Galaxies are preferentially found in groups or larger agglomerations called clusters. The Local Group consists of our own galaxy, the larger spiral galaxy Andromeda (M31) and several smaller satellites, including the Large and Small Magellenic Clouds.
Regular clusters have a concentrated central core and a well-defined spherical structure. These are subdivided according to their richness, that is, the number of galaxies within 1.5 Mpc of the centre (known as the Abell radius). Typically, they have a size in the range 1-10Mpc and a mass M ~ 10^15 solar masses (one followed by 15 zeros, that is, a million billion suns). The Coma cluster shown here is a very rich cluster with thousands of ellipticals inside the Abell radius.
Irregular clusters have no well-defined centre, a similar range of sizes, but they are generally poorer with a mass 10^12 - 10^14 solar masses (i.e. a thousand to a hundred thousand million suns). An example is the nearby Virgo cluster.
Superclusters: These usually consist of chains of about a dozen clusters which have a mass of about 10^16 solar masses (ten million billion suns). Our own Local Supercluster is centred on Virgo and is relatively poor having a size of 15Mpc. The largest superclusters, like that associated with Coma, are up to 100Mpc in extent. The system of superclusters forms a network permeating throughout space, on which about 90% of galaxies are located.
The Great Attractor: Measurements of peculiar velocities---deviations away from the Hubble flow - are achieved by comparing redshifts and galactic distance indicators. These have revealed enormous coherent motions on scales in excess of 60Mpc. Consistent with these flows, our own galaxy is moving at about 600km/s towards a distant object dubbed the `Great Attractor'. This lies at a distance of 45Mpc and has a mass approaching 5x10^16 solar masses.
Voids, sheets & filaments: Deep redshift surveys reveal a very bubbly structure to the universe with galaxies primarily confined to sheets and filaments. Voids are the dominant feature and have a typical diameter of about 25Mpc. They fill about 90\% of space and the largest observed, Bootes void, has a diameter of about 124Mpc. Other features that have been observed are the `Great Wall', an apparent sheet of galaxies 100Mpc long at a distance of about 100Mpc.
A particularly exciting recent development in the study of large-scale structure has been the advent of very deep galaxy surveys, notably those currently being made by the Hubble Space Telescope. These images (below) show galaxies just a couple of billions years after the Big Bang. One of the remarkable puzzles presented by this work is that galaxies appear to form earlier than predicted in most theoretical models.
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