Jonathan Silverlight wrote:


But if you look at _any_ photograph of a deep sky object you see it
much brighter than it really is. We don't have colour vision at that
light level, but the colours are there and usually very pure - they
are from a line spectrum.
The colours in pictures are either an approximation to the actual
colours as seen through Hubble's filters, or, more frequently,
selected for aesthetic and technical reasons and "false" in that
sense. In particular, they often use green for the red hydrogen-alpha
light.


Which is odd, as you'd think they'd use red for the red hydrogen-alpha
light.
But this is science, and is therefore not limited to making sense.
Call it what you like, the photos get retouched to improve their
"gee-whizz" quality for public consumption.

Pat