Online Astronomy eText: Introduction to This eText
False Colors in Astronomical Photographs
  Almost every photograph of astronomical bodies that you see, whether on the Internet, or in a book, magazine, newspaper, or television program, is a 'false' image in one or more ways. For planets, the photographs generally (although not always) show the correct colors, but the pictures usually show those colors far more intensely, and with much higher contrast than they would appear in real life. This is done partly to make the pictures more attractive, and partly to allow low-contrast features which would be hard to discern in a true-color photograph to be more easily observed.
  For extrasolar images, photographs may be 'false' in several ways. Almost every object which lies beyond the boundaries of our Solar System -- whether a group of stars, a galaxy, or a cloud of gas and dust -- is so faint that even if you were up close to the object it would be only a faint grayish or grayish-green blob. As a result, if an image of such an object looks 'nice', you can count on it that it has been altered from the real view to give it, as in the case of planetary photographs, far greater color, brightness and contrast. Unless noted as being in 'true' color, the colors which are shown are almost never the real ones (as an example, visit Behind The Picture (originally on the HST site, but apparently moved or removed, so this link uses its archived version on the Wayback Machine)). Obviously, ultraviolet and infrared wavelengths, which are not visible as any kind of light or color, are assigned arbitrary colors, and it is not unusual for even visible-light colors to be altered where it produces a more pleasing result. A dozen different photographs of the same object may therefore present a dozen completely different appearances, and not a single one of them is likely to be a 'true' view (even 'true-color' views almost always being made far brighter than in real life, as for planets).

Left: (MegaPrime Camera, Canada-France-Hawaii Telescope, apod030429)
Right: (Jay Ballauer (All About Astro, 3RF), apod060324)
On the left, a "red" image of the Rosette Nebula, showing hydrogen emission in red, and oxygen emission in a greatly exaggerated green, producing a yellow hue where both emissions are strong. On the right, a "blue" image of the same region, showing emission from sulfur atoms in red, hydrogen in blue, and oxygen in green. Neither image is a true-color image of the Nebula, but each image, by emphasizing different emissions, reveals slightly different structures.

  This does not mean that the images you see are completely false. The objects might well look as they are shown, if we had eyes which worked quite differently from the way that they do, and their shapes and sizes, and relative brightnesses, are usually more or less as shown. But if you have the opportunity to look through a telescope, you will never see what you see in textbook photographs, and may be very disappointed, if that is what you expect to see. However, actually seeing the object as it was when the light by which you see it left it, is in many ways a far more profound experience than viewing it in a photograph, no matter how nice the photograph may appear, in comparison to the actual view through a telescope.