False colors vs. real colors

Main page▹Observations	9.2.2016 \|

What are real colors of deep sky objects? Philippe Bernhard's unique images of the same nebula acquired through various filters allow to explain what are “false color” images and why are they used to visualize details in the emission nebulae, which shine also in wavelengths invisible to human eye.

Universe is often considered to be just black and white when observed by naked eye or small telescope. That's because our eyes do not distinguish colors when the light intensity is very low and with the exception of bright planets (Mars is reddish, Jupiter and Saturn color is yellow-brown) and stars (for instance Antares looks also somewhat reddish), all star clusters, nebulae and galaxies appear monochrome (maybe it is possible to distinguish magenta hues in the Great Orion Nebula when observed through a telescope, because it is really bright). Long exposure CCD photography can reveal colors of all deep-sky objects and galaxies or planets are often displayed in real colors (well, often enhanced to increase image impact).

Wide field image of the M31 Great Andromeda Galaxy shows real colors

Images of nebulae are sometimes displayed in false colors. This is because emission nebulae shine mainly in three narrow spectral lines, depending on their chemical composition, as opposed to light of stars (and thus whole galaxies), which spectrum is continuous (with the exception of some absorption lines, of course). These lines are named according to the chemical elements, which are responsible for the emission—Hα, OIII and SII. While OIII line is close to the green-blue color, Hα line is so deep red, that it is hard to see by many people and SII line is even redder than Hα, practically invisible for humans. This is why astronomers assign false colors to narrow-band images to show details, which would otherwise remain hidden to us. And we must understand, that what is in reality deep-red, is displayed as green-blue on narrow-band images.

The “North America” and “Pelican” nebulae in real colors (left) and in false colors (right)

The real difference between emission nebula real colors and false-colored narrow-band image is exceptionally well illustrated by the images of the North America and Pelican nebulae above. Both images were combined from individual exposures acquired through red, green, blue and narrow-band filters, acquired with the same telescope and camera.

Remark:

Let us note that even the “real” red color on the left-hand image is somewhat false. In reality the Hα spectral line, which is responsible for the red hue, cannot be displayed on any monitor. While the wavelength of the Hα line is 656 nm, the red color used on monitors (as well as red LED etc.) wavelength is around 630 nm. So in the red should be even deeper displayed.

All images on this page courtesy of Philippe Bernhard. Images were acquired with G4-16000 CCD camera on RH200 telescope.