Expert science writer Giles Sparrow guides you through 21 stars you can see in the night sky and what they can teach us about our universe. On a clear evening, if you look up you can see thousands of stars shining in the dark sky, each with a story of their own. Taking 21 stars (and three imposters, that cheekily aren't technically stars), expert science writer Giles Sparrow offers a complete introduction to what is happening up...