Pattern and structure inform the universe at all levels. Even at the vast scale of a galactic cluster, matter is arranged in complex systems that behave in comprehensible ways. Basic forms such as spheres, disks, rings, ellipses, and spirals seem to be repeated across many levels of scale, from immense galaxies through stellar and planetary magnitudes down to the anatomies of individual organisms.

Our Local Group of galaxies is a hierarchy of 30 to 40 spinning nebulae, large and small, dominated by the Andromeda Galaxy and its younger sibling, the Milky Way. Both of these galaxies exhibit intricate, self-similar spiral structures, and each is attended by a large family of satellite galaxies. The Local Group also includes a third spiral galaxy known as Triangulum, which might be bound to Andromeda or might pursue an independent path through space.

The illustration on this page represents the most prominent members of the Local Group at roughly the same scale. (For an excellent diagram that includes more than 25 of these objects, along with other images that depict the Milky Way and its environs, see Richard Powell's Atlas of the Universe.)

Andromeda is foremost, attended by its two nearest satellites. The smaller is M 32, the larger M 110; both are examples of "dwarf elliptical" galaxies. Andromeda's diameter is estimated to be at least 37 kiloparsecs (120,000 light years) and perhaps as much as 60 kiloparsecs (195,000 light years). It looks like a more mature galaxy than the Milky Way, with many bright middle-aged stars and a relatively low rate of new star formation. Its companion M 32 is also a mature object, but far smaller, with a diameter of about 2.5 kiloparsecs (8,000 light years). M 110 is a more complex elliptical or spheroid galaxy with a diameter of about 4.3 kiloparsecs (14,000 light years).

A bit more distant from the Milky Way than Andromeda is our other spiral sibling, the Triangulum Galaxy (M 33), which is fortuitously positioned so that we see its classic pinwheel structure almost face-on. With a diameter of about 18 kiloparsecs (60,000 light years), it may be one-half to one-third the size of Andromeda.

Our own galaxy's most visible satellites are the two Magellanic Clouds, large and small. Both appear irregular in form, with vestiges of an earlier spiral structure that may have been disrupted by the Milky Way's gravitational pull. The Large Magellanic Cloud ("LMC" or Nubecula Major) is a complex galaxy in its own right, similar in extent to Triangulum and home to frequent supernovae and numerous star-forming clouds (e.g., the Tarantula Nebula). The astronomer Robert Burnham once described it as an "astronomical treasure-house" of star forms and stellar evolution. The Small Magellanic Cloud ("SMC" or Nubecula Minor) has been less studied; its diameter is estimated as 4.6 to 7.6 kiloparsecs (15,000 to 25,000 light years).

With the exception of Andromeda VIII, an associate of the Andromeda Galaxy that is comparable in size to the LMC, the remaining members of the Local Group are quite small (by galactic standards) and either ellipsoidal or irregular in form. All these objects are spread over a volume of space approximately 3,000 kiloparsecs (10 million light years) in diameter. This region's center of mass is located midway between Andromeda and the Milky Way. Andromeda itself – the closest spiral galaxy to ours – is still so distant that its astronomers, if they had imaging technologies powerful enough, would see the Earth as it appeared almost 3 million years ago. At that epoch our furry ancestors were foraging for shoots and grubs in the equatorial grasslands of Africa.

For frequently updated information on the Local Group, see http://www.seds.org/messier/more/local.html, maintained by Hartmut Frommert and Christine Kronberg.