Opening Lecture

Whence Carbon?

irst attempt to explain in the framework of the Big Bang hypothesis the existence of a multitude of chemical elements by George Gamow and Ralph Alpher (1948) was almost a disaster. They were able to predict that two simplest nuclei of hydrogen and helium (with mass ratio 3:1) plus a tiny amount of lithium, beryllium and boron were created in the terribly hot primordial soup (ylem) within few minutes after the Big Bang. However, they failed to find a way towards creating nuclei of carbon and of all other elements of the Mendeleyev table. The disaster delayed the general acceptance of the Big Bang for more than a decade.

The rehabilitation of the hypothesis was a rather painstaking process started by Edwin Salpeter´s discovery (1951) of a strongly temperature-dependent nuclear reaction between two nuclei of helium (alpha-particle) that may enhance creation of beryllium nuclei due to energy resonances. It turned out that accidentally the energy of ground state of beryllium nucleus is almost exactly equal to the sum of energy of two alpha particles. Then Fred Hoyle predicted (1957) by using the so-called anthropic principle that there must exist another resonance when beryllium nucleus meets another alpha particle because the ground state energy of triple-alphas might be close to the excitation energy of carbon nucleus. This prediction was subsequently confirmed by measuring carbon energy by William Fowler in Kellogg Nuclear Laboratory.

In 1957 Fowler, Hoyle and his students Margaret and Geoffrey Burbidge wrote a seminal paper (B2FH) Synthesis of the Elements in Stars. It appears that first generation of massive stars made of hydrogen and helium exhaust rather quickly (million years) their supply of hydrogen in a thermonuclear transmutation to helium and then they started to collapse under their own gravity. The temperature of the core of an aging star rose up due to rising pressure until helium was ignited by a Salpeter thermonuclear reaction and carbon was created due to the enhancement of triple-alpha process. A similar chain of events occurred after the exhaustion of helium in the stellar core etc., until the series of highly efficient thermonuclear reactions ended by creating iron (and a bit of cobalt and nickel).

Massive stars with onion-like layers of burned hydrogen at their surfaces and deeper layers of helium, carbon,...manganese, iron experienced a free-fall of their outer layer upon almost incompressible central core where temperatures reached billions of kelvin. The outer mantle of such star suffers from turbulent inward/upward motions and shock waves. These violent events result in tremendous explosions called supernovae. Upward motions prevail and external layers of debris are ejected by velocities of 15 thousand km/s or more into interstellar space. During this rather short (one day) event the rest of the nuclei of periodical table (copper up to gold and uranium) are created in the turbulent matter by subsequent capture of free neutrons by heavy nuclei. Therefore the total amount of these heavy elements in the Universe is much lower than the fraction of elements created by thermonuclear reactions in stars that live million to many billion years.

In this way carbon and all other elements are dispersed into the interstellar medium and cooled to temperatures below the freezing point. It is obvious that in that medium neutral atoms of carbon are ubiquitous and may form first simple molecules like CO and CO2. However, soon after more complex molecules of organic compounds were synthesized. It appears that the first building blocks of the life were available about 200 million years after the Big Bang.

Large interstellar clouds with mixture of primordial H/He cold soup and ingredients of heavier elements became also cradles for formation of a second generation of stars, slightly enriched by the material created within the first-generation (pure H/He) stars. Almost the same process lead to the birth of third-generation of stars with similar outcome – supernovae that again spread doubly-enriched debris into the interstellar space. Thus, it is now well established by theory and observation that our Sun is a dwarf star of this third generation of stars. The Sun and our solar system is 4.6 billion years old and the relative amount of elements heavier than H and He is still only 2 % of the solar mass. However, solar mass is so low that our life-giving star will end its life without being a supernova. Thermonuclear production of heavier elements in dwarf stars ends either at carbon, or at oxygen, in special cases at neon. It turns out that a dwarf star with final mass less than 8 solar masses would collapse into dense stars called white dwarfs after the cessation of thermonuclear reactions in their interiors.

Planets including our Earth were created almost simultaneously with the Sun from the primordial gaseous and dust disc encircling the Sun. Due to radial gradient of temperatures in the disc the condensation of various compounds was distance-dependent. Selection effects were strong near the Sun where rocky planets in almost identical plane are orbiting Sun but their chemical composition varies because of different temperature during their accretion. Large outer giant planets (Jupiter to Neptune] have chemical composition of their extended gaseous envelopes much more similar to solar abundances.

The Earth is very special planet because of its substantial metallic core that is partly melted. Thus a dynamo effect creates rather strong and stable magnetic field that is essential for protecting the life against penetrating cosmic rays and charged particles of solar wind. We have very low amount of CO2 in our atmosphere in comparison with Venus where the atmosphere is almost pure CO2 creating devastating greenhouse effect; thus the average surface temperature of Venus is even higher than that of Mercury, although Mercury is more than twice closer to the hot Sun than Venus. Terrestrial vegetation employs CO2 for gaining energy through photosynthesis; thus also all fauna is critically dependent on this fundamental reaction.

Carbon is used also as a principal burning component of coal, wood, oil and gas. Expensive diamonds are admired but they also have important technical application in grinding and in extreme high-pressure devices like diamond anvils. Discoveries of carbon fibres and of highly symmetric fullerenes as well as of an almost miraculous 2D material called graphene show that carbon is very unique and may still hides many surprises. It is without saying that all living organisms contain carbon in every letter of their genetic code as well as in every cell. Thus, we may certainly thank very much Mother Nature that she took care on creating carbon from helium in all stars throughout almost all existence of the Universe. Every carbon atom in our bodies started its existence in the deep interior of an anonymous star and after a very long journey landed in our solar system and, eventually, at the Earth. Thus, we all are created from the stardust – this is not a poetical hyperbole but a hard truth.