Select Sky Object

The Image Builder Page

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Sky Object:

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The Instruction / Help Page

Basic Operation

  • Tap the "Select Sky Object" button in the upper left to display the control panel
  • Select an object from the available list of monochrome (grayscale) images
  • From the list of available monochrome images of the chosen object, select which image to display as red, green or blue.
  • A color image combining the three monochrome images will appear on the screen.
  • Adjust the amount of red, green and blue contributions to the color image. The color intensity can be adjusted from 0 (dark) to 100 (full intensity).
  • Save the composite color image using the control panel. Images are saved in jpg format. If the save function isn't working for you, try a different browser.

About Astronomy in Color

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 hydrogen gas is in an excited state. 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.

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.

False 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 false-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. Astronomers also use the term "pseudocolor" when color is used to portray different intensities of light in a monochrome image or different properties of an object, such as temperature or velocity.

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 gas.
  • 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 from million-degree gas in clusters of galaxies.