Is the colour in pictures of nebulas, stars and galaxies their real colour or added?

Question

Is the colour in pictures of nebulas, stars and galaxies their real colour or added?

Answer 1

that depends...



the colour in most pictures is real, in the sense that it accurately reflects the spectra of the objects. they are the colours our eyes would see if they were millions of times more sensitive to light than they really are.



some images map different wavelengths to different visible colours, either for scintific or artistic effect. look up "wood effect" for a common (and beautiful) example.



emission nebulae are particularly troublesome. their light is mainly two wavelengths, a red one due to hydrogen, and a green one due to oxygen. if we see any colour at all we see green, bcuz that's the colour our eyes are most sensitive to.



the most commonly used colour films in the past weren't very sensitive to the particular shade of green, but were very sensitive to red. so emission nebulae photograph as red or pink, but are visually green. modern ccd imaging doesn't care either way, but many nebulae are photographed red to imitate the look of colour film.



ccd imagers, including those on hubble, are black and white devices. to take colour pictures you take individual exposures through colour filters, then add the colour in post-processing. digital cameras do this internally.

Answer 2

The color effect is not what the unaided eye sees. The light from faint objects cannot activate the color cones in the retina. Only the black and white rods are forming the image. And if you were much closer to these objects, particularly gas clouds, they are so thin that you would hardly detect their presence. It is only in the largest telescopes that even a faint hint of color (and sometimes a different one) can be seen by visual astronomers. The stars, however, can exhibit quite striking ranges of colors and combinations if close together. Their light is bright enough for color detection by the eye.

The colors can be false colors--that is, certain parts of the light are from different atoms or molecules. To accentuate that difference, the red of hydrogen line is made stronger, the green and blue from ionized oxygen is boosted, so that there is a greater differentiation for better research.

A long time ago (the 1950s or 60s) an issue of National Geographic showed film photos that were color balanced to a more natural level, but still much enriched. This was strikingly different in some cases, just more of the same in others.

And false color can be added to different levels of intensity of the same radiation to make a map of the intensity.

It all depends upon the purpose of the photos. Naturalness, artistic, or scientific are some of them.

Answer 3

Those are real colours most of the time, but they are too dim to trigger the colour receptors in our eyes; they only show up in long exposure images. Sometimes astronomers will use false colour images to bring out a particular feature, but that should always be indicated in the photo caption.

Answer 4

Stars and galaxies are often seen in more or less true color. However, nebulas are often shown in false color because they simply don't look very spectacular in visible light and you have to go to radio and infrared. Occasionally stars and galaxies may also be shown in false color, especially if they are very far away and hard to photograph in true color.

Answer 5

Good question...



Most pictures use real colour, and if they do not they tell you so specifically. However, most pictures are also taken with long exposure times, allowing colours to develop much better; if you were to look through the telescope at these objects, you would see only a hint of the intensity that comes out through photography. Of course, a lot depends on the instrument being used: images taken by Chandra (X-rays) or Spitzer (IR) are always in false colour. Another reason to use false colour is to bring out contrasts in the image: most standard astronomical images are in FITS format which supports palettes of 256 colours. We pick a palette in which contrasting colours are next to each other; that brings out all kinds of detail in the image. (Try it: SAO-DS9 is a good FITS viewer/analyzer and is available for all platforms).

Answer 6

The camera on Hubble can only take black & white photos. When we want to have the "real" colors: take one photo through a red filter Take a photo through a green filter take one photo through a blue filter Combine all three in a computer, asking the computer to put: red color where the first picture shows white green color where the second picture shows white blue color where the third picture shows white. What we get is an image with the color that we would see, IF our eyes were as sensitive as the camera on Hubble. In reality, when we look at faint galaxies in a telescope, our eyes will detect luminosity (as levels of grey), then a bit a green color (our eyes are a bit more sensitive to green than to the other colors, when the light intensity is low). In addition, when a picture is to be published (on the Web or on paper) we always play with the saturation, hue, contrast, etc, to make the picture look presentable (the action of preparing a picture for publication does cause changes anyways). Then your computer (if you view it on the Web) or the printer (if you print it) will also show the colors based on its parameters. Maybe the blue in your computer is a bit redder than the blue of the Hubble blue filter... and so on. Then, when pictures are taken with a space telescope, they often include light that our eyes cannot see (for example, infrared). In such a case, we have to use "false colors". We can't publish an infrared picture using infrared as the display color: our eyes cannot see infrared! However, when this is done, the caption should tell you. For research, astronomers have access to the original B&W images and the information about the filters and the exposure, so that they can get the information directly.

Answer 7

Many images are false color composites, but any image should have information about what element each color represents is in the caption or credits. Oxygen is often green in many false color composite astronomical images. If you are really interested, Google "additive and subtractive color theory", "8-bit RGB color scales", 8-bit gray color scales" and "8-bit IHS color scales."

Answer 8

The above people are right.



The colors are based off spectral lines, since all color images are made by combining many monochromatic images using different filters.



Typically, red signifies hydrogen emission, blue is oxygen, and sometimes you get other ones in there like yellow for sodium, etc. Usually, if it's a reputable site, you can find out what exactly is going on with the colors.

Answer 9

I thought I read somewhere that most of the time, the colours are just the different gasses being observed using infrared telescopes.

Answer 10

We deduce what the colors are according to their spectral lines and then color them accordingly