 The Cassi is one of the most complex planetary nebula with surprisingly intricate structures including concentric gas shells, jets of high-speed gas, and unusual shock-induced knots. These features made the Cassi nebula perfect for developing a new way to figure out how far away planetary nebula are. The fact is, we don't know a lot about the distance to most of these objects. There may be as many as 25,000 planetary nebula in the Milky Way, but only 300 have distances that have been measured with some reasonable accuracy. This is due primarily to the nature of the nebula themselves. You'll recall that two of our most useful tools for figuring out how far away is it, are standard candles, like cephids, and parallax. But because planetary nebula stars are surrounded by the debris of their own ejection, it is hard to get a good luminosity reading for standard candles, and equally hard to locate a good star nearby to use for parallax calculations.