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Whether you are an omnivore, a vegetarian, or vegan, carbon atoms have traveled long and far from the cores of recycled stars in order to keep you alive. Your nourishment begins with photosynthesis, the process of green plants turning sunlight and atmospheric carbon dioxide into glucose, the source of energy for all organisms. Whereas plants use carbon to make our food, it also turns out that scientists have learned to put carbon to use in many other ways. This is the story of one of those uses.
The story of carbon is complex, but it is interesting because it is our history. Stars are our most distant ancestors.
When our universe began with the Big Bang some 13.8 billion years ago, it produced three fundamental particles: electrons with a negative electric charge and two types of quarks, up quarks with a positive charge and down quarks with a weak negative charge. These 3 particles became the fundamental elements of atoms. Within a few minutes after the Bang, the two types of quarks, came together to form protons and neutrons. Two up quarks and one down quark formed a positively charged proton, while two down quarks and one up quark formed neutrons which were electrically neutral.
For many years the universe was too hot for any interaction between positive protons, neutral neutrons, and negatively charged electrons. It was not until 380,000 years after the Big Bang, that the universe had cooled enough to allow protons and electrons to join together to form the first and most basic atom, hydrogen, with one negatively charged electron and one positively charged proton. The atom was electronically neutral and so small there was no room for a neutron in its nucleus.
Once the process of electrons and protons attracting each other to form hydrogen atoms got started, the number of atoms multiplied rapidly. Within a few million years, hydrogen atoms made up most of the universe, and still do today. Eventually there were so many, that they bumped into each other forming small clumps. As the clumps grew larger, their gravity increased and began pulling in more and more atoms to the point that the high pressure in the center of the mass of atoms created so much heat that the hydrogen atoms, each with their one electron and one proton, began fusing together to creating larger helium atoms with two protons and two electrons. The larger atom was able to fit in two neutrons as well. With the onset of the hydrogen to helium nuclear fusion, the clumps of atoms became stars. We can see star formation, that is, the onset of the nuclear fusion process going on inside stars across the universe today. For example, one area where this is taking place is the Orion Nebula, a large cluster of hydrogen and other atomic elements, 1,350 light years away, where new stars are being created.
This fusion process, called stellar nucleosynthesis, continues inside stars as long as sufficient heat and pressure are maintained, which is usually millions or billions of years depending on the size of the star, and as long as the hydrogen fuel lasts. Our sun, considered a medium sized star, has been fusing hydrogen for about five billion years, and has about five billion more years of fuel left in it. Larger stars use up their hydrogen quicker, sometimes in just a few million years. Betelgeuse, in the Orion constellation, for example, is a red giant star some 880 times larger than our sun, and at 10 million years old, is already considered an old star in the last phases of its life.
The complex process of hydrogen stellar nucleosynthesis not only produces another type of atom, helium, it also produces a photon, a tiny packet of electromagnetic energy that exhibits both wave and particle traits. All photons start life as gamma rays, very high frequency electromagnetic waves/particles with enough energy to be dangerous to humans. But the interior of a star is so dense that gamma ray photons bounce into millions of protons on their way out of the star’s core and most of them lose energy and frequency along the way. A gamma ray’s zig zag journey from the core to the surface of a large star could take thousands of years.
Bouncing from proton to proton, a large percentage of the gamma ray photons lose energy and frequency but not speed. Thus, the spectrum of frequencies that leave the surface of a star can range from low frequency radio waves to extremely high frequency gamma rays, all of them traveling at the speed of light. In the middle of this electromagnetic spectrum of frequencies is found the visible spectrum, the range of frequencies that can be seen with the human eye. Our eyes can perceive light frequencies from the low frequencies just beyond infrared to higher frequencies just short of ultraviolet. Frequencies lower and higher than these are all around us but invisible to us. Without nucleosynthesis, stars would not produce photons of light and the entire universe would be dark, and life as we know it would not exist.
In our sun, photons have already traveled several years and some 430,000 miles, the radius of the sun, before they escape from the sun’s outer surface. A small percentage of them wind up heading toward Earth. Then, traveling unencumbered at 186,000 miles per second, the last 93 million miles of the photon’s journey to us takes only 8.3 minutes. When photons in the visible part of the electromagnetic reach our eyes, the rods and cones in them see them as light.
Fortunately for us, the dangerous high frequency gamma rays and X rays that make it out of stars are absorbed by molecules in Earth’s upper atmosphere before they reach the surface. Many ultraviolet rays do get through, however, and that is why we need sun screen in the summer. The lower frequencies, from radio through violet, with longer wavelengths, get through but do not harm humans.
