HuBi 1 is a very rare born-again planetary nebula discovered in 1990. This nebula was named after the astronomers J. Y. Hu and E. A. Bibo. HuBi 1 was discovered using data from the Nordic Optical Telescope, which is located at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands. HuBi 1 has a heliocentric distance of 6.88 ± 2.19 kpc.
The outer shell HuBi 1 possesses an outer shell emitting H and He recombination lines, weak heavy-elements, and excited lines, indicating that the outer shell is recombining. This outer shell has a radius of ~10" or ~15", making it the largest component. It is also the oldest structure and was ejected ~6700 years ago, expanding at a velocity of 50 km s-1. Notably, the [O III] λ5007 emission line in the outer shell may be more intense than the Hβ in the inner shell.
The inner shell The nebula features an inner shell whose emission of intense excited lines of N+, O+, and S+ demonstrates an inverted ionization structure, This inverted ionization structure cause by the C-rich population of stars at the center. The inner shell has a smaller radius of ~3" and was ejected ~1700 years ago. Its expansion velocity of 40 km sec-1 provides evidence that an intense [N II] λ6548,λ6584 emission produced by shocks occurred. [O III] emission lines were discovered along the Northeast direction, uniquely defines the inner shell of HuBi 1. This emission has a ring-like appearance extending out to a radius of ~2."5, with two distinct peaks along the west-east direction. The [O III] emission also surrounds both the [S II] and [N II] emission zones, while a He II emission line encompasses the [O III] emission. Among these features, the He II λ4686 possesses the most ionization potential and is also the most spatially extended. The inner shell encompasses emission lines such as Hα moving at -14 to +22 sec-1 and Hβ moving at -96 to +171 sec-1. the Hα emission region is obscured by the nearby [N ii] λ6584 line.
The outflow or jet Obscured at the center of the nebula is a fast outflow or jet spanning ~2". It is the fastest kinematic component, traveling at 150 km s-1, and a possible maximum velocity of 250 km s-1. It is also the youngest feature, having been ejected ~335 years ago.
The central star The central star has experienced born-again events, a process where, instead of evolving to a typical PN, it undergoes a late thermal pulse, or more likely a very late thermal pulse. This event forces the stars evolutionary track to reverse, causing its outer atmosphere to expand and cool. After looping back in the H-R diagram through the AGB zone, the star enters the PN zone a second time. It has been proposed that after returning to the PN stage the star becomes a Wolf–Rayet star, ejecting a H-poor nebula. Interaction with the older, H-rich old outer shell hindered the H-poor nature of the nebula. Alternatively, the system has been proposed to be a binary star system. The central star of HuBi 1 exhibits an unusually late, WC10 spectral type, corresponding to an effective temperature of ~35,000 K, which supports observations that the star has been fading significantly over the past 40–50 years.
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