For those who can fit it into the busy semester, here's something that might create one of the "wow" moments in some student's head and they decide they want to "do" science. It compares a 20th century photo with a 2012 photo. Notice the stars are fixed, but the gas tendrils in the Crab aren't. Given the diameter of the Crab, some eager students could give an estimate of the speed of the gas (with uncertainties, of course!). http://vimeo.com/71117055