‘Stepping stones to the Universe’ outlines the work of Edwin Hubble. Hubble was convinced that spiral nebulae were separate galaxies, and used the advances in telescopy and photography during the 1920s to prove himself right. He identified different shapes of galaxy: spirals (comprising several different subtypes), and ellipticals, with no obvious internal structure. Hubble's discovery of Cepheids in Andromeda proved that the nebula was in fact a galaxy much like our own, and was very far away. Hubble collaborated with Milton Humason to find a link between redshift and distance, and used this to establish the Hubble constant.