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What is astrochemistry?

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  • Chemistry in stars. The chemical composition of most stars is dominated by hydrogen, with helium in second place and the remaining elements a long way behind. The relative proportions (or abundances) of the elements are quantified either by the number of atoms or the mass involved. In terms of mass, average material from outer layers of the Sun and the rest of the Solar System contain 70.7% hydrogen, 27.4% helium and 1.6% of all the other elements. These three quantities are called mass fractions and are conventionally labelled X, Y and Z. In terms of the number of atoms, hydrogen dominates even more, with 92.0% hydrogen, 7.8% helium, and all the rest just 0.12%. Elements other than hydrogen and helium are often termed the HEAVY ELEMENTS.

  • These figures, which are representative compositions throughout the Universe, are changed by NUCLEOSYNTHESIS reactions inside stars. Thus at the centre of the Sun, the abundance of hydrogen has currently fallen to 38% by mass, and that of helium has increased to about 60%. Stars that have completed their MAIN-SEQUENCE lives, will have cores of almost pure helium. Solar mass stars will go on to synthesize carbon and oxygen, and higher mass stars will create the elements all the way to iron. The remaining elements are produced during SUPERNOVA explosions.

  • Nucleosynthesis implies that hydrogen is continually being converted into heavier elements, and so abun
  • dances change slowly. But only a few per cent of helium by mass can have been produced this way during the lifetime of the Universe. Most helium, therefore, was produced during the early stages of the Universe (see COSMOCHEMISTRY). The remaining elements have been produced inside stars, and then ejected to mix with the interstellar medium through stellar winds, supernova explosions and so on. The mass fraction, Z, of the heavy elements is therefore increasing with time. The oldest stellar POPULATIONS, known as Population II stars, found in the galactic nucleus and halo and in globular clusters, have values of Z around 0.5%. Stars in the disk of the Galaxy are younger, have a higher proportion of heavy elements and are called Population I stars. Thus the Sun, which formed about 4.5 billion years ago, has Z = 1.6%, while stars forming today have Z ≈ 4%. 

  • Some stars are exceptions to the norms, the most important being the WHITE DWARFS. These stars, which are at the ends of their lives, have lost their outer layers and so have their cores exposed. The cores contain the products of nucleosynthesis, and thus the composition of white dwarfs ranges from helium-rich through relatively pure carbon to calcium. WOLF–RAYET stars subdivide into the WN stars, formed of helium and nitrogen, and the WC stars, within which helium, carbon and oxygen predominate. T TAURI stars have an over-abundance of lithium. AP STARS are over-abundant in elements such as mercury, strontium, silicon, europium, holmium and chromium. It is thought that these latter peculiarities occur just in a thin surface layer and are perhaps brought about by diffusion. Beneath that thin layer the remaining parts of the star are of normal composition. CARBON STARS have over-abundances of lithium and carbon, while S-type stars additionally have over-abundances of zirconium and yttrium. Technetium has been detected in both carbon and S stars, and since all isotopes of technetium are radioactive with short half-lives, it must have been produced inside the stars. Nucleosynthesis products are thus being brought to the surface in these stars. There are other anomalies to be found, and such stars are termed CHEMICALLY PECULIAR STARS (CP). 
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