Here’s what Molloy tells us about how the images are created:
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.
You can find more of Malloy’s work over on Flickr and on 500px. He has an entire set devoted to images created with this photo stacking technique, including many photos of stars.
Read more at http://www.petapixel.com/2012/11/10/stacked-photos-in-which-clouds-look-like-brush-strokes-across-the-sky/#46gjtE1H1LLzC8yf.99