This is episode 60 of the Urban Astronomer podcast! Is that a milestone? Sure, but we'll save the party hats for episode 75. Today we'll just get on with the job at hand, and bring you a science explainy bit. Today's question: How can astronomers be so certain about what things in space are made from? On Earth it's relatively easy to send geologists out to different places with their hammers, and have them collect samples from interesting rock formations and bring them back to the lab for analysis, but astronomers hardly ever get to do that with planets, comets, the Sun and distant galaxies. So how do they know, and how can they sound so confident? Listen below to find out!