Because I am usually occupied by the documentation of wild birds, I rarely point my lens heavenward. However, when there is a sunrise or cloud formation that I feel I can use for montage purpose, I will bang off a few shots.
When I was viewing this image on my camera screen, I noticed a black dot embedded in the image (thinking of course dirty sensor). I have subsequently discovered that the spot never moved, and was in fact a sunspot.
Perhaps of interest, sunspots appear as a visibly dark area on the sun, caused by intense magnetic activity. They can be as large as 80,000 square kilometers in size.
In closing, if you see this as a montage on my photostream, please note - I do not have a dirty sensor!!