2020 Jul 31: Phil Plait wrote an article about a supernova in the Circinus Galaxy that no one noticed until well after it'd already exploded. www.syfy.com/syfywire/the-nearby-supernova-no-one-saw
The Circinus Galaxy is an active spiral galaxy that lies near the plane of our Milky Way so that it is rather heavily obscured by intervening stars and whatever other gas and dust lies along our line-of-sight to it. The brightly glowing filaments are yellowish this time because I had two (!) sets of narrowband filters to work with instead of just one or none. Previously, my other images showed them as blue. In reality, I think they are likely a slight greenish color, but the coloration of such things as they might appear to our eyes is unimportant and an unrealistic expectation to have, as fun as it may be to consider.
What I was pleased to see here is the stark contrast in features between the H-alpha and [O III] emission features. The H-alpha features, presented here in red, which are commonly associated with star formation, are present in both star-forming areas and the light cones of the active nucleus. The [O III] features, presented in green, are present almost exclusively in the light cones. This intersection results in a yellow color, clearly illustrating the difference between light-emitting nebulas.
This was a difficult dataset to work with not only because of that, but because there were not very observations available. What images were there are also not rectangular or square in shape, but rather are the stairstep shape of the old WFPC2 camera. There were a few gaps and especially problematic were the yellowish filaments, because they were abruptly cut off.
Regarding the filaments: I was debating exactly how to show the entire nucleus without having the awkward cutoff, and finally arrived at a conclusion that is as far away from fictional as I could get it. Because the wideband data also overlaps with the narrowband data, I was able to recover the last of the details that were cut off and accurately portray the structure as it should appear in a whole image. I confirmed this later by comparing it to some ESO data. The image halfway down this page shows them in whole in blue: www.atnf.csiro.au/people/bkoribal/circinus/circinus.html
Some areas of the image contain cloned data because there were coverage gaps. I have uploaded am image with no cloned data to illustrate those areas: flic.kr/p/24fsf1X
Data from the following proposals were used to create this image:
Gray: WFPC2 F606W, WFPC2 F814W
Red overlay: WFPC2 F656N
Green overlay: WFPC2 F502N
North is up.