Last week I described how a boulder-sized meteorite exploded in the skies over Murchison, Australia, forty years ago. The remarkable mix of organic compounds discovered in samples of the meteorite, which included amino acids, confirmed that some of the compounds required for the origin of life could have an extraterrestrial origin, as John Oro had proposed  ten years earlier. But where did the organic compounds come from, and how were they synthesized? 

    At this point I should make clear what I mean by the word organic. In common usage, something that is organic usually has to do with life processes, such as organic food. But if you hear scientists talk about organic chemistry, they are specifically referring to compounds that contain carbon, such as oil products and plastics, that have little to do with biology. The organic compounds in meteorites are carbon-based, but were not products of anything alive. Instead they are products of synthetic reactions in which carbon reacted with hydrogen, oxygen and nitrogen, but where carbon does itself comes from?

    The first hint of an answer was put forward in 1946 by Fred Hoyle, at the time a young, brash British astronomer. Hoyle was born in 1915 in Yorkshire, England, and his career is a wonderful example of how ideas become grist for the mill of science, and how bad ideas disappear into dust while the rare gems of good ideas survive scientific grinding to become touchstones for future generations of scientists. Fred Hoyle was full of ideas, and was bold enough to publish all of them. Here are some examples -- see if you can pick out the gem:

    1. The fossil archaeopteryx (a small, feathered bird-like dinosaur) in the British Museum of Natural History is a fake.

    2. Vast molecular clouds in outer space contain microorganisms that brought the first forms of  life to  the Earth.

    3. Flu epidemics occur when the Earth passes through the tails of comets loaded with virus particles.

    4. All the carbon required for life to exist is synthesized in stars.

    5. The universe has no beginning or end: The idea that the universe has a beginning is nonsense, and deserves a silly name: The Big Bang.

    6. The requirements for the synthesis of carbon are so precise that life could not have accidentally arisen. There must have been an intelligence at work to make it happen.

    If you decided that idea #4 is the gem, you got it right. It is now the scientific consensus that all the carbon circulating throughout the universe, including every carbon atom in life on the Earth, was synthesized in extremely hot interiors of dying stars (100 million degrees!) then blasted out into space when the star reached the explosive end of its life.

To understand how this could happen, it helps to have a brief primer on atomic nuclei, so if you have forgotten your high school physics, here it is again. All matter is composed of atoms, and all atoms have a tiny nucleus composed of elemental particles called protons and neutrons. Hydrogen is the lightest element, with a single proton in its nucleus, and helium is the second lightest, with two protons and two neutrons in its nucleus.

If we could somehow grab a one gram sample of the universe and use it to fill a balloon on the Earth, the balloon would float away, because the visible material in the universe is 98% hydrogen and helium. Stars like our sun are essentially a gas of naked atomic nuclei, mostly in the form of hydrogen and helium but with heavier elements in the mix as well. Helium is produced by the hydrogen fusion reactions that make stars shine, but sooner or later stars exhaust their supply of hydrogen. At that point the star collapses and the temperature increases from 10 million to 100 million degrees, then a second round of fusion takes over.

    What Hoyle realized is that at sufficiently high temperatures, two helium nuclei can fuse to form a nucleus of the lightest metallic element, beryllium, which then fuses with a third helium nucleus to produce carbon. Because three helium nuclei combine to make one carbon, this reaction is called the triple alpha process. (Helium nuclei are also called alpha particles when they are emitted from a radioactive element.) Hoyle had his revelation about the origin of carbon in 1946, but earlier theoretical models had already shown that if carbon is somehow made available, nitrogen and oxygen can be formed in a process called the carbon-nitrogen-oxygen (CNO) cycle, which turns out to be the primary source of fusion energy in large, hot stars on their way to oblivion as novas and supernovas. The CNO cycle was independently published by Carl von Weizsäcker and Hans Bethe in 1938 and 1939. They did not know how the carbon was made, and this is where Hoyle’s idea filled in a significant gap in our knowledge a few years later.  

    When Hoyle published his idea, he did not include a mathematical analysis, although he hinted at it. In 1957 Hoyle got together with William Fowler, and Margaret and Geoffrey Burbidge at CalTech to publish an article in Reviews of Modern Physics which became a classic. The review incorporated Hoyle’s original idea in a brilliant analysis of nucleosynthesis of elements in stellar interiors, and should have won a Nobel prize for someone. It did, but not for Hoyle. Discovering the treasures hidden in the scientific landscape is a chancy business, but getting credit for your discoveries is even chancier. The Nobel Prize in 1983 went to Fowler, who certainly deserved it for his many contributions, and Fowler shared the Prize that year with Subrahmanyan Chandrasekhar, who developed a theory of stellar evolution. A recent analysis suggested that if Hoyle had only included an equation in his earlier paper, he might have been awarded a share of the prize. Sir Fred had to be satisfied with a knighthood.

    To sum up, we can now account for all of the major elements of life in terms of nuclear reactions in stars, a process called stellar nucleosynthesis. The atoms of carbon , nitrogen, oxygen, sulfur and phosphorus that comprise life on the Earth were once in the center of stars like our sun, forged at temperatures hotter than any hydrogen bomb. Most of the hydrogen atoms in living organisms are as old as the universe, which burst into existence 13.7 billion years ago when time began. We are not in any way separate from the rest of the universe. Instead we borrow a tiny fraction of its atoms for a few years, incorporate them into the patterns of life, and then are forced by the inexorable laws of entropy to release them again.

The primary elements of life -- CHNOPS -- are referred to as the biogenic elements, and next week I will describe how they can gather on the surfaces of interstellar dust grains in dense molecular clouds, then react to form organic compounds when activated by photons of ultraviolet light.