Looking at answers to this question, people seem pretty convinced that human naked eye should be able to see the colour of ionised gas/dust cloud when they are inside an emission nebula.

And we know Solar system is surrounded by Interplanetary dust cloud, and possibly inside an Interstellar cloud. Why don't we see a colorised space backdrop?

Does it have to do with density of gas cloud? or how ionised they are? Does camera with long exposure see some background colour our naked eye can't?

(Excuse me for any wrong terminology.)

  • $\begingroup$ "Colorised" does not mean what you think it means. $\endgroup$ May 26, 2017 at 11:53
  • $\begingroup$ @Carl-Witthoft : A link to the dictionary may help to OP. However I confess to not knowing what the appropriate phrase might be in this case - any English experts care to enlighten me. :-) $\endgroup$ May 26, 2017 at 14:46
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    $\begingroup$ @StephenG I'd suggest "perceived in color" $\endgroup$ May 26, 2017 at 16:06

1 Answer 1


This is really more of a biology question, I think, so: The Mark-I Eyeball is far less sensitive to color than to intensity. There are rods and cones in the retina. The neural signals from the rods are used to report intensity only. There are different spectral sensitivity curves for different cones (typically three types in humans), so the relative outputs from all cones are used to generate color information. However, cones are less sensitive than rods, so for dim light such as that reaching us from nebulas, only the rods produce a neural output.

  • $\begingroup$ This is also why, when you look at stars with the naked eye, they're all basically white, even though in reality their color varies widely. $\endgroup$ May 29, 2017 at 4:31

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