A coal glowing red in a barbecue is cooler than our Sun, which appears yellow, which is cooler still than some stars which appear bright white. Visible light runs from the familiar blue to green to yellow to orange to red. Red light is the least energetic of the colors of visible light, and blue is the most energetic. Beyond the red end of the visible part of the spectrum lies the infrared and radio radiation. Many scientists are interested in studying the invisible universe of ultraviolet light, since the hottest and most active objects in the universe give off large amounts of ultraviolet energy.
Radiation travels in waves.
It's a convenient shorthand to avoid writing lots of zeroes when talking about the wavelengths of light. In everyday terms, a sheet of paper is approximately 1,000,000 angstroms thick. The ultraviolet portion of the spectrum being studied by Astro-2 covers the portion of the spectrum from about 900 angstroms to about 3,000 angstroms. Visible light, on the other hand, covers the range from 4,000 to 8,000 angstroms. Updated July 18, 1995.