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Ask Astro: Why don't we see green stars? |
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January 2004. Why can't we see green stars? From Astronomy Magazine, "Ask Astro" p. 73. [question is from] Bill Howe and Patricia Linden, Mahwah, New Jersey [Answer]: You can't see green stars because there aren't any. You can, however, see stars as green. It's not the same thing. As you proceed through the colors of the rainbow from red to orange, yellow, green, blue, and violet, you also go up in energy. Deep violet light carries about twice the energy of deep red. A star is basically a hot body that radiates its energy into space. The colors of the light it emits depend on its temperature, which is a measure of the energy of the radiating surface gases. Stars can range in temperature from under 1000 kelvins (Celsius degrees above absolute zero) to over 100,000 kelvins. At low temperatures, stars can emit only low-energy radiation. A 1000 K star will radiate only in the infrared where we cannot see it. At 2000 K, however, a star radiates higher-energy visible red light as well as infrared. As temperature rises, the star emits more and more toward the violet end of the rainbow. If hot enough, it will radiate in the ultraviolet. At the same time, it is emitting ever-more low-energy radiation also. The eye takes all these colors and combines them into more subtle shades. Cool stars will appear reddish or orangered. Warmer ones, 4000 to 5000 K, may take on a yellowish or yellow-orange cast. At a temperature where the star produces a lot of green light, the human eye just combines all the colors into white. At the high-temperature end, we see a star as bluish-white. Green, in short, just gets skipped over. The only solitary star that appears green is Zubeneschamali (Beta Librae), but it looks just white to me. However, the eye can be fooled. If you place two subtly-colored stars in proximity (as in a double star), and contrast effects make the colors appear richer, then one of the stars (usually the fainter one) looks green. However, it's an illusion. The classic case is Antares, the red supergiant that hovers over a much fainter blue-white companion. The latter consistently has the reputation of looking green. But this is caused by the eye reversing itself due to the overwhelming reddish light of Antares. - Jim Kaler, University of Illinois (check out Jim's "Star of the Week" at http://www.astro.uiuc.edu/-kaler/sow/sow. html) |