Hi all, I'm a math major with decent training in physics (through quantum and statmech), and I'd like to learn more about astrophysics. Not so much the theoretical cosmology kind of stuff, though, as how scientists take observational data and deduce things like "the mass of the galaxy" or how old the universe is. Right now it's beyond my imaginative capacity to understand how that kind of quantity could be deduced, and I'd like to get more of an idea. Hopefully my intent is clear enough; if not, please help me be clearer. Thanks for your help!