Galactic Blues

This is very much true in our galactic companion, the Andromeda galaxy, where you often see an exaggeration of the blue intensity well beyond what the photometry would suggest. The same would apply on the average galaxy out there which would need to appear more yellowish than the current fad in over-emphasize color contrasts would otherwise lead one to believe. Having said that it is true that the OB clusters form in the arms of the galaxies and showing them isn't supposed to raise any eyebrow.

My understanding is that blue in galaxy arms is an indication of blue giants which in turn indicates star formation because blue giants are the shortest lived, so if they’re around they must’ve been formed recently. Giant, so bright, and maybe few because they come and go so quickly, relatively speaking.

Cheers,
Scott

That's a good question.

In my pictures I don't tweak anything in the colors after SPCC is applied, yet in most cases galactic arms are blue. 

So there should be some truth in this.

John Hayes:
The type O and B stars found in the spiral arms of galaxies are super luminous so they are 3-6 orders of magnitude more luminous than the old yellow/red stars found in and around the core.  A lot of these stars form in the spiral arms and that's what gives the arms their characteristic blue-ish color.

John

That's true but O and B stars are very rare with type O stars making up about 0.000013% of main sequence stars, B type at 0.13. I'm also wonder how much of the x-axis of the H-R diagram is cut off by a typical RGB or uv/ir filter set. I mean, 400 nanometers equals peak blackbody emission of around 7400K. I know stars aren't black bodies but still, a large amount of the radiation from these super hot stars is far outside of anything we image. I'm not challenging the idea that the different stellar population types will display a different hue, I'm just trying to get a handle on how much given the sensitivity of our cameras and the narrow bandwidth of our filters.

Alan Brunelle:

John Hayes:
The type O and B stars found in the spiral arms of galaxies are super luminous so they are 3-6 orders of magnitude more luminous than the old yellow/red stars found in and around the core.  A lot of these stars form in the spiral arms and that's what gives the arms their characteristic blue-ish color.

John

Tony Gondola:

John Hayes:
The type O and B stars found in the spiral arms of galaxies are super luminous so they are 3-6 orders of magnitude more luminous than the old yellow/red stars found in and around the core.  A lot of these stars form in the spiral arms and that's what gives the arms their characteristic blue-ish color.

John

That's true but O and B stars are very rare with type O stars making up about 0.000013% of main sequence stars, B type at 0.13. I'm also wonder how much of the x-axis of the H-R diagram is cut off by a typical RGB or uv/ir filter set. I mean, 400 nanometers equals peak blackbody emission of around 7400K. I know stars aren't black bodies but still, a large amount of the radiation from these super hot stars is far outside of anything we image. I'm not challenging the idea that the different stellar population types will display a different hue, I'm just trying to get a handle on how much given the sensitivity of our cameras and the narrow bandwidth of our filters.

To respond to both, what John says is key.  Yes, these stars are rare, but if you look at my image of M31 here  and here (RASA14), where blue, white and yellow giants are clearly resolved, even the rarity that you, Tony, point to, is compensated by the magnitude difference.  In the second image, very many of the stars seen are resolved, even the fainter stars that make up the galactic halo.  Yet the spacing between these clearly blue giants is not that great, relative to their brightness.  Examples, especially the OB clusters (NGC206 a prime example) throughout, and the fact that these higher densities of blues giants are able to illuminate much of their surroundings, including reflection nebulosity as the light reflects off of local molecular clouds.  I have seen any number of locations, while carefully working up this data that shows such blue reflections adding to the blue cast.  In any case, your point of 0.000013% does not mean that there are only a handfull of blue giants!  Its all about the numbers and the visibility.  0.000013% of maybe even a trillion stars is still a lot of stars.  Also of related interest, is that it is estimated that of all the stars in the Milky Way, dwarf stars make up over 90% by number of the total stars.  Yet how many dwarf stars are even shown on any star chart.  Not many.  They basically are mostly invisible to our instruments.

Just to add, and to respond to the original post, a lot of the blue is actually excessive boosting of the color saturation and/or blue color.  As Andrea was alluding too, but maybe trying to be politically sensitive.  Fact is blue is really nice in these images, and it is easy to make these adjustments, so as art, many make them look nicer?  If you look at my two images, neither is particularly blue.  Yet the blue giant stars are blue, the red giant stars are red and yellow giant stars are yellow.  I specifically confirmed that from known data.  Yet these images use only SPCC for color correction.  On the other hand, the SPCC yielded slightly different hue casts for the two different data sets.  Something that is particularly troubling to me.  In any case, what other images you see of M31, maybe you can judge just what M31 should look like.  

BTW, never automatically use the professional images as a judge for color accuracy.  Data from the "Agencies" that is published is often enhanced for public consumption.  It is the point of these publications to make the public interested in the science, and ultimately excited as to what the taxpayers get for their moneys spent.  So the pressure to "pump it up a notch" is always present.

That was going to be my next question. It's sometimes hard to tell because of processing but most wide field images of our own galaxy really don't have the vibrant blue we're talking about. I think most of us can agree that in a lot of cases, it's the processing, not nature. I think that changes my question a bit. If blue, how much. I suspect that in a realistic representation it really should just be a very light blue tint and not Las Vegas neon blue. Alan's M-31 is I think, closer to the truth.

