There are only two basic ways in which abundances, or the relative quantities of each clement in the Universe, may be determined: the direct chemical analysis of samples, and deduction from analysis of absorption and emission-line spectra. We include here the parts of the spectrum outside the visible range, because some of the most exciting discoveries about the interstellar medium have resulted from microwave and radio astronomy.

Direct chemical analysis has so far been possible only for the surfaces of the Earth and Moon, and for meteorites . Its great advantage is the accuracy with which the composition of any individual sample can be assessed under laboratory conditions; even the relative amounts of different isotopes can be determined, However, finding the compositions of the Earth and Moon as whole objects is more difficult because of the chemical separation that has taken place since they formed. The Earth’s interior must have a composition different from its crust as the average density of the Earth (5500kgm-3) is considerably greater than that of typical rocks in the crust (2400 kg m-3). The existence of several layers in the Earth’s interior is confirmed by studies of the way earthquake waves travel through the Earth. The core must be predominantly iron and nickel to account for the average density of the planet, and the presence of a metallic, core, is confirmed by the existence of the Earth’s magnetic field. The layer between the crust and the core, the mantle, is probably made of olivine rock, a mineral of iron arid magnesium silicate. 98 per cent of the crust is composed of only eight element: oxygen, silicon, aluminium, iron, calcium, magnesium, sodium and potassium.

Apollo astronauts left seismometers on the Moon. Studies of the weak naturally occurring moonquakes, and artificially-created once show that the Moon too has several layers, but no metallic core. The surface rocks that have been analysed are similar in nature to terrestrial rocks, but the\ have a significantly different composition, in particular. more titanium, uranium and rare earth elements are present. This is one example of abundance data teach¬ing us about astronomical history. In the face of the composition difference, it seems unlikely that the Moon was part of the Earth, except perhaps in t he very earliest phase of t he Solar System.

The Earth, Moon, and inner planets have very little of the lightest and volatile elements, presumably because these elements (e.g. hydrogen and helium) rapidly escape into space from a small body under the influence of solar heat. The giant planets, on the other hand, are further from the Sun so the heating is weaker and they are largely composed of the light elements. Because of the different evolutionary history of each planet, it is not possible to use planetary abundances to gain a completely precise picture of the abundance of the elements in the material from which the Solar System formed. For this we have to turn to the Sun. The composition of the Sun itself is probably the best clue to the nature of the material from which the Solar System formed. The composition of the outer layer that is responsible for the Sun’s absorption-line spectrum remains unchanged despite the nuclear processes going on in the interior. From the lines in the spectrum of the Sun it is possible to deduce the relative composition of the solar atmosphere. The solar- abundances are confirmed by results from a surprising source, namely the relative proportions of the elements found in certain meteorites: one type of meteorite, the CARBONACEOUS CHONDRITES, contains 20 per cent water and compounds of volatile elements, the presence of which suggests that these meteorites have never undergone heating, so that they presumably have their original composition. Their name arises from the small spherical bodies, or chondrules, that they contain, and only a handful have ever been discovered.

When the effects of heating and consequent chemical separation have been taken into account, the results from the Sun, meteorites, Earth, Moon and planets are not remarkably different, and the abundances of all the elements in the Solar System have been drawn up on the basis of this data. plotted on a graph against atomic mass, A. The general features of this curve tell us a great deal about how the elements could have been created. Note that there is a general decrease in abundance towards heavier elements. On top of this general trend, there are two particularly notable features. One is the great dip corresponding to the light elements lithium, beryllium and boron. The other is the spike around A— 56; this is the so-called IRON PEAK.

Perhaps it is surprising that most stars whose spectra have been examined have compositions similar to the Sun’s. Some of those that do not can be explained by circumstances peculiar to the particular stars. However, the metal-deficient stars form an interesting group. These are stars that have significantly less of the elements heavier than helium (metals) than the Sun and they have been found to belong exclusively to Population II. These are all old stars in globular clusters, or in high-velocity orbits perpendicular to the plane of the Galaxy. Then* is a clear indication that younger Stars have richer compositions, though even old stars near the galactic centre are not metal deficient. The composition of a star depends on both its age and its location in the Galaxy, a reflection of the changing corn posit ion of the Galaxy as a whole as time progresses

It not possible to gain a detailed picture of the chemical composition of other galaxies, but we can infer from their spectra that they cannot be grossly different from our Galaxy The ratios of the abundances of some elements can be determined , especially for galaxies with emission lines in their spectra, and these do not show spectacular deviations from the Solar system abundances

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