Herschel's Ring looks like it might be a supernova remnant, with its loculated, bubbles-within-bubbles wall structure, especially visible toward 2 o'clock.
But it isn't, not yet. It's a nebula created by a pre-supernova Wolf Rayet star whose strong stellar wind has slammed into and shock-jocked the pre-existing medium.
Big one (2MB) here
How do we know without a spectrograph? Two strong hints. The first is the extreme paucity of SII (here mapped to red), compared with acutal supernova remnants such as the Pencil Nebula or the area around Pismis 4 that we posted recently. The second is (perhaps) the very large amount of pre-existing medium hanging around, waiting to be scooped up, as seen as purish hydrogen alpha [green] toward the bottom left edge.
The complex bubble structures toward 2 o'clock are worth a close look. There are also some nice, fainter shocks glowing in OIII toward the left.
Q: Why is the image so green when we know that is sinful?
A: We are being true to the light, and true to the astrophysics. As explained before, this region is not a post-supernova spectacular. It is a relatively peaceful (greenpeace?), youthful (greenhorn?) dainty (Daintree rainforest?) area, with very little SII. Trying to make it battle red [SII] when it isn't would be unfaithful to the light, and unfaithful to the physics. Embrace the green, for what it is telling us.
Field 36 min arc, North up. Ha: 5hrs, OIII 6hrs; SII 10hrs, all in 1hr unbinned subs mostly at -30C. Aspen 20" PlaneWave. MI-760 fork. Half moon. Processing using our very own GoodLook 64.
Best,
Mike and Trish