The first molecule to be discovered in space at radio wavelengths was the hydroxyl radical (OH) in 1963. Starting in 1968, there then came a rush of new molecule discoveries. Most of these were made at wavelengths of less than one centimeter by groups of astronomers using the 11 –metre radio telescope of the US National Radio Astronomy Observatory at Kitt Peak, Arizona. In some cases molecules were identified on the basis of a pattern of spectral lines, but sometimes only a single transition has been seen. In most cases the lines are observed in emission rather than absorption.

The interstellar molecules seen by radio telescopes up to early 1977. Although a few inorganic molecules have been found (e.g. NH3, SiO, H2S), most of the molecules are organic. They range in complexity from diatomic molecules such as CH, CO, CN and CS to molecules with as many as nine atoms (e.g. (CH3)2O, C2H5OH). The number and complexity of the molecules discovered in the early seventies was a great surprise to astronomers and chemists alike, who up to that time had believed that none but the simplest molecules could form and survive in space.

Interstellar radio spectroscopy has now become so sophisticated that it is in some cases easier to study the spectrum of a molecule in space than in a laboratory. Such a case is illustrated in figure 13.13; on Earth the N2H+ion is so reactive that at the time of its discovery in space it had never been possible to isolate enough of it for spectroscopic study. In the low-density regions of space, however, the molecule can survive in large enough quantities that spectral lines can be seen that are sufficiently clear for scientists to measure the detailed structure of the molecule for the first time.

Interstellar molecules are very unevenly distributed in space. Most of the molecules listed in table 13.1 have been seen in than a dozen locations in the sky and many in only one place -the giant interstellar cloud Sagittarius B2, very close to the centre of our Galaxy. Molecules are plentiful only under certain inter¬stellar conditions, chief of which are a high gas density (typically 1010 molecules nr3), and sufficient dust to provide a shield from the destructive effects of ultraviolet starlight. A few molecular clouds are associated with old stars, but most of them are regions where new stars are condensing out of the interstellar medium. These clouds include the most massive objects in the Galaxy; although the clouds themselves are usually invisible at optical wavelengths, they are often found associated with known H+ regions.

Most of the radio lines are caused by rotational transitions. A simple molecule, such as carbon monosulphide, spends most of its time with no angular momentum; sometimes, however, as a result of a collision with another molecule (most probably an H2 hydro¬gen), it can be made to spin in an end-over-end fashion. If un¬disturbed, it will (on average) then remain in this rotational state for just a few hours before emitting one or more millimeter-wavelength photons and reverting to its ground state. Because carbon monosulphide molecules drop out of their excited states so rapidly they are seen only in regions where molecular collisions are very frequent, namely where the molecular hydrogen density is 1012m-3 or more. The molecule carbon monoxide behaves in many ways much like carbon sulphide, but does not have to be excited by molecular collisions nearly as frequently as does carbon sulphide. It can therefore be seen in regions of comparatively low density, typically 108 molecules m~3. For this reason, carbon monoxide is used for mapping out the moderate-density clouds in our Galaxy, whereas carbon sulphide and certain other molecules show us the location of the densest parts of these clouds.

In the places where several different molecules have been seen, estimates can be made of their relative abundances by comparing the strengths of their spectral lines. Although molecular hydrogen is very difficult to observe directly, there is little doubt that it is by far the dominant species in these clouds. Of those molecules observed at radio wavelengths, carbon monoxide is much the most widespread, with something like one molecule for every 10 000 of hydrogen. Carbon monoxide is so common, and is seen in so many directions in space, that it can be used to study the structure of our Galaxy in a way that complements the observations of atomic hydrogen. The next most abundant molecules are hydroxyl and ammonia, although they are only one per cent as common as carbon monoxide. All other known molecules are rarer still, though the as-yet unobserved nitrogen molecule (N2) is probably wide¬spread. As described in the next section, the relative abundances of different molecules give us clues to the processes of interstellar chemistry. A more speculative calculation was performed by the discoverers of interstellar ethyl alcohol in 1974; they estimated that the ‘proof (a measure of the ratio of alcohol to water) in the molecular cloud Sagittarius B2 was somewhat less than 1°. Despite this dilution, the cloud contains enough alcohol for 1028 bottles of whisky.

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