For going on two decades, Scientific American's "Ask the Experts" column has been answering reader questions on all fields of science. We've taken your questions from the basic to the esoteric and reached out to top scientists, professors and researchers to find out why the sky is blue or whether we really only use 10% of our brains. Now, we've combed through our archives and have compiled some of the most interesting questions (and answers) into a series of eBooks. Organized by subject, each eBook provides short, easily digestible answers to questions on that particular branch of the sciences. The second eBook in our series – Astronomy – looks skyward and explains a variety of universal phenomena and theories. Are you curious about how planets acquire rings or what creates those gorgeous spiral arms around galaxies? Or maybe you want to know why the Big Bang didn't collapse into a black hole. Astrophysicists, professors and scientists tackle questions about stars, planets, asteroids, galaxies and nebulae, the expanding universe as well as the oddities – black holes, wormholes and dark matter. Look inside and find out what we know and what we don't know about these wonders.