In astronomy, appearances can be deceiving, especially when you're looking at objects that are both distant and complex. Glowing clouds of gas are often grouped under the catch-all term "nebula," but the processes that light them up can be radically different.
A planetary nebula, for example, marks the late-life shedding of material by a sun-like star. (It has nothing to do with planets; the name comes from the object's passing resemblance to a planet, as seen through early telescopes.) A stellar nursery signals the opposite: the messy, energetic beginning of a new star's life. Untangling which story you're seeing usually requires detailed measurements of how the gas is moving and what it's made of.
