Free Astronomy Magazine September-October 2020

27 SEPTEMBER-OCTOBER 2020 SPACE CHRONICLES histories of shock waves in them. Such shocks are typically generated when fresh, fast stellar winds slam into and sweep up more slowly expanding gas and dust ejected by the star in its recent past, generating bubble-like cavities with well-defined walls. Researchers suspect that at the heart of each nebula were two stars or- biting around each other. Evidence for such a cen- tral “dynamic duo” comes from the bizarre shapes of these nebulas. Each has a pinched, dusty waist and polar lobes or outflows, as well as other, more com- plex symmetrical patterns. A leading theory for the generation of such struc- tures in planetary nebulae is that the mass-losing star is one of two stars in a bi- nary system. The two stars orbit one another closely enough that they eventu- ally interact, producing a gas disc around one or both stars. The disc then launches jets that inflate polar-directed lobes of outflowing gas. Another, related, popular hypothe- sis is that the smaller star of the pair may merge with its bloated, more rapidly evolving stellar companion. This very short-lived “common en- velope” binary star con- figuration can also gen- erate wobbling jets, form- ing the trademark bipolar outflows commonly seen in planetary nebulae. However, the suspect companion stars in these planetary nebulae have