Common envelope evolution: From binary star chrysalis to cosmic butterfly

Planetary nebulae are some of the most strikingly beautiful astrophysical phenomena known, gracing many a glossy-paged, coffee-table book and earning them the nickname «cosmic butterflies». While classical stellar evolutionary theory states that planetary nebulae are the end products of intermediate mass stars, forming as the star leaves the Asymptotic Giant Branch and evolves towards the white dwarf phase, it is now clear that a significant fraction of planetary nebulae originate from a binary evolutionary pathway. As the immediate products of the common envelope, close-binary central stars of planetary nebulae offer a unique tool with which to study this rather poorly understood phase of binary evolution. Furthermore, as the nebula itself represents the ionised remnant of the ejected common-envelope, such planetary nebulae can be used to directly probe the mass, morphology and dynamics of the ejecta. Here, I will summarise our current understanding of the importance of binarity in the formation of planetary nebulae as well as what they can tell us about the common envelope phase – including the possible relationships with other post-common-envelope phenomena like novae and type Ia supernovae.