More than twenty different elements have now been detected by their visible or, more commonly, by their ultraviolet interstellar absorption lines. Most of these elements exist both in their neutral and their ionized state; sulphur, for example, has been identified as S°, S+, S++, and S++,. Since the number of each kind of atom can be calculated, from the depth of its absorption lines, we can compare the constitution of the interstellar medium with that of the stars.

The two most abundant elements are, of course, hydrogen and helium. Together these elements make up over 99 per cent by mass of the interstellar medium, as they do in most stars. It is in the abundances of the rarer, heavier, elements that differences be¬tween the interstellar medium and stars are found. The most care¬fully studied section of the interstellar gas is that in the line of sight between the Earth and the star £ Ophiuchi. Here it is found that almost all the elements are much rarer as compared with hydrogen than is the case in stars. The interstellar gas contains only a tenth of the sodium, a hundredth of the iron and less than a thousandth of the calcium that stars do.

What is the reason for this difference ? The answer is that most of these ‘missing elements’ are in the interstellar grains, which we take no account of in studies of absorption lines. £ Ophiuchi has a visual extinction of about one magnitude. Calculations show that if all the missing elements from the gas are combined chemically to produce a mixture of suitable grains, the dust that would result would provide almost exactly the observed amount of extinction. The actual ratio of gas to dust in the interstellar medium is about 200:1 by mass. This ratio is fairly uniform over a large scale, but anomalously high or low ratios are sometimes found where grain destruction or grain formation processes are particularly lively.

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