Most of the stars we see in our night sky are within a few hundred light years of us. There's one in Cassiopeia right at the edge of naked eye visibility that's more than 16,000 light years away. And you don't see the individual stars in the Andromeda Galaxy, but it's more than two million light years away. Just how do astronomers know how far away any stars are when we can't travel to them, let alone unroll a tape measure?

Also learn about Gemini, the twins Castor and Pollux.