The strange case of HuBi1, an inside-out stellar corpse

The physical structure of a planetary nebula resulting from the final evolution of a solar-like star is contrary to that of all other similar sources. A research led by IAA-CSIC concludes it resulted from a born-again event of its central star


Planetary nebulae are the descendants of low- to intermediate-mass stars, like our Sun. They form at late evolutionary stages, when their progenitor stars expel their envelopes, which are then photoionized by the hot white dwarf at the nebular center. The regions closer to the central star have species with higher ionization than the farther, cooler regions. This is exactly opposite to HuBi1, whose inner shell inverted ionization structure results from a peculiar stellar evolutionary path, a born-again star, according to the conclusions of a study published today in Nature Astronomy.

HuBi1 looks like a typical double-shell planetary nebula, with a diffuse outer shell and a bright inner shell, but this research reveals nothing is like it seemed to be: the outer shell is unusually recombining and the central star, which is surprisingly cool, has been “switched off” about 50 years ago and is now 10,000 times fainter than in 1971.

"The inverted ionization structure of the inner shell of HuBi1, with lower excitation ionic species at the inner regions and highly excited ions at the outer regions, is simply amazing. It defies the most basic thermodynamics laws in photoionized nebulae and points out to a peculiar stellar evolutionary event”, according to Martín A. Guerrero, the researcher at the Instituto de Astrofísica de Andalucía (IAA-CSIC) who has led the study.

The ionization structure is, indeed, typical of bow-shocks generated by fast-moving material interacting with the surrounding medium. This, together with the low ionizing flux from the cool central star, unveils a truly unique scenario.

"In a few thousand years, planetary nebulae disperse into the interstellar medium and their central stars dim. Instead, the central star of HuBi1 recovered through a very late thermal pulse that processed all hydrogen on its surface", points out Marcelo M. Miller Bertolami, a researcher of the Instituto de Astrofísica de La Plata (Argentina) who contributed to this study.

HuBi1 has been caught at the exact moment when its central star underwent a born-again event, becoming a hydrogen-poor [WC] star. About 15% of planetary nebulae so far discovered have [WC] central stars, but their evolutionary path had remained unidentified until now. This brief born-again process ejected large amounts of fast-moving highly processed material, which now interacts and shocks the outer nebula to produce the observed double-shell morphology.

Being one of the very few known born-again planetary nebulae, the evolution of HuBi1 will be followed. “As the progenitor star of HuBi1 had a mass similar to our Sun, it provides a glimpse of a potential future (and the ultimate fate) for our solar system”, concludes Martín A. Guerrero (IAA-CSIC)..


M. A. Guerrero et al. "The inside-out planetary nebula around a born-again star". Nature Astronomy.


More info: 
Martín Guerrero mar[arroba] -  958230622

Instituto de Astrofísica de Andalucía (IAA-CSIC)
Unidad de Divulgación y Comunicación
Silbia López de Lacalle - sll[arroba] - 958230532