These beautiful photos of the sky were created by stacking images of the same scene together by Photographer Matt Molloy. Usually photo stacking is used either to create HDR images or to increase depth of field of the camera or a time-lapse video. Mr Molloy used the same technique, albeit in a different way, to produce extra ordinary images of the sky that looks like smears or brush strokes.
Matt Molloy, explains his techniques to PetaPixel:
To make these ‘photo stacks’, I first shoot a timelapse, taking a photo every 5 seconds or so. (Settings differ depending on the subject and lighting conditions). I then merge several photos into one image using Photoshop. I start with the first image from the timelapse as a normal photo and then blend the rest of them with the ‘lighten’ blending mode. This only adds things that are brighter than what was in the first photo, and so you can see things like the paths of stars as they move across the sky. (The movement of the stars is actually from the earth’s rotation).
I think it’s an interesting way to observe time. You can see a time frame of several hours in one image, something our eyes can’t do naturally.