Abell 78 (PK 081-14.1, PN G 081.2-14.9) is a planetary nebula in the constellation Cygnus discovered in 1966 by George Abell. It spans 1.4 light years in diameter and is located roughly 5,300 light years away. Planetary nebulae are formed by the expanding gas shell around a white dwarf progenitor star, the late-stage evolutionary phase of small to medium size stars, not massive enough to explode into a supernova. Having exhausted the nuclear fuel in their cores, these stars collapse to become dense white dwarfs.
What is unusual about the white dwarf star visible at the center of Abell 78 is that it has reignited and come back to life, hence the moniker 'born-again star'. Only a handful of such born-again stars have been discovered. Although nuclear burning of hydrogen and helium had stopped in the core of the dying star, some of the star's outer layers became so dense that fusion of helium resumed there.
The renewed nuclear activity started another much faster stellar wind, blowing more material away. The interplay between the old and new outflows has shaped the cloud's complex structure.
Abell 78 is a small planetary nebula (113"x88"), the smallest deep-sky object I have imaged so far. In spite of its reasonable brightness, I collected a significant amount of data to allow me to stretch the background and select low-FWHM subs to highlight the intricate structure of this small DSO. The hydrogen and faint oxygen emissions in the background sky surrounding the nebula are not often visible in amateur images.
References:
http://www.esa.int/ESA_Multimedia/Images/2015/07/Born-again_planetary_nebulaJ.B. Kaler & W.A. Feibelman, The central star of the planetary nebula Abell 78, Astrophysical Journal 282 (1984), 719-727.
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