If we were close enough to the Eagle Nebula would we see it like this? If we were close enough, is this what we would see with the naked eye stretching over (say) 90 degrees of the night sky? Or would the reality be more disappointing?

 A: A partial answer from Universe Today

Nebulae never get any brighter even as you get closer. In optics, there’s a rule called “the conservation of surface brightness”. As you get closer to a nebula, it also gets bigger in the sky. The increased brightness is spread out over a larger area, and the average brightness remains exactly the same. You could be right beside the Orion Nebula, and it wouldn’t look any brighter or majestic than we see it from here on Earth. 
Astronomers generally use black-and-white CCD cameras to make their observations. Then they’ll put filters in front of their cameras to only let through very specific wavelengths of light. Those filters can match the specific colors that make up the visible spectrum: red, blue and green. But usually they’re using filters that reveal scientific information. For example, astronomers want to detect the presence of hydrogen, oxygen and sulfur in a nebula. They’ll use one filter that reveals each one of the elements. And then in a program like Photoshop, they’ll assign red to hydrogen, blue to oxygen and green to sulfur. The resulting image can look beautiful, but the colors have nothing to do with reality. 

 
Filter Images Combined

True color images typically have no value for astronomers, but occasionally they’ll throw us a bone. They’ll produce an image using red, blue, and green filters which roughly match the capabilities of the human eye. And NASA’s Curiosity rover has a pair of color cameras, which allow it to capture images of the surface of Mars that match what we would see.

