This image captures a series of HII-dominated emission nebulae located in the southern constellation of Vela at a declination of -48 degrees. My mouseover here outlines the 5 Gum nebulae in this image.
These nebula also have RCW designations as the result of Alex Rodgers, Colin Campbell, and John Whiteoak's nebula catalog of 1960, which was an expansion of the 1955 Gum catalog. RCW38 consists of Gum 22 and Gum 23 (not Gum 24, as reported in some atlases). RCW 39 is Gum 24, and RCW 40 is Gum 25.
I think the nebulae are easier to visualize and interpret when one understands that we are looking at a region of the sky with many different superimposed HII nebulae, located at different distances away from us. The Gum 12 nebula (also just called the Gum Nebula) is only 1500 light years from us and is the largest nebula in the sky, spanning 36°. Gum 21 of my image seems to simply be a part of Gum 12, as shown below. Gum 22, 23 and 25 of my image are further away, probably around 5000 light years. Gum 24 is still further away at about 10,000 light years.
The Gum image below (click to expand) is an annotated overlay of a crop of a wonderful 2018 Astrobin IOTD by
@John Gleason :
The image above also includes other Gum nebulae for reference, as well as the unrelated superimposed Vela SNR (the closest object at only 800 light years away):
The Vela SNR********************************
Reminder – These objects and 3100 more are included in the Imm Deep Sky Compendium, a free planning resource to help astrophotographers find interesting targets in their night sky. Foreign language translations in Spanish, German, French, Italian, Polish, and Chinese are now available at the link above.
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