The stars in space are not evenly distributed. They group together in great star islands, with empty space in between. We call these islands galaxies. The Sun and all the other stars we can see in the sky belong to our Galaxy, which we call the Milky Way. The Milky Way is the name we also give to the milky white band we can see arching across the heavens on a clear night. When we look at the band through a telescope, it resolves into thou- sands upon thousands of stars. This star band is in fact a cross-section of our Galaxy, which takes the form of a flattened disc with a bulge at the centre. The disc measures about 100,000 light-years across and contains about 100,000 million stars. These stars are not spread evenly throughout the disc, but are concentrated into a number of spiral arms, curving out from the central bulge, or nucleus. The Sun is on a spiral arm about 30,000 light-years from the centre. The whole Galaxy spins round bodily in space: from a distance it would look rather like a flaming Catherine wheel firework. It takes the Sun about 225 million years to circle the Galaxy, a period of time called the cosmic year. There are many other spiral galaxies in the Universe, including one we can see with the naked eye in the constellation of Andromeda, the Great Nebula. It lies over 2 million light-years away and is much bigger than our Galaxy. Some galaxies in the Universe are termed elliptical: they have the shape of an ellipse - oval or spherical. Others have no particular shape and are termed irregular. Observers in the Southern Hemisphere can see two irregular galaxies with the naked eye. They are fuzzy white patches near the south celestial pole, which we call the Large and Small Magellanic Clouds (LMC and SMC). The LMC is the nearest galaxy to us, about 170,000 light-years away.
