Not a spectacular image, but an interesting phenomenon. The AT2019abn SuperNova in M51, the whirlpool galaxy. Discovered on 22 January 2019 with the ATLAS telescopes on Hawaii.
In order to get the before and after you have to be lucky enough to have older photographs available. I happened to have photographed this object in April/May last year, so had a decent set of luminance data. I added another 3.5h luminance to it now, and compared the two images. It took a while before I had the two images more or less equally processed, so that they can be compared well. The main difference you see now is how bad I focused a year ago...
In the insert, the images are inverted for a better view.
DeepSky imaging usually assumes very static objects, as movements take place on such an astronomic scale. So to see a dynamic event like this happen in a DeepSky object is a nice bonus.
UPDATE: As Kevin Moorfield pointed out, recent research indicates that this is likely to be an Intermediate-Luminosity Red Transient (ILRT). This phenomenon can occur in double-star systems where the stars come close enough to develop a single envelope, where a lot of plasma is emitted. The cooling down of the plasma and re-joining of electrons and protons to atoms sends out a lot of red light with brightness somewhere between a nova and a supernova. More info:
https://escholarship.org/uc/item/9r8010tw.
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