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I was trying to photograph some star trails last night and I kept noticing that a number of stars in the upper half of the photo were showing up as blue. I'm really not too sure as to what's going on here. Is there anything I can do to try to prevent this?

  • Camera: Sony A99
  • Lens: Sigma 24-105, f4.0(set to 24mm, f8.0)
  • Exposure: 10mins
  • ISO: 250

enter image description here

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Some stars are showing up as blue because some (actually, a lot of) stars are blue.

You can:

  • adjust white balance (making more stars appear orange instead)
  • reduce color saturation (this could make the image closer to what we really see at night (when our color vision is impaired))
  • increase the exposure (time or sensitivity), turning more stars overexposed and white (but it also makes fainter blue stars more visible)
  • Aha, thank you. I had just figured I was doing something wrong or there was something wrong with my camera or lens. That puts my mind at ease. – Philip Gibbons Feb 2 at 18:14
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    One reason it's easy to end up with WB adjustments that bring out the blue in stars is that light pollution on a hazy sky tends to be fairly yellow, so balancing this out tends to amplify the blue. – junkyardsparkle Feb 3 at 5:09
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    @junkyardsparkle That really needs to be an answer because I believe it is the prime explanation for the example in the OP. I was about to write such an answer myself when I saw your comment. – Michael C Feb 3 at 15:13
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    might be, however in this photo white balance seems to be adjusted for the stone blocks in the foreground – szulat Feb 3 at 16:19
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    Shoot Orion and you'll get blue and red stars in the same photo. – Jim MacKenzie Feb 5 at 3:03
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To expand on the reference to white balancing in @szulat's answer: If you don't explicitly white balance the image to a reference temperature, arbitrary light sources in the scene may end up being the reference for the chosen white balance, either visually or due to using "auto" white balance. In the posted image, it may have been the lighting on the rocky foreground. I would guess that it had a lowish color temperature and perhaps a slight greenish cast, and balancing for this would produce a bluish-purple cast to the rest of the image, including the stars.

For a somewhat "objective" reference, this shot of the sky over Los Angeles, full of light pollution and helicopters, is balanced to ~6500K, roughly what you might get from a flash tube or overcast sky. The stars range from bluish to yellow-orange colors...

enter image description here

Due to the light pollution, many of the fainter bluish star trails are visible due more to contrast in chroma than lightness (and might not be visible at all to someone with blue-yellow vision deficiency), while fainter trails close to the color of the light pollution might be completely obscured. Now, if we balance the image to make the clouds a little more neutral...

enter image description here

...the similarly colored trails have also become more "white", while the bluer ones have become even bluer. Since modern light sources can be all over the place in terms of spectrum, their effect on auto white balance for images of things like sky objects can be somewhat unpredictable.

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