As its nuclear fuel runs out, a Sun-like star balloons into a red giant whose outer layers are blown farther outward by the stellar wind. The naked carbon–oxygen core left behind contracts under gravity into a white dwarf, its surface becoming hot enough in the process to ionize its surrounding envelope of dust and gas. The glowing shells, known somewhat confusingly as planetary nebulae, are among the most beautiful sights in the night sky.
The vast majority of the thousands of such nebulae cataloged by astronomers have shapes that seem inconsistent with the stellar wind’s spherical symmetry. For the past two decades, theorists have been trying to figure out why. Although the issue remains unsettled, most believe that the presence of a pair of orbiting stars is the most efficient way to generate sufficient angular momentum to break the symmetry and shape the wind into an outflowing equatorial ring, sometimes...
