Until the early-20th century it was widely believed our Solar System was part of an ‘island universe’ – a nebulous mass of stars with nothing beyond its boundaries. What is now known as the Andromeda galaxy 2.6 million light years away was thought to be a ‘spiral nebula’ on the edge of this island. That was until 1923, when Edwin Hubble (the Hubble telescope’s namesake) found a star in Andromeda with a regular cycle of brightening and dimming. It was to become V1: a Cepheid variable, the first named variable star and a reliable distance marker. It helped prove that Andromeda wasn’t part of our ‘island universe’, but was in fact a completely separate entity – another galaxy. From here, astronomers went about looking for other variable stars and discovering hundreds, thousands and, today, millions of other galaxies observable from Earth.