The molecular universe

nation. Clearly, the universe is not entirely made of molecules and we are exaggerating. In fact, most of the mass in the universe is probably not even made of familiar matter, i.e. the atoms and molecules of which we, the Earth, the solar system, and all the stars in our own and other galaxies are composed. “Dark matter” contains about 90% of the mass of the universe and is detected only by the gravitational effects it exerts. The composition of this matter is unknown. The remaining 10% or so of matter in the universe is the familiar “baryonic” material, mostly locked in stars. Only about 1% of all matter is gaseous and distributed between the stars. And of this 1%, perhaps half is molecular. Therefore only about 0.5% of the total mass of the universe is composed of molecules. Why then do we emphasize in the title of this article the importance of molecules? Surely this is a case of the tail wagging the dog? In fact it is not. This gaseous component is important because it is the reservoir of matter that remains to be processed into galaxies, stars and planets. For example, in the early universe protogalaxies were formed from gas clouds that contracted under their own weight to form galaxies of stars, but in the present era there is too little gas left in intergalactic space for galaxy formation to continue. Within any particular galaxy, star and planet formation continues while sufficient gas remains in the interstellar medium. However, when this reservoir is empty, a galaxy has little opportunity for further development and can only await the death of the stars that it contains. The interstellar medium of galaxies is replenished to some extent by material expelled from stars in winds and explosions, so the interstellar gas is continually enriched with heavy elements and dust that are the ashes of nuclear burning. Stars that form from the enriched interstellar gas will be richer in these heavy elements and, as the gas from which these stars form becomes richer in dust, the opportunity to form planets as a by-product of star formation increases. Many of the most interesting astronomical phenomena (e.g. the formation of galaxies, galactic collisions, formation of stars and planets and the injection of material into the interstellar medium through stellar winds and explosions) occur in (or are best traced through) matter that is at a higher-than-average density. For example, the average number density of the gas in the interstellar medium of the Milky Way is about one hydrogen (H) atom per cm (i.e. a million per cubic metre), compared to 2.7×10 molecules cm in the air that we breathe on Earth. Processes that initiate star formation occur in clouds that are about one thousand times denser than this average interstellar density; in gas that is about a million times denser, star formation is inevitable; and the processes that control planet formation occur in gas that is about 10 times denser than the mean interstellar gas. Material injected into the interstellar medium from stars is also, initially, very much denser than the interstellar medium. These are the kinds of regions that many astronomers wish to study. High density implies a high collision-rate in the gas between atoms, molecules, radicals and dust grains, stimulating a complex chemistry that produces a wide variety of new molecular species (see table 1). In fact, emission of electromagnetic radiation from (or sometimes absorption by) molecules is the most effective way of studying the properties of the denserthan-average gas, wherever it may be found. Molecules usually have electronic transitions in the optical/UV wavelengths, vibrational transitions in the infrared and rotational transitions in the radio. There is nearly always a suitable molecular transition to use to diagnose the conditions of a molecular region whether its temperature is as low as 10 K or as high as several thousand K. Emission in the radio regime is a particularly useful probe of dense and dusty regions that are opaque to UV and optical radiation. Often, the radiation emitted by the molecule represents an important loss of energy from the gas, so molecules can be important coolants. Cooling may prevent a cloud that is collapsing under its own weight from heating up, as gravitational potential energy is converted into heat. If the temperature remains sufficiently low, then the gas pressure will not rise enough to prevent the collapse. Something like this process must happen at the birth of both galaxies and stars. The chemistry that gives rise to the coolant molecules also tends to reduce the level of ionization in the gas, thereby reducing the ability of the magnetic field to support the gas against collapse. Consequently molecules play a key role in regions where much of the astronomical action takes place; through molecular emission we can trace the physical conditions during these events and this radiation may itself be important in modifying the physical conditions that allow these changes to continue. The motivation for studying astrochemistry is strong and although molecules are a minor component of the mass of the universe they exert a profound influence on its development because they affect and trace the transition to high density.