In artistic renditions of space scenes, there were stunning depictions of gasses or galaxies with bold red and blue colors immediately adjacent to one another, in contact as water is with oil in a container. I wonder, are there known arrangements of gas like that, with both ends of the color spectrum adjacent to one another? If so, what are some examples, what is known about them, and what causes the two colors?

One reason I'd call it into question is that I'd imagine the properties of boldly blue gasses are different than those of red gasses. Maybe that assumption is wrong because the property of color isn't necessarily associated with other properties. Another assumption would be that gasses with very different properties wouldn't find themselves collecting adjacent to one another in space. That assumption could also be wrong - I'm not an astronomer and only have basic understanding of physics and chemistry.


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Images that you see of nebulae show more intense colour than you could ever see with your eyes. Most nebulae are too dim to for our colour vision to be very active when looking at them. Cameras are designed to try and mimic human sight, to produce an image with colours that is close to what we would see. Telescopes are doing science and the colours are there to enhance features that we couldn't otherwise see.

A telescope like Hubble will image a region at several wavelengths. Some of these wavelengths might be in the infrared or ultraviolet, beyond human sight. Then a colour is assigned to each wavelength, and the images are mixed. (See this description of how to achieve this with photoshop) This brings out details that would be lost in natural colour.

Now there are two main reasons that dust and gas in a nebula might shine. One is that it is reflecting the light of nearby stars, the other is that it has been energised and is emitting its own light.

Hydrogen gas will emit light with a red colour (in the right conditions), and some stars shine in blue-white light. If an emission nebula is next to a reflection nebula then you will get blue-on-red. A good example of this is the Trifid Nebula in Sagittarius. As in this photo by Scott Rosen

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The lower, reddish part of the nebula contains the emission nebula — hydrogen gas which is emitting red hydrogen light since it is being heated by the hot young stars within the nebula. The upper blue portion of the Trifid is a reflection nebula — it consists of dust particles that are reflecting the light from the hot stars contained within the emission nebula. Because the hot stars are mostly blue, we see the reflection nebula as having a blue hue.


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