Extraordinary advances have occurred in the last decade in our ability to process images in order to gain maximum information from them. Most of this is due to the advent of cheap and powerful computers. The widespread use of computers has permeated every field of science. Astronomy makes extensive use of digital information. This marks a profound change in our way of viewing the world. It is now easy to convert each picture element or pixel in an image into a number. Such a digital image may come not only from a CCD; an ordinary photo can be scanned and measured and converted into such pixels, which is called digitizing the image. The position of each pixel and its brightness is stored in a computer. Then the image can be processed in many ways. A technique that helps to emphasize subtle details of images is to assign different colors to each brightness level. In this way, we could produce an image in which the colors have nothing to do with the original colors but are used merely as a code to allow us to separate features of slightly different brightness. Such images are called false color images. When you see images made at radio or X-ray wavelengths they are by definition false color since the eye cannot detect those wavelengths. Mapping an invisible wavelength of radiation into a visible color is an excellent way for us to appreciate what is revealed by observations across the electromagnetic spectrum.
False color composite image of the Orion Nebula as seen by the Hubble and Spitzer Space Telescopes. Click here for original source URL.
False color can be misleading, because it does not necessarily contain any physical information. For this reason, optical astronomers prefer to use "true color" images, though many texts and magazines are full of false color imagery. "True color" is in quotes because at some level of detail it is difficult to reproduce all the nuances of natural color, especially in faint objects. Scientists working at the Space Telescope Science Institute made a conscious decision long ago to give Hubble Space Telescope images as much fidelity as possible to what the eye would see. So when you see HST images, especially from the "Hubble Heritage" project, their colors are quite realistic. How can we use a CCD to make a "true color" image? As with photography, this is done by combining two or three CCD exposures through different color filters into one final color image. Images made at wavelengths outside the visible spectrum are by definition "false color", since the eye cannot detect these types of radiation! However, some attempt is made to adhere to the sense of the electromagnetic spectrum, so that longer infrared wavelengths are shown in red and shorter infrared wavelengths are shown in blue, for example.