Sh2-71 (PK 36-01.1, PN G035.9-01.1) was discovered in 1946 by Rudolph Minkowski, who described it as a diffuse and peculiar nebula. In 1959, Stewart Sharpless cataloged it as a possible planetary nebula (Sh2-71). In 1961, Soviet astronomer Boris Aleksandrovich Vorontsov-Vel'Yaminov confirmed that Sh 2-71 (V-V 1-9) was indeed a planetary nebula and described it as follows: “Overexposed in the red, with a kind of dark band in the middle and with extremely faint appendages; the edge, however, is sharp. The brigt part measures 2.5' x 1.3'. In the blue, this is evidently a planetary nebula in the form of a broad ring, one-half of which is far brighter than the other. There are two stars in the center, one of which should be the nucleus." Sh2-71 is a complex, multipolar nebula with several bipolar lobes in different orientations, implying the presence of more than one progenitor star and varying events throughout its formation.

Inspired by the presence of SII signal in some images on Astrobin, I projected my shots in Hα, OIII, and SII, with RGB reserved only for stars. It took me 22 nights to accumulate the total 42 hours and 35 minutes of exposure. In processing, the SHO palette did not yield attractive results, but HSO, with slight modifications, produced beautiful and demonstrative images. I partially shifted the violet that dominated the center toward a more blue tone. For luminance, I used the same HSO combination in PixelMath, but reinforced Hα across all three channels.

Noteworthy aspects of the photograph:

The Progenitor Star: The first candidate was the brighter central star, a variable binary with a magnitude of 13.5, showing 0.7 magnitude changes every 68 days (Star A in the image). However, it is a B8VA-type star, too cool to cause the ionization of the nebula’s gases. Nonetheless, Cuesta and Phillips (1993) proposed a mass-transfer hypothesis (cataclysmic hypothesis) to explain the process. Alternatively, Frew & Parker (2007) suggested that a nearby small blue star, 7.4 arcseconds northwest with a magnitude of 19 (Star B in the image), might be the origin, but this too proved inadequate. In 2015, attention returned to Star A, with the hypothesis modified based on a detailed analysis of its spectroscopic variations (Močnik and coworkers ). Currently, the leading theory is of an evolved initial triple system. In 2019, David Jones and coworkers proposed that initially, the A binary and Star B formed a triple system, and that Star B, initially more massive than A, drifted away and lost mass, forming the nebula and inducing changes in the A binary system to explain its current behavior. In my image, I achieved good differentiation of the small blue 19-magnitude star, perhaps due to an accidental 132 exposures of 30 seconds with filter B while I slept.



Distant Emission Regions: In his article, D. Jones also describes the 2011 discovery of extended emission regions several arc minutes east and west of Sh2-71. These regions consist of gas from the nebula and appear as filamentous structures against the diffuse HII emission background to the east. They were discovered with Hα+[NII] filters, though questions arose regarding their nature due to the wide capture band (6568 Å with an FWHM of 95 Å). Recently, amateur astronomer @Kevin Morefield captured these structures in an image using ultra-narrowband Hα, SII, and OIII filters , with a signal only relevant in Hα (Image). I too could only detect them in the Hα master (790 minutes in Hα, 790 minutes in OIII, and 840 minutes in SII, using Baader filtres 6.5 nm optimized for CMOS).



Irregular Structure: Accustomed to the balanced, serene beauty of most planetary nebulae, it is challenging to imagine order in the irregular shapes and colors of Sh2-71. Thus, I find the analysis by Muhammad Fajrin et al. (2021), which created a three-dimensional model based on optical and spectrographic images, astounding and very appealing. The two-dimensional appearance is explained by the combination of a toroid and three pairs of bipolar lobes with different axes. I reproduced this model over my image but recommend setting it aside and exploring Fajrin’s article.