l o c a l   g r o u p
The three principal galaxies of the Local Group, along with a selection of their largest satellite galaxies and four major
independent group members. A total of 30 to 40 galaxies comprise our neighborhood cluster.
NASA/courtesy of nasaimages.org and Sloan Digital Sky Survey; layout based on Atlas of the Universe
atlas of the
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 down to individual organisms.
Our Local Group of galaxies is a hierarchy of 30 to 40 spinning star clouds, large and small, dominated by the Andromeda Galaxy and its younger sibling, the Milky Way. Both of these galaxies exhibit intricate 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 is evidently bound to Andromeda. 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 (M31) is the largest member of the Local Group, with a diameter between about 37 kiloparsecs (120,000 light years) and 60 kiloparsecs (195,000 light years). It hosts about 500 globular clusters (3 times as many as the Milky Way) and more than a dozen satellite galaxies. Most of its satellites are classified as dwarf elliptical or ellipsoidal galaxies. Andromeda appears more mature than the Milky Way, with many bright middle-aged stars and a relatively low rate of new star formation. Over the course of its long evolution it has ingested many smaller galaxies, thereby increasing its mass and size.
Triangulum (M33) is the smallest of the three spiral galaxies in the Local Group. It is positioned so that we see its classic pinwheel structure almost face-on. With a diameter of about 18 kiloparsecs (60,000 light years), Triangulum may be one-half to one-third the size of Andromeda. Its extended stellar halo overlaps with that of its more massive neighbor.
The Milky Way’s largest satellites are the two Magellanic Clouds, Large and Small. Both appear irregular in form, with vestiges of an earlier spiral structure that has been disrupted by gravitational interactions. The Large Magellanic Cloud (“LMC” or Nubecula Major) is a complex galaxy in its own right, home to frequent supernovae and numerous star-forming clouds (e.g., the Tarantula Nebula). The astronomer Robert Burnham described it as an “astronomical treasure-house” of star forms and stellar evolution. Its diameter is about 9 kiloparsecs (30,000 light years). The Small Magellanic Cloud (“SMC” or Nubecula Minor) is more distant, and has been studied in less detail. Its diameter is about 6 kiloparsecs (20,000 light years).
Still more remote is NGC 6822, another dwarf irregular galaxy that may be similar in extent to the SMC. Although it is clearly a member of the Local Group, it is not a satellite of any of the three major spiral galaxies.
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.
Sources: Carigi et al. 2006 (ApJ 644:924-939), Ibata et al. 2007 (ApJ 671:1591-1623), Noel & Gallart 2007 (ApJ 665:L23-L26), Staveley-Smith 2002 (ASP Conference Series). For more information on the Local Group, see http://www.seds.org/messier/more/local.html, maintained by Hartmut Frommert and Christine Kronberg.
All text is copyright Raymond Harris 2006-2009. Image credits appear in the accompanying caption.