=>Cr-48 +photon     Cr-48+He-4<=>Fe-52+photon       Fe-52+He-4<=>Ni-56+photon     Ni-56+He-4<=>Zn-60+photon

      But, try as they might, supergiant stars can’t prolong their life for long. Radiation pressure falls off quickly in about 24 hours. The above quasiequilibrium fails as attempts to fuse Ni-56 and larger mass nuclei drain such stars of the energy needed to defeat the pull of gravity from the mass of elements accumulating in their dense central cores. 

      Everything in a dying supergiant’s solar atmosphere comes crashing down in a critical instant to figuratively bomb and literally blow the giant’s incompressible central core to smithereens until all that is left is a super dense neutron star! And in case you wondered, as I have,  if neutrons could ever get together on their own, here’s an example. 

      The resultant blast of energyfrom a supernova  is so great that it is believed to be responsible for fusing, in an instant of endothermic chaos some of the nuclei of the quasiequilibrium into ones of larger mass. In fact, most of the truly large nuclei in the periodic table, including gold, platinum and uranium, are assembked in such cataclysms.

      But this story is about calcium. Did you notice it’s assembly in the quasiequilibrium? Obviuosly, some  of it’s nuclei escape fusion annihilation to become members of the nubular clouds left behind by supernovae.


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