M51 is another name for the Whirlpool galaxy in the constellation of Ursa Major. It is visible with small telescopes and forms part of the catalog that Charles Messier made in the 1700's while looking for comets.
M51 is an amazing object to observe because it has that classical face-on spiral structure, but additionally, it is considered a merger, with a smaller galaxy getting engulfed by the large one.
My initial problem painting galaxies in watercolor was that I could not make the background as black as I wanted, and still be able to paint the fine shape of the spiral arms. In this particular case, I wanted to shoe the subtle trail of gas bridging both galaxies.
To study fine structures, astronomers use image processing software that allow to manipulate real data and enhance the places where some dim gas is connecting merging galaxies, like in the case of M51.

I used scientific software to turn the real image into a negative, and then added false colors to distinguish the light coming from different parts of the galaxy, the nuclei and the interstellar medium. False colors are more fun and easier to paint on a white background. I finished the work splattering some stars randomly.