I am going to inaugurate the "What is happening here?" post series. I am interested on knowing what is happening physically in some aspects of flow around aircrafts. And surely, due to the high level of the co-forum-ers people :thumbs_up , you could provide me an answer, if you want of course, and discuss the reasons. First of all, please open the attached file *.jpg. It shows the exhaust zone of the Shuttle at launch. Why does it appear that blue colour?. All of us know that it is inside the combustion chamber where the flame and chemical reaction takes place. So that, what do you think is that?. I think it cannot be a detached flame. It is too outside to be a combustor's flame. It is a singular effect that is puzzling me. Surely it has to do with shocks waves interactions at the outlet. We have seen it a lot of times in TV, but in particular I'm not able to explain it.