Brief description of the Universe

Our Sun is one star in a collection of about 1011 stars forming our Galaxy. The Galaxy is shaped roughly like a pancake – approximately circular in ‘plan’ and with thickness much less than its radius – and the Sun is situated towards the outside of this distribution, not far from the central plane. The Galaxy is about 100 000 light years (ly) across. Almost all the stars visible to the naked eye at night belong to our Galaxy, and looking at the Milky Way is looking into its central plane, where the density of stars is greatest. The Andromeda Nebula, also visible to the naked eye, is a separate galaxy about 2 million light years away, and in fact is a member of the Local Group of galaxies. The construction of large telescopes in the first decades of the twentieth century led to the discovery of many galaxies and groups of galaxies and it is now known that there are about 1011 galactic clusters in the visible Universe. Considering these clusters as the ‘elementary’ constituents of the Universe, on scales larger than that of the clusters their distribution in space appears to be homogeneous and isotropic. This is the first – and very remarkable – feature of the Universe.