Winter, Spring, Summer, or Fall – The Four Seasons of the Iris Nebula (NGC 7023/Caldwell 4)

The Iris Nebula is a reflection nebula in the Cepheus constellation located approximately 1,400 light years away. It’s about six light years in breadth and primarily comprises microscopic dust particles that scatter light from the bright star near its center. The Iris is part of the Cepheus Flare—a giant molecular cloud complex above the galactic plane.

Although the Iris presents as largely blue like many other reflection nebulas, it also contains some areas of red that have puzzled scientists. They believe this to be the result of the presence of some unknown hydrocarbon compounds. Study on this continues.

Meanwhile, inspired by Carl Bjork’s image of the Iris, I decided to pursue shooting a significant amount of data with both hydrogen (which yields a red color) and oxygen (which yields more blue color) narrowband filters. Very few people have done this, and I was very surprised about how very strong the signal was in both bands. So I’ve prepared four images of this: (1) a true color image; (2) true color combined with the narrowband signal; (3) hydrogen and oxygen only; and hydrogen and oxygen expressed using a dynamic color palette called Foraxx. Playing the slideshow above will cycle through all four images. But you can stop on any one of them for a closer look.