Less than a century ago, the bright arc of our Milky Way was thought to contain all the stars in the universe. But, astronomers were perplexed by a cigar-shaped object in the autumn sky called the Andromeda nebula. Some astronomers thought it was another galaxy like our Milky Way. It too had a flattened-disk shape. But how far away was it? In the early 1920s, astronomer Edwin Hubble photographed the nebula in detail. He found a pulsating star hidden deep in one of its spiral arms. Edwin Hubble used it to calculate the distance to the Andromeda nebula. This showed it was far beyond our Milky Way. Andromeda in fact was really a separate galaxy. The Hubble Space Telescope has recently observed the star as a tribute to the landmark discovery.
Image Credit: A. Mellinger (Central Michigan University), T. Rector (University of Alaska, Anchorage), Digitized Sky Survey (DSS), STScI/AURA, Palomar/Caltech, and the Hubble Heritage Team (STScI/AURA)