The discovery of a structurally ‘inside-out’ planetary nebula - the ionized material that surrounds white dwarf stars - is reported online this week in Nature Astronomy. The inversion is thought to have resulted from the star undergoing a ‘born-again event’, ejecting material from its surface and shock-exciting the nebular material.

As low-mass stars age, they typically shed their outer layers, forming a ‘wind’. As the star transitions from its red giant phase to become a white dwarf, it grows hotter, and starts ionizing the material in the surrounding wind. This causes the gaseous material closer to the star to become highly ionized, while gas further out is less so.

Studying the planetary nebula HuBi 1, however, Martin Guerrero and colleagues find the reverse: HuBi 1’s inner regions are less ionized, and the outer regions more. Analysing the central star, the authors show that it is surprisingly cool, and that its optical brightness has diminished rapidly over the past 50 years.

The authors suggest that the inner nebula was excited by the passage of a shockwave caused by the star ejecting matter unusually late in its evolution. The stellar material cooled to form dust, obscuring the star. In the absence of ionizing photons from the central star, the outer nebula has begun recombining - becoming neutral. The authors conclude that, as HuBi 1 was roughly the same mass as the Sun, this finding provides a glimpse of a potential future for our star system.