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The pictures of nebulae are always depicted as vividly colored and with a great variety of colors:

Omega nebula Source
Omega nebula

Eagle nebula pillars Source
Eagle nebula pillars

According to the English wikipedia article on nebulae, under the section Formation, «as the material collapses under its own weight, massive stars may form in the center, and their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths

This means that we can see the nebulae, but not whether we see them colored or not.

Furthermore, in this article on Wired.com, there is a photograph of the Omega Nebula, which states: «Looking more like a painting than an astronomy photograph, the Omega nebula glows with vivid colors in this new image from the European Southern Observatory’s Very Large Telescope. [...] The heart of the Omega nebula contains roughly 35 stars, the hottest of which spew out ultraviolet radiation that causes the surrounding hydrogen gas to glow red and pink.»

On another site, seasky.org, the article states: «They are also among the most beautiful objects in the universe, glowing with rich colors and swirls of light. Stars inside these clouds of gas cause them to glow with beautiful reds, blues, and greens.»

What I want to know is: if we could observe directly these nebulae, either by telescope or with our own eyes, would they appear like this? If not, how?

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I can't add the tag [nebulae] due to my reputation. Can someone add it, please? –  Alenanno Apr 13 '12 at 10:55
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+1 I've also heard that the color of Mars in rover-based photographs doesn't quite match what a human observer standing on Mars would see. –  Andrew Grimm Apr 13 '12 at 12:41
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@Alenanno sorry, but I can't remember. –  Andrew Grimm Apr 13 '12 at 12:44
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They surely look pretty though! –  Ivo Flipse Apr 13 '12 at 16:01
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Is this really a question for Skeptics? You're not challenging a claim. This is more about you know knowing the answer to a simple scientific principle. This seems better suited to the Astronomy site... maybe. –  Robert Cartaino Apr 13 '12 at 19:28
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2 Answers 2

up vote 43 down vote accepted

From HubbleSite - Behind The Pictures:

The Meaning of Color

Meaning of Color

The colors in Hubble images, which are assigned for various reasons, aren't always what we'd see if we were able to visit the imaged objects in a spacecraft.

We often use color as a tool, whether it is to enhance an object's detail or to visualize what ordinarily could never be seen by the human eye.


Example: The Eagle Nebula

Eagle Nebula

Note:
The three B&W images each correspond to a certain wavelength (=color).

Three Colors

  • Left: Oxygen - Green
  • Middle: Hydrogen - Red
  • Right: Sulfur - Red

Each of these B&W images are then (re-)assigned a unique color, because otherwise the red light from hydrogen and that from sulfur would be hard to tell apart.


Choosing Colors

Color as a Tool


Why are the inital images B&W?

The digital camera you have at home basically works the same way: Light is "converted" into electrons.

From Panasonic:

Since a CCD can only distinguish whether light is strong or weak, color filters are used to obtain each color's light and dark information.

RGB

(I modified the above picture to make it more clear)

One can see that the yellow flower emits more in red and green (light) and less in blue (dark).

RGB [RGB Color Model on Wikipedia]


Hubble's Color Filters

Hubble uses special filters to "screen out" the types of light from an object that astronomers are not currently studying.

These filters allow only a certain range of light wavelengths through. Once the unwanted light has been filtered out, the resulting light is allowed to fall onto one or more light-sensitive detectors.

Spec Wheel

This produces a "picture" of the star in the selected wavelength.

Since the detectors can detect light outside the visible light spectrum, the use of filters allows scientists to see "invisible" objects — those only visible in ultraviolet and infrared wavelengths.

[Source]


Example: The Eagle Nebula

M16

(Source: ESA)


The Eagle Nebula in Visible Light (Wide Angle)

M16 visible

Three-colour (blue, green, red) composite mosaic image of the Eagle Nebula (Messier 16, or NGC 6611), based on images obtained with the wide-field camera on the MPG/ESO 2.2 m-diameter telescope at the La Silla Observatory.

At the centre, the Pillars of Creation can be seen.

This wide-field image shows not only the central pillars, but also several others in the same star-forming region, as well as a huge number of stars in front of, in and behind the Eagle Nebula.

(Source: ESA)



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What would make this a killer answer would be natural color photos of the Omega and Eagle nebulas for comparison. Anyone? –  Oddthinking Apr 13 '12 at 17:27
    
@Oddthinking The "visible" light images in the above seem to be what you're asking for, no? –  Larry OBrien Apr 13 '12 at 19:25
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@Larry: Yes, Oliver stepped up after my comment and expanded his answer. I had already upvoted it, so to upvote him again, I've had to use a special supervote (you may need a custom camera to see it - it only emits in ultraviolet wavelengths.) –  Oddthinking Apr 14 '12 at 1:04
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A “tl;dr: Yes.” sentence at the top would be very helpful … –  Konrad Rudolph Apr 16 '12 at 11:15
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Added clarification about B&W images. BTW, I never took this question as "Do nebulae have color?" but as "Are the colors in the images "true" colors?" –  Oliver_C Apr 17 '12 at 10:36
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To the human eye, nebulae always appear overwhelmingly B&W. Human dark-adapted eyes are much more sensitive to B&W (wikipedia) and their color sensitivity shifts. There are only a few nebulae that are reliably visible to the naked eye and your pupil just can't pick up enough photons to detect color.

This discussion of amateur astrophotography breaks down the colors that you would be able to perceive with enough aperture: essentially there's a lot of very faint red up there (from hydrogen emissions) and there's some amount of relatively "bright" blue that's caused by scattering similar to what happens in our atmosphere during the day. You can reliably perceive blue nebulosity in, for instance, the Orion nebula with moderate amateur equipment. With exceptional equipment and conditions (a 24" Starmaster at 9000' on Mauna Kea) I've perceived faint red and green. Modern professional telescopes do not allow for direct visual observation :-( .

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