The Pumpkin Patch in Auriga...

Sh2-232, Sh2-235, Sh2-231, Sh2-233

Objects: Sharpless 2-232 (Pumpkin Nebula), Sh2-235, Sh2-231 and Sh2-233
Type: Emission Nebulae 
Constellation: Auriga
Distance: 5870 light years

Dates: 11th, 13th and 16th. February 2020
Equipment: ATIK 460EX with EFW2, Skywatcher f5.5 Esprit 100 ED refractor, Avalon Linear mount, guiding with Lodestar X2/PHD

Subframes: 12 x Red (300s), 12 x Green (300s), 12 x Blue (300s), 20 x 600s H-alpha, 12 x 600s OIII (2x2 binned), no flats/darks (hot pixel removal in Astroart).

Stellarium map showing location of Sh2-232

Between Theta Aurigae and Iota Aurigae (the two southernmost stars of the “kite” of the constellation of Auriga) lies a string of emission nebulae. The above image shows four faint areas of nebulosity lying just to the east of the brighter and better known nebulae IC 410 and IC 405.  Of these, Sh2-235 is the most central and the brightest nebula of an H II region known as G174 + 2.5; it is observed in the direction of the northern part of the OB Aur OB1 association and includes the nebulae catalogued as Sh2-231, Sh2-232, Sh2-233 and Sh2-235, identified in the 1959 Sharpless catalogue of H II regions.

Although in optical images they appear as distinct nebulae, in reality they all belong to a single giant molecular cloud, some parts of which appear illuminated by young and hot stars. The cloud is found in the Milky Way spiral Arm of Perseus at a galactic latitude which places it slightly off-centre with respect to the centre of the galactic disc.

Sh2-232 is very faint in comparison to Sh2-235 and much larger, covering an area slightly greater than that of the full moon. Its photographic appearance has given it the nickname of the Pumpkin Nebula.  Although it appears in the above image as similar in brightness to its companions, this is an artefact of image processing: it has been selectively “stretched” to bring out detail.  The annotated unstreched hydrogen alpha image (below) shows the true relative brightnesses of the nebulae.

Annotated H-alpha frame showing true relative brightnesses

There is very little OIII detail to be had, although it did highlight the little planetary nebula lying just off-centre in Sh2-232 (circled opposite). The OIII stack was stretched and combined with the blue filter stack, with the H alpha data being combined with the red filter stack. The Ha and OIII data were combined as layers in “multiply” mode and then combined with the green filter stack. The three colour channels were then RGB combined and a partial Ha layer added over it in luminance mode, with some smoothing and star reduction plus other selective boosts to colour saturation, sharpness and brightness to give the final image.

Sky conditions were not great on any of the three nights I collected data. Attempts to boost the brightness of Sh2-232 have resulted in a rather noisy image, but it is a rather more colourful rendition of this area than is usually seen and I quite like it.