In astronomy, the fixed stars are the lights, mainly stars visible to the naked eye, that appear not to move relative to each other against the dark background of the night sky. They are defined in contrast to lights that appear to move relative to each other and to the fixed stars, which include classical planets and comets. The fixed stars include all the stars visible to the naked eye other than the Sun, as well as the faint band of the Milky Way. Due to their star-like appearance when viewed with the naked eye, the few visible individual nebulae and other deep-sky objects are also counted among the fixed stars. Approximately 6,000 stars are visible to the naked eye under optimal conditions.