Shades of opinion
Colour in astronomy is a difficult and often controversial topic. Rather than just lump all the photons together, we like to use separate filters which allow a standard set of wavelengths through; that means that when I talk about a ‘blue’ image, then I will certainly be talking about the same ‘blue’ as anyone else. Each colour encodes information; blue represents young stars, red old stars or emission from dust. This is all very well, but the problems start when we try to produce colour images. Which filters do we use? No camera has the same response as the human eye, so this can be a controversial question. The Galaxy Zoo images are based on three of the Sloan filters – blue is blue, but red is green and infrared is red. That’s why the Voorwerp appeared as a blue blob even though it’s really green.
Yet it’s worth the effort; colour images are a natural way for us to process information. So much so that I’ve been jumping ahead of the careful work that Bill and Anna have been doing in processing the images and producing rough and ready colour versions of a few. Here’s a standard field; the colours are blue=blue and red=very near infrared, with green being an artificial combination of the two.
Of course, the power of this approach is in looking in detail at our overlapping galaxies. Remember this system? From the monochrome image it’s hard to tell which galaxy is on top of which, but in (exaggerated) colour it’s obvious.
You can see the blue spiral arms crossing in front of the edge-on galaxy, whose dust lanes are highlighted in red. One of the arms is clearly thicker than its tenuous neighbour; quantifying the difference between the two is one of the results we’re after.