A doppelganger of the planet Saturn floats down the southwestern sky on November evenings. Through a telescope, it looks like the outline of the planet Saturn seen with its rings nearly edge-on. Yet that appearance is deceiving. The Saturn Nebula is really the brilliant final gasp of a dying star. All stars of a certain mass exhale a similar nebula as they die. For most of their lives, these stars produce energy by “burning” the hydrogen in their cores to make helium. When the hydrogen is gone, the core shrinks and gets hotter, allowing the star to burn the helium to make carbon and oxygen. But when the helium is gone, nuclear fusion in the core shuts down. For a brief time, the star continues to burn a thin layer of hydrogen around the core. Radiation from that shell pushes away the star’s outer layers, surrounding the core with an expanding bubble of gas. It’s known as a planetary nebula because, when viewed through a telescope, its outline can resemble a planet. The exposed core zap
