The most noticeable manifestations of the interstellar medium are NEBULAE. The term ‘nebula’ is used fairly loosely in astronomy; the word is derived from the Latin for cloud, and can refer to almost any object that appears fuzzy or extended through a telescope. One of the early catalogues of nebulae was that of Messier, who in 1781 published a list of about 100 nebulous objects. His designations, for example M 42 for the Orion Nebula, are still in use for some of the brightest objects. Messier’s list includes many different kinds of astronomical phenomena, many of which have nothing to do with the interstellar medium; about a third of his objects are galaxies, and over half are clusters of stars. The term ‘nebula’ is nowadays not much used for either of these classes of object. Thirteen of the Messier objects are bright clouds of inter¬stellar matter; one is a reflection nebula, four are planetary nebulae, seven are ionized hydrogen (H+) regions, and one is a supernova remnant. The last three types are sometimes referred to as GASEOUS NEBULAE.

The difference between these four types of nebula lies mainly In the way they are illuminated and on whether their dust or their gas makes them more prominent. A reflection nebula contains dust grains which reflect and scatter light from nearby stars, The light from planetary nebulae and ionized hydrogen regions, on the other hand, is generated by a fluorescent process, in which ultraviolet photons from a very hot star excite interstellar atoms to an ionized state, whence they recombine and emit visible light. Supernova remnants, of which the Crab Nebula is a famous example, are the result of gigantic stellar explosions.

There is another important class of nebula not represented in Messier’s catalogue; these are the dark nebulae. These are .clouds of gas and dust that are so opaque that they prevent light from stars or bright nebulae passing through them. They can only be detected if they are large enough and thick enough to obscure so many stars that a prominent ‘hole’ appears in the sky. Some of these dark nebula have names, such as the ‘Horsehead’ and the ‘Southern Coalsack’. They contain approximately the same sort of mixture of gas and dust as the bright nebulae, but have no suitable stars to illuminate them.

There are, of course, many more nebulae than are listed in Messier’s catalogue. Some are simply very small or very faint. Others, hidden from us can be detected by their emission or absorption of radio, microwave or infrared radiation rather than visible light. Such objects are commonly called interstellar clouds; the term refers to a concentration of gas or dust whether or not it is visible to us on Earth as an optical nebula.

The variety in types of interstellar cloud is an indication of the variety of conditions found in the interstellar medium. For ex¬ample, tempera tiny and density variations of a factor of & million are found between different regions, with the faintest and most extended objects losing their identity as individual nebulae and blending into the general background of the interstellar medium. Other important variables include the ratio of gas to dust, and the state of ionization of the hydrogen in a cloud; different regions of space may be either predominantly molecular (H2), neutral (H°) or ionized (H+), depending on their density, temperature, and proximity to bright stars. The various different sorts of interstellar cloud are discussed in chapter 14; their importance is that these are the regions of the Galaxy where there is the strongest interplay between the stellar and interstellar components of the Galaxy.

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