For most of its life a star maintains equilibrium, that is, the energy generated by the fusion process inside it creates enough outward pressure so that the force of gravity will not cause it to collapse. Just imagine the sun’s enormous amount of gravity, strong enough to hold the planets in place, being countered by the pressure created by thousands of hydrogen bombs exploding inside it every minute.
Each young star begins nucleosynthesis with a finite amount of hydrogen available for fusion. So eventually, after millions or billions of years, a star will have fused most of its hydrogen atoms into helium. When the fusion process reaches a low enough level, the internal pressure drops and the external gravitational pressure causes the star to collapse leading to the next phase of stellar nucleosynthesis.
The sudden collapse of a star’s outer material into the core hits with such force that it causes the material to bounce back from the center, greatly expanding the radius of the star into what is called a red giant. At this point it is relatively cooler at its outer edge, made mostly of hydrogen, helium, and small amounts of other elements, but still very hot at its core which is now mostly helium. In about 5 billion years, our sun will become a red giant and expand to be beyond the orbits of Mercury and Venus and possibly Earth. Life on Earth will more than likely cease to exist. At this time, there are several red giant stars visible to us in the Milky Way, the most famous of which is Betelgeuse in the Orion constellation. It is large enough that it is expected to go supernovae in about one million years.
The red giant phase can last millions of years during which the core of the star heats up to the point that the fusion process reignites causing the helium in the star’s core to begin fusing into heavier atoms. It is at this point that we get carbon.
In a sequence of events called the triple alpha process, 3 helium atoms each with their two protons, two neutrons, and two electrons, are fused into carbon atoms with 6 protons, 6 neutrons, and 6 electrons. Add another helium atom and you get an oxygen atom with 8 protons, 8 neutrons, and 8 electrons and so on. However, our sun, being only a medium sized star, does not get hot enough to generate much fusion beyond oxygen. Small amounts of some heavier elements from neon to iron are made but not in large quantities. In most stars, when the fusion process reaches the element iron, fusion ceases.
In about a billion years after its red giant stage, our sun will cool and collapse into a very dense type of star called a ‘white dwarf’ not much larger than the Earth and made mostly of carbon and oxygen atoms. It will then begin to cool into a ‘brown dwarf’. At that time, with the sun no longer giving off heat and light, our solar system will cease to exist as it is today. I suppose one could call this the ultimate climate change.
In the world of stars, size matters. Stars many times larger than our sun can explode into gigantic supernovae that generate enough heat and pressure in their core to produce many other elements heavier than iron, such as gold, platinum, and uranium. When the supernova explodes, these elements are scattered throughout the universe and could wind up being part of a planet or a living creature. The elements in our bodies came from exploding supernovae. In the largest stars, hundreds or thousands of times larger than our sun, the force of gravity can be so strong that they can collapse into neutron stars or even become black holes.
This process has been going on across the universe in billions of stars for many years before the coalescing of our sun and the formation of our solar system. In that time, an enormous amount of carbon, which is considered the most important element for life on Earth, has been created and dispersed throughout the universe, laying the groundwork for carbon-based life here on Earth and possibly other planets as well. This basic carbon atom is called carbon 12 since it contains 6 protons and 6 neutrons.
Without the carbon atom there would be no life on Earth or just about anywhere else it may happen to be. It is one of the four essential atoms that make up all living things: carbon, hydrogen, oxygen, and nitrogen. Carbon primarily acts as a bonding agent attaching itself to other atoms to create an innumerable array of molecules, both organic and inorganic. As science writer Natalie Angier put it: “Water may be the solvent of the universe, but carbon is the duct tape of life”.
Carbon is unique in that it has the ability to form more compounds than all the other elements combined, hundreds of thousands of them. For example, when electrons from carbon electrons form covalent bonds with the electrons of 2 oxygen atoms they make a molecule of carbon dioxide, CO2, which plants absorb from the air and use in photosynthesis. In fact, carbon can form so many bonds with other atoms that an entire field of study called organic chemistry is based on the numerous compounds and molecules that carbon forms.
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We are familiar with how nature uses carbon in photosynthesis to make glucose. But modern science also has learned many ways to make use of carbon. One of these is to determine the age of fossils using a process called carbon 14 dating.
