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In this practical, you will use observations of two galaxy clusters to  estimate several key cosmological parameters. The observations were  made with the 1.2 metre UK Schmidt Telescope (UKST)  at Siding Spring Observatory in NSW; photos of this telescope are displayed in the lab.  Galaxies are attracted toward one another due to the force of gravity,  
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a rather insignificant cluster we call the Local Group. This cluster  has two big galaxies, our own and the Andromeda Galaxy (M31), in  orbit around each other. Each of these two big galaxies is a spiral  galaxy, with a radius of about 10 kpc (kilo-parsecs, (kilo-parsecs;  one parsec = 3.26 light years $= 3.1  \times10^{16}$ m).  We live about 8 kpc from the centre of our galaxy, 
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are part of another colossal supercluster of galaxies, known as  the Hydra-Centaurus Supercluster, which along with the Virgo Supercluster  and others forms part of an even larger structure called the Laniakea Supercluster.  The scale of structure doesn't end with superclusters, superclusters --  even these unfathomably large objects are observed to group together along filaments and sheets,  separated by relatively empty `voids'.  The network of filaments and voids form what is known as the `Cosmic Web', itself  a relatively recent and extremely important discovery in Cosmology. cosmology.  In Figure 3 we show an image of the cosmic web constructed from the Sloan Digital Sky   Survey, which measured the positions and redshifts (a cosmological unit of distance) of  millions of galaxies.