Like beauty, color is in the eye of the beholder. As an example, consider the binary star system known as Izar. It forms the second-brightest star of Boötes, the herdsman, which is high overhead at nightfall.
Many stargazers describe the colors of its two stars as orange and green. But that's actually a bit of an illusion. One of the stars is yellow-orange, while the other is blue-white. The contrast between them tricks the brain into thinking that there's a greater difference in color than there really is.
The color itself comes from the temperature of the two stars -- the surface of the blue-white star is thousands of degrees hotter than the surface of its companion.
The blue-white star is about twice as massive as the Sun. It's in the prime of life, "burning" the hydrogen in its core to make helium. The energy from this nuclear engine heats the surrounding layers of gas, making the star glow blue-white.
The other star is about four times as massive as the Sun. Because it's heavier, it's already used up the hydrogen in its core, and is now burning the helium to make carbon. This change made its outer layers puff up like a giant balloon. The thinned-out gas cooled off, so the star shines yellow-orange.
Arcturus, the brilliant leading light of BoÃ¶tes, stands high in the south at nightfall. Fainter Izar is to its northeast. To the eye alone it shows no color at all. But a telescope reveals the true nature of this colorful binary.
Script by Damond Benningfield, Copyright 2009
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