Since the Hubble Space Telescope began collecting data from the far reaches of the universe more than ten years ago, the public has been treated to striking images of stars forming, galaxies merging and other cosmic phenomena. But Hubble itself does not produce the colors in these amazing images. Astronomers add the jewel-toned hues, using software such as Photoshop.

Critics contend that the practice constitutes false advertising of the universe. But the Hubble astronomers say that they are not drawing colors on the celestial objects, but rather that they are drawing colors out of the Hubble images.

In other words, Hubble gathers data about the universe's features through layered images in gray. Hubble astronomers make multiple long exposures to draw out whatever color is present in the celestial object, using filters to block all but certain wavelengths of light. The filters, like those of television screens, are red, green and blue, because they can produce all the colors of the rainbow.

Once the unwanted light is filtered out, astronomers record the remaining light, producing images with colors that are more intense -- or sometimes less intense -- than the original tones, according to Zoltan Levay, imaging resource lead at Space Telescope Science Institute, which produces many of the Hubble images. Astronomers can also assign colors based on the object's chemical composition or other physical characteristics, he added, or if an image is taken outside the visible light spectrum, the processors can choose colors arbitrarily.

In some instances, a color is selected to better present the information, as was the case with the Eagle Nebula, Levay explained in an interview with the Online NewsHour. The towers of gases in the popular 1995 image were depicted in green -- even though the predominant gas, hydrogen, fits into the red part of the spectrum -- in order to separate it visually from sulfur, another element with a red wavelength.

These choices are a concern to some. Kenneth Brecher, a professor of astronomy at Boston University, said at a 2002 meeting of the American Astronomical Society in Albuquerque, N.M., that while the Hubble astronomers are not doing anything deceptive, the fact that the colors are drawn out is not always communicated well in the media. The process is also highly subjective, he said.

"The color of objects that astronomers release are not really representative of a thing one might imagine exists, which is the objective color of a star or a galaxy," Space.com quoted Brecher as saying. "Color is a very, very subjective phenomenon. Color is in the eye of the beholder."

Indeed, if people were to travel to some of the cosmic phenomena that Hubble imaged, they would often find much fainter, mostly white objects. That's why humans use telescopes, which are designed to capture light and color in ways the human eye cannot, Levay said.

The images from Hubble are not meant to produce a literal interpretation, but rather a picture that represents as much information as possible -- such as temperature, velocity or an object's material composition, he said. And, "sometimes it's just a matter of aesthetics," he added.

Another aspect of Hubble images -- the oddly shaped stair-step effect -- comes from the use of the Wide Field and Planetary Camera 2, or WFPC2. WFPC2 is made up of four cameras, one of which magnifies the view of the object. When the magnified view is reduced to match the proportion of other three images, the shape of the entire picture changes from a rectangle to a stair-step shape as seen in the Eagle Nebula (left).

Other orbiting observatories provide images that are later colorized, including the Chandra X-ray Observatory. And after the James Webb telescope is launched, more information will be gathered from even earlier reaches of the universe that astronomers will then seek to explain in a way that can be visually understood.

-- By Larisa Epatko, Online NewsHour