Carbon, Hydrogen, Oxygen, Phosphorus, Sulfur, and Nitrogen—The Elements of Life
Almost all the elements in the periodic table, with the exception of hydrogen and lithium, are generated in stars and then “recycled” by being integrated into cosmic objects of the next generation. This describes the overall chemical evolution in the Universe. As has become apparent from the research into metal-poor stars, the amount of the elements present in the early Universe did not suffice, by a long shot, to yield a planet such as Earth, upon which life could eventually develop. Earth, composed of iron, oxygen, silicon, magnesium, and other elements, could actually form only once the Universe had sufficient amounts of these particular elements. However, we know that the right amount was produced sometime within the first ~9 billion years after the Big Bang. Otherwise the Sun and the Solar System could not have formed from the presolar nebula 4.6 billion years ago.
Chemical evolution is one of the most complex processes in the Universe and even the current best models and simulations can only coarsely describe exactly how it proceeded in detail. It is still unclear at what point the Universe was “mature” enough, from the chemical point of view, for the emergence of the first planets. In addition, planet formation itself is a fairly complicated and complex process. Moreover, all planets of the Solar System have different chemical compositions, showing that there is no simple “recipe” for planet formation.
Current extensive searches for planets around other stars are being followed with excitement and suspense by the public at large. Stars resembling the Sun need to be observed over long periods of time to reveal whether they host planets. In their selection, extremely metal-rich stars are mainly sought whose metallicities are higher than that of the Sun. A large amount of metals present in the birth gas cloud increases the chance of finding a star being orbited by one or even several planets, one of which could perhaps host life. In the past few years, the Kepler space telescope has already discovered hundreds of planets, although none among them is very similar to Earth. Finding the right one seems, however, only a matter of time. Hence, even for the search for Earthlike planets, the analysis of the chemical composition of the host stars is of great importance.
How did those elements crucially involved in the emergence of life as we know it evolve? The most important elements in this context are carbon, hydrogen, oxygen, phosphorus, sulfur, and nitrogen. The human body is mainly made of these elements. Due to the large percentage of water in the body, oxygen takes the lion’s share of the body’s mass, at 61%. Carbon comes in second at 23%, then hydrogen at 10%, and finally nitrogen at 3%. The remaining 3% constitutes many different trace elements. The chemical characteristics of carbon make it combine easily with other elements to form molecules, particularly with hydrogen, nitrogen, and oxygen. This way complex molecules needed in all organisms, such as proteins, nucleic acids, carbohydrates, and fats, can be formed. Consequently, these three elements are extremely important for life, and for maintaining a breathable planetary atmosphere containing enough oxygen to sustain life.
Phosphorus and sulfur also play an important part in the human body. Both elements fulfill fundamental tasks in every cell by enabling the formation of important molecules. Without phosphorus there would be no DNA and RNA molecules. The metabolic exchange of energy in a cell does not operate without phosphorus. So what is known about the cosmic history of these two elements? First of all, it is difficult to detect phosphorus and sulfur in metal-poor stars. Some of their absorption lines are hidden in the near-i nfrared region among many lines of H2O (i.e., water) and other molecules. These are generated in the terrestrial atmosphere, not by the star itself. Such contamination complicates precise line measurements in the spectrum. Moreover, the generally weak lines are not detectable in extremely metal-poor stars. However, some phosphorus lines are present in the UV spectral range for which data can be obtained only with space telescopes. New results have recently shown that phosphorus can be measured from the UV lines even in extremely metal-poor stars, enabling a complete reconstruction of the cosmic evolution of this element.
Phosphorus can be made from silicon atoms by neutron-capture. It occurs mainly in later burning stages during the evolution of massive stars. Newly synthesized phosphorus is ejected into space by the subsequent supernova explosion. As for sulfur, it technically is an a-element and is thus built up, just like magnesium or titanium, by the capture of helium nuclei. Accordingly, sulfur is likewise produced in core-collapse supernova explosions and then dispersed into the Universe. Consequently, phosphorus and sulfur have been present in the Universe since the earliest times. As they cannot form in low-mass stars or in other burning or evolutionary stages, their production rates have changed little with time, if at all.
Let us finish with an amusing little story. The main task when working with metal-poor stars is to determine the metallicity of each object. Can we do the same for the human body, too? A colleague once posed this question, asking his audience to raise their hands to indicate whether they thought that human beings are metal-poor or metal-rich. Both sides received 50% of the vote. Any body’s metallicity is, of course, defined by the ratio of iron to hydrogen. Our iron is in our blood, and hydrogen is a component of water, which makes up more than half the mass of our bodies. Therefore, we are metal-poor compared to the Sun—we have a metallicity of [Fe/H] = -0.5, which is three times less iron than hydrogen compared to the solar ratio.