To an observer in the southern hemisphere, the existence Of galaxies beyond our own should be as evident as the existence of stars beyond the Sun: the two nearest external galaxies, the LARGE and the SMALL MAGELLANIC CLOUD, (LMC and SMC) are readily visible to the naked eye . At their distances of 50 and 60 kpc respectively, they lie certainly (but only just) outside our Galaxy, of which they are satellites. From places on Earth above latitudes 30°N the Magellanic Clouds are always below the horizon. But another external galaxy can be seen, without any telescope, as a fuzzy patch of light about a third of a degree across, in the constellation Andromeda. This object is not a satellite of the Galaxy and it is 690 kpc away. These distances, of course, cannot be determined with the unaided eye; it is only with hindsight that we know the Clouds and the Andromeda Nebula, (also known as Messier 31 or M 31) to be extragalactic.

In fact, it was not until the late 1930s that most astronomers admitted the existence of galaxies, similar to our own, elsewhere in space. The distance to the Magellanic Clouds had been known since the investigation, early in this century, of Cepheids in the Clouds. Because M 31 is a dozen times more distant, Cepheids and novae in it were not discovered until the construction of sufficiently large telescopes in the late 1920s. The last doubts as to the extragalactic nature of M 31 were finally dispelled in 1943, when the advent of fast red-sensitive photographic plates allowed the central region of this galaxy to be resolved into individual stars for the first time ; their apparent brightnesses left no doubt as to their comparatively great distance. But more than two decades before that time, some observers already believed M31 to be a galaxy like ours: long-exposure photographs showed it to be quite unlike any galactic object and also different from the Magellanic Clouds.

A search for nearby galaxies reveals that they occur in small clumps. For example, the Galaxy, the LMC and the SMC are evidently bound together by gravitational attraction. Likewise, the Andromeda Nebula has a number of satellites, some of which even have a nucleus, as is the case with elliptical M32. These clumps, in their turn, form a small swarm of galaxies with 500 kpc radius. No galaxies are found within the immediate vicinity (a thousand kiloparsecs, say) of this swarm, which therefore is considered to be a separate entity, the LOCAL GROUP OF GALAXIES. By the end of 1977 28 members were known. It is interesting to compare their distribution in space to that of the nearest stars . Note firstly, that the distribution of the galaxies is evidently less random than is that of the stars. Secondly, we see that the average distance between the dominant galaxies (M 31, the Galaxy and M 33) is only about a dozen galactic diameters, whereas the corresponding distance between stars is some 30 million stellar diameters! Clearly, galaxies are very close neighbours in relation to their size. Also, the Local Group is very diffuse as compared with a galaxy: the average density of matter in the galactic plane, smearing out the stars evenly in space, is about 10 million particles per cubic metre, but the average density in the Local Group, smearing out its galaxies is only about 15 particles m-3.

Because we live in the plane of the Galaxy, galactic structure can be determined only in a roundabout and ambiguous way. Therefore, it is of advantage to study external galaxies, and obviously the nearest ones — the Local Group — are of prime importance for a comparison with the Galaxy. But immediately a problem arises because merely looking at the member galaxies reveals a baffling variety of appearances: tight spirals (M 31 and the Galaxy), an open spiral (M 33), irregulars (LMC and 1C 1613), ellipticals with nucleus (M32), spheroidals without nucleus.

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