This photo of the Iris Nebula has the longest integration time of all my pictures so far. In the description I'm trying to give a bit more detail on the circumstances of creating this photo.
The Iris Nebula
The Iris Nebula, most frequently referred to by NGC 7023, was discovered by William Herschel in 1794. It is located in the constellation of Cepheus. NGC 7023 is a combination of a reflection nebula and an open cluster of stars. The shape of the reflection nebula glowing in blue resembles the plant Iris, hence the name of the object. The distance of the complex is about 1300-1400 light-years from Earth, the reflection part has a diameter of about 6 light-years. The blue light which is being reflected by the nebula is coming from the young hot star at the centre of the bubble. The bubble was created by the star itself, sweeping the interstellar dust and gases out from its vicinity by its intense stellar winds. The star looking extremely bright in the picture is barely visible for the naked eye, even for an experienced observer at very dark skies with good atmospheric transparency. As it can be seen in the picture, the dust cloud is much bigger than the part being lit by the star, actually it is even bigger than the field of view of this photo. The dust cloud consists of very fine dust of solid particles, gases and simple molecules. Although the density of the cloud is rather low, it is still opaque in the visible wavelengths because of its vast proportions. It blocks the light coming from stars and other kinds of objects behind it.
The nature of light
Recording the image of dim objects is a very difficult task, because of the nature of light. The photos from a source of constant intensity does not arrive uniformly separated in time, but according the Poisson distribution. If we count the photons arriving (more precisely: detected) in equal time intervals (basically that's what each pixel of a digital camera do), we will get different results. Sometimes we detect more photons, sometime we detect less in same length of intervals. This behaviour appears as noise in the images we record. The noise decreases by the increasing number of photons detected. In case of bright objects the number of detected photons can be sufficiently high to have a low-noise image even with short integration times, but more dimmer the source, the more longer integration times are needed to keep noise down. There are additional contributors to noise too, but this kind of nature of light is the most significant one. Scattered light from light pollution is another big contributor to noise, degrading image quality even more, that's why astrophotographers try to find the darkest places to take images from.
Circumstances of creating this picture
My camera collected photons for this picture for more than 44.7 hours altogether, almost twice as long as for the second longest Cigar Galaxy which has "only" 23.5 hours of integration time. I started photographing the Iris Nebula in early July and finished it in late September. I spent 11 Moon-less nights under the sky in this period at four different dark sites. 10 nights I was able to take pictures, but one of the nights was totally useless because of bad weather (despite the good forecast). I took almost 500 sub-frames, 6 minutes long each, out of which I kept 447, the rest had to be scrapped due to weather or technical issues. The length of the subframes were determined to avoid over-exposition of the brightest parts (except the stars), but also to have enough photons collected in the darkest areas to outgrow the read-out noise of the camera. This ensured to record valid data, real details in the entire dynamic range of the camera.