That's a lovely image Greg and 20hours WOW
Maybe you did it to show the clear border between your 2 data sets, and please ignore my comment if that's the way you wanted to go, but if it were me, I would try to meld the "diagonal" border somewhat. I did a quick check in PS (here's me suggesting to you -sorry
) and you could use something as simple as the black point eyedropper in Adjustments> Levels area by say setting the black point in the inner framed image in the black surrounds of the galaxy by sampling the black point eyedropper with the darkness in the 1-2 o'clock zone. Doing this quick adjustment really integrated the surrounding space without any penalty that I could see to the galaxy detail, in fact it punched it out more contrast wise.
PS Greg: All suggested with the greatest of respect - I would have no hope of producing something similar gear and experience wise, so feel somewhat vexed in putting this suggestion forward.
Best
JA