The planetary nebula HuBi 1, a rebel with a cause

Planetary nebulae, the descendants of low- and intermediate-mass stars, have characteristic onion-like ionization structure, with the highest ionization species closer to the central star. This is true for all planetary nebulae, but HuBi 1, which shows an inverted ionization inner shell. There is a reason for this oddity, a peculiar stellar evolution of its central star, which makes HuBi 1 the missing link of the population of cool C-rich central stars of planetary nebulae.