Expert: Courtney Seligman - 5/10/2009

Question
Dear Courntey Seligman,


could you tell me what the TRUE color of Cat's eye Nebula is?


Is it that: http://karlrichard.files.wordpress.com/2009/03/the-cats-eye-nebula.jpg   ?


or that: http://image.guardian.co.uk/sys-images/Arts/Arts_/site_furniture/2007/06/11/cats...

?






thank you

Answer
I presume you mean, if you were looking at the nebula directly, without any optical aid or photo-editing, what color would it be? In that case, the answer would probably be pale gray-white, or perhaps in the brighter regions, greenish-white. That is the appearance which planetary nebulae have in general, when seen through very large telescopes. The reason for this is that the light from a nebula is too faint to trigger the color receptors in our eyes, in which case we see no color, but only shades of gray, or just barely bright enough, in which case we see greenish-gray, because our eyes are most sensitive to the yellow-green part of the spectrum.

If, however, you mean what color would the nebula have if it were bright enough to correctly stimulate the color receptors in our eyes, then the color would depend upon the exact mixture of wavelengths radiated by the nebula, which I haven't seen discussed in any detail. Given the temperature of the central star, the main radiation would probably be by hydrogen, which would give it a faint pinkish glow, but radiation by rarer atoms might add enough light to alter the color. I just don't have enough information to give you an exact answer.

Despite that uncertainty, I can state with confidence that the paler and less colored an image is, the more likely it is to approximate the actual color of the nebula. The brighter and more spectacularly colored it is, the more likely it is to be a 'false-color' image. So the 'former' image is probably a bit closer to the truth; but neither that nor any other color image is likely to show the nebula as you would actually see it.