Tony Gondola:
That was going to be my next question. It's sometimes hard to tell because of processing but most wide field images of our own galaxy really don't have the vibrant blue we're talking about. I think most of us can agree that in a lot of cases, it's the processing, not nature. I think that changes my question a bit. If blue, how much. I suspect that in a realistic representation it really should just be a very light blue tint and not Las Vegas neon blue. Alan's M-31 is I think, closer to the truth.

I think your question opens a can of worms.  Yet it is a logical question.  Some of the questions raised in this thread have objective, definitive answers, even if those answers are still not completely resolved by the best of science.  But the question of what the color should be, as it relates to our astrophotography likely has no objective answer, even if parsed by science.  The reason I say this is because we all work in visible light with our amateur gear.  And if we are limiting ourselves to the visual spectrum, such as we are in this thread, then the fact is, none of these non-stellar objects show any color if we just look up at the sky.  Even some of the largest telescopes that one can use for visual inspection can't gather enough light to excite our color sensing retinal cone cells.  But I don't want to sound so negative, because our gear has the ability to act like much larger light buckets, such that they can deliver enough light to our eyes/cameras to show us their color.  However, how we represent that image is always couched in the qualifier as to how whatever technical assistance was use to present the image to the viewer.  The best way to try to understand how astro colors really look to the human eye is to go to a Bortle one site when the Milky Way is high, especially the Sagittarius region.  Look up first with naked eye and see the subtle colors.  Yes the brightest closest stars are bright enough to see real color, but even these colors are muted when compared to what most star colors are presented here on AB.  Then the star clouds can be seen and some of the brightest ones might show some slight differences in color (especially when viewed through binoculars), but only when in comparison to nearby other star clouds.  Now imagine, you are hovering above the Milky Way in such a way that it looks when sized like M31 is usually presented by astrophotography.  Under no circumstances will you see it in any way like any image of M31 that is presented here on AB.  No HII region will be visible.  Nor any of the other typical narrow band excitation regions, even though all of these narrow band lights fall squarely within our visual capability.  At best, the nebulosity that will show up, is some of the reflection nebulae associated with the brightest nebulae, such as Orion and the Lagoon.  The emission nebulae of these brightest nebulae only start to be seen in telescopes of decent size, and even those are still overwhelmed by the reflection components.  

That said, for a balanced OSC or LRGB image of galaxies, they often show distinct color biases.  And I believe that these are real.  In contrast to the blue boosted images you see of M31, M33 appears to be truly blue in nature.  It shows as such in almost all astrophotos (which is no objective measure).  But I see the blue even before I start processing it.  Objectively, M33 is known to be a relatively star-poor galaxy compared to M31.  It is also a much more active star producing galaxy.  I have read that it is likely producing stars at a relative rate much in excess of 5X that of the Milky Way and the Milky Way being even more active than M31.  That means many more young stars/area and many more of these are blue and blue giants.  So M33 has a reason for being so blue.  It also has no central bulge, which would add more yellow/red stars to the body of the galaxy.  M51 is also one that consistently is presented as strikingly blue and is nicely contrasted by its entangled partner, which is decidedly yellowish, thereby lending credence to the colors presented.  Now, then, your question asks really, how should these examples (and all other galaxies) be presented by the author of the art?  That is where the subjectivity imparts an endless spectrum of results, including color contrast, exact hue of the blue, etc.  Lately, I have adopted using SPCC in PixInsight to let the color be what it is.  I also have used either no color saturation or very light on the touch.  I think most people here would consider my presentations to be rather dull because of this.  And that suits me just fine.  If my red stars are a believable red, and my blue stars are a ..., and so on then I am happy that the galaxy looks as it does.  If I have to differentially color saturate my galaxy from my stars to make the galaxy look like everyone elses, then I know that I am working in a space I choose not to.  At least of the present moment.  I like what others do here, so I have no problem when I see pumped up the color in all of this stuff.  I do what I do for my own purposes.

andrea tasselli:
As a somewhat unrelated issue but mentioned here above, yes you can see colors in extended objects with amateur instruments. Or at least I could in the right conditions (read dark skies with night adapted vision).

Maybe some.  A few.  I can see just a hint of color in the Orion.  The Ring, which is bright, and not really extended...  And as I mentioned above, I can see slight color in Milky Way star clouds when under Bortle One skies.  But hardly would one suggest that color is the memorable part of the experience considering the dull color seen.  As per the topic targets, M31 or any other galaxy, which I routinely can see naked eye from even a Bortle 4 sky, I have never really seen any color of significance in my 5 inch.  Nor even in my 12 inch.  Maybe I can argue that I see a bit of a brownish hue throughout?  That was years ago.  Sadly now, at my age, I wonder if I can see much of anything other than the floaters in my eyes  I hear from written accounts from people who have looked through 30 inch amateur reflectors that they for the very first time start to see some color in some of the common extended objects we all photograph.

I am confused... Is this just  fake color that Bray Falls conjured up?
https://app.astrobin.com/i/dtmpz6

Yes, a lot.

I suppose artistic rendition would account for the latter, with a somewhat heavy hand in raising the saturation while for the former it is well over the bar.

Dan Kearl:
I obviously won't link all the Galaxy top Picks but  all of Wolfgang Promper galaxies look like this as well as most all the top pick Galaxies.
If it just artistic license then most Galaxies on this site are artistic renditions since blue is the rule and not the exception.

*Yes, by and large we all do artistic rendition of night objects. If you really want to see what the "natural" color would look like stick a DSLR on the back end of the scope and take a longish exposure. You'd surprised what will come out of it...