APOD Robot wrote:
The featured image from the Gemini North Telescope shows the galaxy in great detail, color-coded so that pink highlights gas while white and blue highlight stars.
This is a very imprecise piece of information. Which stars are color-coded white and which stars are color-coded blue?
Do not confuse astronomical "blue" (a ratio of filtered intensities) with visual "blue".
Is it at all useful to talk about "visual 'blue'" when it comes to the outer parts of galaxies? Aren't they so faint that they would generate no color response to the human eye whatsoever?
I'd expect the blue saturation to be high on the outside and low on the inside, even though the individual stars have the same photometric blueness.
They don't have the same photometric blueness. I have spent hours after hours checking up the B-V color of thousands of A-, B- and O-type stars. The bluest stars (the ones with the most negative O-B index, around -0.25 or so) are typically stars of spectral classes around B0, O9 and O8. Still hotter O-type stars are usually less blue, but there is a reason for that. The hottest and most massive O-type stars are so rare that they are typically so far away that they are always significantly dust-reddened. Still, I'm willing to at least consider the possibility that "maximum blueness" might be reached in stars of spectral classes B0, O9 and O8.
It is however absurd to claim that A-type stars and cool B-type stars are as blue as the hottest B-type stars and late O-type stars. That is very obviously not true. A-type stars are pretty common, so many are nearby, and it is overwhelmingly clear that they so rarely have a more negative B-V index than -0.01, and hardly ever as negative as -0.10.
Adam Block's image strongly suggests that the outer features of NGC 3310 are dominated by old and intermediate stars, which are non-blue in color. So the outer parts of NGC 3310 may be dominated by the light of stars of spectral classes F, G and K. Many galaxies have halos dominated by non-blue stars, so it would be no surprise whatsoever if this was true for NGC 3310.
And the bluest parts of NGC 3310 are found in its inner arms. Why should that fact not be respected? Why should the visual rendering of this galaxy be tweaked to resemble what we would see if we were almost blinded by the brilliance of spiral arms dominated by the light of OB stars, but could see and enjoy the color of the outer parts dominated by A-type stars? Galaxies are faint objects, the stars are always far apart, and the outer parts of galaxies are exceedingly faint. We are frankly never blinded by the brilliant light of galaxies.
The blue and white colors in this image are rendered in a way that doesn't let us see or understand why they look they way they do. There seems to be no scientific reasoning behind it. To talk about something as imprecise as visual color response in most humans is nonsensical in my opinion when it comes to an object as visually faint as a galaxy.