V1 in the Great Andromeda Galaxy

All the bright stars and almost all the dim ones are foreground stars in the Milky Way, but the starclouds, the red nebulae, and that one marked star are in a galaxy far, far away. The star at the intersection of the red lines is a Cepheid variable identified by Edwin Hubble and called by him "V1". It and a few others very much like it proved that the spiral nebulae are not local objects but are "island universes" distinct from the Milky Way. In the early 20th Century, the scale of the universe was redrawn, millions of times larger than before. The modern view of an incomprehensibly vast cosmos began here.
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