Astronomy in Color
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How does Astronomy in Color Work?

Astronomy in Color allows you to create color images of astronomical objects from a selection of monochrome (black and white) images taken in different wavelengths of light using a variety of astronomical facilities and color filters.

Different sets of monochrome images are available for each object. The images labeled violet, blue, green, red, and far red are taken with filters that transmit a broad range of colors centered in the violet, blue, green, etc. Other monochrome images are taken using filters that transmit a narrow range of colors centered on wavelength emitted by particular atoms in hot gas. The H-alpha filter, for example, isolates hydrogen emission and indicates regions where the gas has temperatures in the range...

X-ray, gamma-ray, and ultraviolet images are taken from space telescopes designed to observe the sky in those wavelength regions. Images at radio wavelengths are obtained using radio telescopes on the ground.

Astronomers use multi-wavelength images to compare and contrast an object's appearance at several wavelengths. Stars, gas, and dust all glow in different parts of the electromagnetic spectrum. By combining images from different wavelength regions, astronomers can visualize how the gas, dust, and stars in a nebula or a galaxy interact, and can see how physical conditions vary within the object.

Basic Operation

  • Click "continue" on the dropdown blackboard to see a list of images available
  • Select an image and click continue
  • From the list of available monochrome images of the chosen object, select which image to display as red, and click continue.
  • Repeat to select which images to display as green and blue.
  • The three monochrome images will appear on the left side of the screen, and the combined images will appear as a composite color picture in the large image box.
  • Use the sliders next to each monochrome image to adjust the brightness of each monochrome image in the composite.
  • To save the composite image, click on the word "Save" in the lower right corner of the screen. Images can be saved either in PNG format or in JPG format. The images are all 512x512 pixels.
  • Clicking on "Choose again" on the right edge of the screen will allow the user to select a new image.

Choosing Colors

Natural Color - Astronomers often want to produce a natural color image that represents how the object might look to the eye. To produce a natural color image, select the available red, green, and blue monochrome images, placing the red image in the red color box, the green filter image in the green color box, and the blue filter image in the blue color box.

Pseudo-Color - To produce images of astronomical objects observed at wavelengths the eye cannot see, astronomers assign the monochrome images to be displayed as red, green or blue. For example, for the Horsehead Nebula, images in x-ray, infrared, and radio bands are available. The X-ray image could be displayed as red, the infrared image as green, and the radio image as blue. Such pseudo-color images do not portray how the object might look to the eye, but do allow astronomers to compare how the X-ray, infrared, and radio light from an object are distributed in space.

What information do we get from different wavelength regions?

  • The blue, green, and red filters tell us how bright an object is in each of those visible wavelength regions. Visible light comes from stars, and the relative amounts of blue and red light determine the color an object appears to be. Objects with more blue than red light appear blue, while objects that are brighter in the red than in the blue appear yellow, orange, or red.
  • Infrared light comes from cool stars and warm dust, with temperatures between 10K and 1000K (degrees Kelvin). Star-forming regions are usually bright in the infrared.
  • Radio light comes from cold hydrogen in interstellar gas and from molecular clouds in space. Temperatures are typically 10K or cooler.
  • Ultraviolet light comes from very hot stars and quasars.
  • Gamma-ray and X-ray light comes from highly energetic regions - stellar flares; accretion disks around white dwarfs, neutron stars, and black holes; pulsars; and from regions where fast-moving gas encounters slower-moving gas creating a shock front. X-ray light also comes fromr million-degree gas in clusters of galaxies.


Support from the National Science Foundation through the CCLI program is gratefully acknowledged.



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Last updated: 20 November 2010
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