Carbon 14 is created when neutrons in cosmic rays from our sun or other areas of our galaxy come into Earth’s upper atmosphere and collide with nitrogen 14 atoms, each of which contain 7 protons and 7 neutrons. This neutron-nitrogen collision knocks out a proton and adds a neutron to the nitrogen atom. The nitrogen atom then has 8 neutrons and 6 protons, and because of its 6 protons, it is now made into an isotope of carbon called carbon 14. The carbon 14 atom, which had been stable carbon 12, does not now have an equal number of neutrons and protons. Atoms without the same number of neutrons and protons are unstable and attempt to reach stability by ejecting a proton, neutron, or an electron. The process of particle ejection makes them radioactive.
These carbon 14 isotopes, which are only a small percentage of all the carbon in the atmosphere, then bond with oxygen atoms to form slightly radioactive molecules of carbon dioxide which are absorbed by Earth’s plants along with the regular carbon 12 atoms. Plants then combine the radioactive carbon dioxide molecules with water and photons from the sun in the process of photosynthesis that produces the glucose the plant needs in order to grow. When animals eat plants or other animals that eat plants, their bodies take in both the carbon 12 as well as the radioactive carbon 14 atoms that had been absorbed by the plant. Through this process, all living things become slightly radioactive and remain that way for many years even after they die as long as any bone or tissue remains. Scientists have learned to put this small amount of radioactivity to good use.
Although we do not think of ourselves as being radioactive, all organisms give off small amounts of carbon 14 radioactivity. In an organism that is still alive, a carbon balance is achieved when the radioactive electrons emitted by carbon 14 atoms are continually replaced by the carbon electrons in the food and air the organism takes in.
In February 1940, two scientists at the University of California at Berkeley, Martin Kamen and Sam Ruben, noticed that even dead plants continue to give off radioactive electrons from their carbon 14 atoms through a process called beta decay. In this process, when electrons are emitted, one of the neutrons in the carbon atom changes to a proton; thus, beta decay is a process that turns an unstable carbon 14 atom with 6 protons and 8 neutrons back into a stable nitrogen atom with 7 protons and 7 neutrons.
The scientists also determined that with sensitive instruments the amount of radioactivity being emitted could be measured. They could tell that the carbon 14 atoms emitting the radiation had a long half-life but could not at that time determine how long it is. Half-life is a term that refers to the amount of time for a radioactive material to lose one half of its radioactivity.
Going from the research done by Kamen and Ruben, in 1945 chemist Willard F. Libby, then at the University of Chicago, determined that by using a device that accurately measured beta decay, the half-life of carbon 14 radiation emitted from any carbon-based organic material that is no longer living is approximately 5,730 years. That is, when the organism dies and stops taking in more food, the carbon 14 radiation continues but decreases at a measurable rate, half of it every 5,730 years.
For example, in a dead plant, bone fragment, or piece of wood, half of its radiation is given off in the first 5,730 years after it dies. Half of that remaining half is given off in another 5,730 years, then half of the remaining radiation is given off in another 5730 years, and so on until the amount of radiation becomes too small to measure. Thus, doing the math, one finds that Carbon 14 dating is accurate up to an age of about 50,000 to 60,000 years. Matter older than that, such as dinosaur bones, and rocks, since they can be millions of years old, require a different kind of dating.
The accuracy of radioactive carbon 14 dating, or radiocarbon dating, has been tested by comparing the carbon 14 date of an object with written records that describe how old an object is. For example, when carbon 14 dates were compared with written records describing when an Egyptian mummy was buried, the carbon 14 date proved to be accurate.
Given that Carbon 14 dating is fairly accurate up to an age of about 50,000 years, it is used extensively by anthropologists and archaeologist to determine the age of human remains and artifacts. Its value in dating matter was recognized early, and in 1960, Willard Libby was awarded the Nobel Prize in chemistry for its discovery.
It is interesting to note that the radiocarbon created by thermonuclear explosions is identical to radiocarbon created naturally in the atmosphere. This has resulted in higher-than-normal levels of carbon 14 in plants and animals, and people, who have lived since the first nuclear bomb test in July 1945. The United States, Russia, England and other countries tested dozens of nuclear weapons during the 1950s and early 1960s raising the carbon 14 level in earth’s atmosphere by about 3%. This increased amount of radioactive material was absorbed by plants and put into our food chain. Thus, carbon 14 measurements made after 1950 tend to be inaccurate. However, since the signing of the Nuclear Test Ban Treaty in October 1963 ended all atmospheric testing, radiocarbon levels have been slowly returning to normal pre nuclear bomb levels. We see then that radioactive carbon 14 is with us from the beginning of our lives and remains with us in our teeth and bones until our 50,000th birthday.
