The Formation of Elements
What can we tell from the distribution of elements?
Quite a lot. Just by looking at the rankings listed above, and not even paying close attention to the percentages, we can deduce the following:
From the Bacteria/Human rankings:
From the Crust/Bacteria rankings:
- There is virtually no elemental difference between bacteria and us (in fact bacteria make up a large percentage of our dry weight).
- The one element, calcium (Ca), is different by a factor of 6 and is excreted from bacteria due to its limited usefulness. This exclusion led eventually to calcareous shells (shellfish) and masses, which led to bones (fish), allowing greater mobility and eventually the ability to live on land (land animals, including us). We have a skeleton – bacteria don’t.
From the Solar/Crustal rankings:
- Carbon is vastly preferred by living organisms to silicon (Si), even though the latter is much more abundant in the Earth's crust. Carbon has more variety in its interactions and they are not too hard to break, lending flexibility to its chemical molecules. The carbon ring is central to organic chemistry. Silicon is not nearly as useful, so it is doubtful if we will ever encounter silicon-based naturally-evolved life (we are excluding robots here) elsewhere in the universe.
- Carbon, hydrogen, oxygen, and nitrogen (the most abundant materials in living materials) are all high in the Solar list, although C and N are not high in the Crustal list, as they come to us from the atmosphere.
- Earth is neither a star nor a gas giant planet: all the gaseous hydrogen and helium have risen to the top of the atmosphere and been sputtered off into space by incoming radiation – the Earth’s gravity is not strong enough to capture or retain these gases this close to the Sun. Some hydrogen remains locked in chemical compounds, but helium does not form chemical bonds and is essentially all lost. Likewise, neon, another “noble gas” has been lost from Earth.
- There is surprisingly little carbon in Earth’s crust. It is largely in carbon dioxide gas in the atmosphere, sequestered away in limestones or in living organisms. The same must hold for nitrogen, to a lesser degree.
- Aluminum is relatively abundant in the Earth's crust; together with silicon it is a light element that makes light minerals that float over the heavier materials of the mantle and make up much of the crust. The result of these density differences is plate tectonics, which appears to be vital in keeping the planet habitable.
Now, as we have seen in the table above, only 9% of our bodies are made of hydrogen; carbon (C), oxygen (O), nitrogen (N), sulfur (S) and phosphorus (P) make up 90%, while the remaining 1% consists of various trace elements. The Big Bang is theorized to have only created hydrogen, helium and traces of lithium; where did these heavier elements (astronomers call them all "metals") come from? They were created in the nuclear fusion fires of past generations of stars. We are all made of star dust.
The M16 “Eagle Nebula” – a birth place for new stars (Courtesy: NASA
The cosmic cycle consists of interstellar clouds collapsing, the formation of stars, the death of stars with dust and enriched matter going back to interstellar clouds can be imagined from the Space telescope images of the star-building regions of the Orion Nebula and similar places. Places in our galaxy with the oldest stars include stars with very low levels of heavy elements, but no stars have yet been discovered without any heavy elements at all. We no longer see stars being formed from primordial matter.
Stars like our Sun fuse together four protons (hydrogen nuclei) to make one helium nucleus, with the release of about 27 million electron volts (MeV) of energy per helium nucleus formed. This is essentially the same reaction that takes place in a hydrogen bomb - only the Sun’s powerful gravity keeps it from exploding. This is also the goal of fusion energy reactors, which may one day provide energy (probably after we run out of fossil fuels). This so-called “proton-proton chain reaction” (often abbreviated as the “pp cycle”) provides 98.5% of our Sun’s energy, which powers most life on earth and all of the forms of life with which we are most familiar.
The Sun gets the remaining 1.5% of its energy from a complex web of interacting nuclei, termed the “CNO cycle.” Stars more massive that our Sun (signified with the letters F, A, B and O) get most of their energy from the CNO cycle while smaller stars (signified with the letters K and M) get most of theirs from the pp chain. Below is a diagram illustrating most of the reactions taking place in stars. The first line represents the pp cycle, the dominant reaction in stars like our Sun and in the great number of stars smaller and cooler than it. The series of six steps shown in the second line of the figure represent the CNO cycle that is found in stars more massive than our Sun. The later lines in the diagram indicate the reactions occurring in aging stars that have used up most of their “easy” hydrogen fuel and have accumulated heavier elements. For example, a star that is about 20 solar masses burns hydrogen for 10 million years, helium for 1 million years, carbon for 1000 years, oxygen for 1 year, silicon for a week, and iron for less than a day. After fusing atoms to create iron-group elements the energy released reaches its peak – after this no more energy can be extracted. If the mass of the star is large enough, it will then collapse and then rebound, resulting in a supernova whose intense particle flux adds neutrons and protons into the atomic nuclei and produces all the higher elements up to uranium.
Since we are on the topic of stellar evolution, a brief note ought to be made on so-called “black holes.” Black holes are theorized to come about when a massive star uses up the last of its nuclear fuel and collapses due to its own gravity into something called a “singularity” – a place where the pull of gravity is infinite and where space and time cease to exist. Nothing can escape the gravitational pull of a black hole, not even light itself. Albert Einstein’s General Theory of Relativity predicted the presence of black holes although they have not been detected directly. Some indirect evidence of black holes includes intense x-ray emissions from gases that are being sucked into a black hole and being heated by the intense gravitational pull, as well as the radiation emissions resulting from the rapid motion of gas and dust surrounding a black hole. Black holes are thought to exist in the center of many galaxies.
The pp and CNO cycles in the Sun.