As its nuclear fuel runs out, a Sun-like star balloons into a red giant whose outer layers are blown outward by the stellar wind. The naked core left behind contracts under gravity and shrinks to a white dwarf, becoming hot enough to ionize its surrounding envelope of dust and gas. The glowing structures, known as planetary nebulae, are among the most beautiful sights in the night sky. The vast majority have shapes that don’t match the stellar wind’s spherical symmetry. Theorists have long realized that if a nebula contained a pair of orbiting stars, the stars’ angular momentum could be sufficient to break the symmetry and elongate their common envelope. Recent sightings and analyses of close binary stars have borne out that hypothesis. Miguel Santander-García of Spain’s National Astronomical Observatory, Romano Corradi of the Institute of Astrophysics in the Canary Islands, and their collaborators have now discovered an especially intriguing pair of companions inside the planetary nebula Henize 2-428. Using powerful telescopes to monitor the Doppler shifts and variations in brightness inside the nebula’s pair of extended lobes, shown here, the team found two pre-white-dwarf stars of nearly identical mass orbiting with a period of just four hours. What’s more, the mass of each star is about the same as our Sun; so when the two finally merge in 700 million years, their combined masses will exceed the Chandrasekhar limit of 1.4 solar masses and possibly trigger a type-Ia supernova. Before the new observation, scientists had only theorized about the circumstances that lead to such an explosion. (M. Santander-Garcia et al., Nature, in press, doi:10.1038/nature14124.)
